Convert a single-flight panic into an error

- flightGroup.Do recovers a panicking fn so the leader and every waiter get a non-nil error instead of a zero-value success served as 200 []
- Note in the README that facts_cache_bytes budgets body bytes only

.woodpecker/docker.yaml

@@ -1,23 +1,20 @@
-# Build and push the pdbmux container image on a v* tag. pdbmux is a k8s-only
-# daemon (deployed via argocd-apps), so it ships as an image. Mirrors the estate
-# convention: the woodpeckerci/plugin-docker-buildx plugin pushes to the Gitea
-# registry using the droneci / DRONECI_PASSWORD credentials.
+# plugin-docker-buildx is the CA-baked variant; artifactapi's cert is not in the default trust store.
 when:
   - event: tag
     ref: refs/tags/v*
 
 steps:
   - name: docker
-    image: woodpeckerci/plugin-docker-buildx
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/plugin-docker-buildx:latest
     settings:
-      registry: git.unkin.net
-      repo: git.unkin.net/unkin/pdbmux
+      registry: artifactapi.k8s.syd1.au.unkin.net
+      repo: artifactapi.k8s.syd1.au.unkin.net/docker-internal/pdbmux
       dockerfile: Dockerfile
+      buildkit_config: |
+        [registry."artifactapi.k8s.syd1.au.unkin.net"]
+          ca = ["/etc/docker/certs.d/artifactapi.k8s.syd1.au.unkin.net/ca.crt"]
       build_args:
         VERSION: ${CI_COMMIT_TAG}
-      username: droneci
-      password:
-        from_secret: DRONECI_PASSWORD
       tags:
         - ${CI_COMMIT_TAG}
         - latest

Dockerfile

@@ -1,6 +1,3 @@
-# Container image for pdbmux, the merging PuppetDB proxy daemon. pdbmux is a
-# k8s-only service (deployed via argocd-apps), so it ships as a distroless
-# static image rather than an RPM.
 FROM golang:1.25-alpine AS builder
 
 RUN apk add --no-cache git

Makefile

@@ -9,7 +9,6 @@ ARCH    ?= $(shell go env GOARCH)
 
 all: build
 
-# Build the single static binary into dist/.
 build:
 	CGO_ENABLED=0 GOOS=$(OS) GOARCH=$(ARCH) go build $(GOFLAGS) -o $(DIST)/$(BINARY) .
 
@@ -28,8 +27,6 @@ clean:
 install:
 	go install $(GOFLAGS) .
 
-# Bump helpers — read the latest semver tag and create the next one.
-# If no tag exists yet, start from v0.0.0.
 _LATEST := $(shell git tag --sort=-v:refname | grep -E '^v[0-9]+\.[0-9]+\.[0-9]+$$' | head -1)
 _BASE   := $(if $(_LATEST),$(_LATEST),v0.0.0)
 _MAJ    := $(shell echo $(_BASE) | sed 's/^v//' | cut -d. -f1)

README.md

@@ -1,21 +1,21 @@
 # pdbmux — merging PuppetDB proxy
 
-`pdbmux` is a small HTTP daemon that fronts **two** PuppetDB backends and serves
-a single, merged PuppetDB v4 query surface on one address. Point `node-lookup`,
-`pblastreport`, or anything else at `pdbmux` instead of a raw PuppetDB and it
-sees one consistent view spanning both.
+`pdbmux` is a small HTTP daemon that fronts **several** PuppetDB backends and
+serves a single, merged PuppetDB v4 query surface on one address. Point
+Puppetboard, or any other PuppetDB API client, at `pdbmux` instead of a raw
+PuppetDB and it sees one consistent view spanning all of them.
 
 ## Why
 
-During the VM→k8s Puppet migration there are two PuppetDBs:
+Running more than one PuppetDB — during a migration between two of them, or
+across regions — means a given node's current data lives in exactly one at any
+moment, and consumers have to know which, or query each in turn. `pdbmux`
+merges them all so consumers don't have to know (or query twice) which PuppetDB
+a node currently lives in.
 
-- **old** — the legacy Consul-registered `http://puppetdbapi.service.consul:8080`
-- **new** — the k8s `https://puppetdb.k8s.syd1.au.unkin.net` (TLS terminated at
-  the gateway; backends are plain PuppetDB on 8080)
-
-Nodes move from old to new as they migrate, so at any moment a given node's
-current data lives in exactly one of them. `pdbmux` merges both so consumers
-don't have to know (or query twice) which PuppetDB a node currently lives in.
+All backends are equal — `pdbmux` is never told which one to favour. Backend
+names are arbitrary labels and there is no fixed number of them. The configured
+order is used only as a tie-break, so output is reproducible.
 
 ## Endpoints
 
@@ -24,133 +24,232 @@ not PQL) is forwarded verbatim.
 
 | Path | Behaviour |
 |---|---|
-| `GET /pdb/query/v4/nodes` | Fan out to both backends, dedupe by `certname`, keep the record with the newer `report_timestamp`. |
-| `GET /pdb/query/v4/facts` | Fan out to both, and per `certname` keep **all** facts from the backend that owns that node (see merge semantics). |
-| `GET /pdb/query/v4/*` (any other) | Transparently proxied to the **primary** backend, unmerged, streamed verbatim. |
-| `GET /healthz` | Per-backend reachability. `200 {"status":"ok"}` if all reachable, `200 degraded` if some fail, `503 down` if all fail. |
+| `GET /pdb/query/v4/nodes` | Fan out to all backends, dedupe by `certname`, keep the record with the newer `report_timestamp`. An `extract`/`count` query is **summed** instead. |
+| `GET /pdb/query/v4/facts` | Fan out to all, and per `certname` keep **all** facts from the backend that owns that node (see merge semantics). |
+| `GET /pdb/query/v4/resources` | An `extract`/`count` query is fanned out and **summed**; any other query is an unmerged pass-through. |
+| `GET /pdb/query/v4/reports` | Fan out to all and serve the **union**, deduped by report `hash`, re-ordered and re-paged across backends. |
+| `GET /pdb/query/v4/events` | Fan out to all and serve the **union**, deduped by record identity, re-ordered and re-paged. |
+| `GET /pdb/query/v4/event-counts` | Fan out to all and **sum** each subject's counts into one row per subject. |
+| `GET /pdb/query/v4/aggregate-event-counts` | Fan out to all and **sum** the summary object's counts. |
+| `GET /pdb/query/v4/reports/<hash>/{events,logs,metrics}` | Ask every backend; serve the answer from whichever backend actually holds that report. `404` when none does. |
+| `GET /pdb/query/v4/*` (any other) | No merge rule, so backends are tried in configured order and the first success is streamed back verbatim; if all reject it, the first upstream error response is replayed. |
+| `GET /pdb/meta/v1/version` | Fan out to all and report the **lowest** version any backend runs. |
+| `GET /pdb/meta/v1/server-time` | Fan out to all and serve the first reachable backend's clock. |
+| `GET /metrics/v2/read/<mbean>` | Fan out to all and merge the Jolokia response; numeric attributes are **summed** by default (see merge semantics). |
+| `GET /metrics/v2/list` | Fan out to all and serve the **union** of the backends' MBean trees. |
+| `GET /metrics/v1/mbeans[/<mbean>]` | Same merge, applied to the legacy envelope-less body. |
+| `GET /healthz` | Per-backend reachability plus cache state. `200 {"status":"ok"}` if all reachable, `200 degraded` if some fail, `503 down` if all fail. |
 
 Fan-out is concurrent. If one backend errors or times out, `pdbmux` serves the
-survivor's results and logs a warning; a merged endpoint only returns `502` when
+surviving backends' results and logs a warning; a merged endpoint only returns `502` when
 **every** backend fails. Response records are passed through as raw JSON so
 unknown fields survive untouched.
 
 ## Merge semantics
 
 - **`/nodes`** — dedupe by `certname`; the record with the strictly-newer
-  `report_timestamp` wins. On a tie (or when a node exists in only one backend),
-  the **preferred** backend's record is kept.
-- **`/facts`** — node-level granularity. For a `certname` present in both
-  backends, `pdbmux` keeps **all** of that node's facts from **one** backend and
-  drops the other's, chosen by the merge strategy:
+  `report_timestamp` wins. On a tie, the backend listed first in `backends`
+  supplies the record — a tie-break only, so the merged output is deterministic.
+- **`/facts`** — node-level granularity. For a `certname` present in more than
+  one backend, `pdbmux` keeps **all** of that node's facts from **one** backend and
+  drops the others', chosen by the merge strategy:
   - **`freshness`** (default) — attribute each `certname` to whichever backend
     holds its newer `report_timestamp`. `pdbmux` derives this from a per-certname
-    freshness map built by querying `/nodes` from both backends, cached for
-    `freshness_ttl` (default 30s). Ties/fallbacks use `prefer`.
-  - **`static`** — always keep the `prefer` backend's facts for shared nodes.
-    No extra `/nodes` query.
+    freshness map built by querying `/nodes` from every backend, cached for
+    `freshness_ttl` (default 30s).
+  - **`static`** — skip the extra `/nodes` query and take each shared node's
+    facts from the first backend in configured order that holds it.
   - A node present in only one backend always appears (falls back to whichever
     backend actually returned facts for it).
+- **`/reports`, `/events`** — **union**, not a per-node winner. Reports are
+  immutable history, so a node's reports can legitimately exist in more than one
+  backend and all of them belong in the merged view. Reports dedupe on `hash`;
+  events, which carry no id of their own, dedupe on the verbatim record (a node
+  reporting to more than one backend stores identical records in each).
+- **Aggregates** — `extract`/`group_by` rows are counts, not records, so each
+  backend returns a partial answer that has to be **added**, not deduped. This
+  covers `/event-counts`, `/aggregate-event-counts`, and any `/reports`,
+  `/nodes` or `/resources` query whose `extract` carries a `["function", ...]`
+  column.
+  - The grouping key is the row's non-aggregate fields: for `/reports`,
+    `/nodes` and `/resources` they come from the query — the plain `extract`
+    fields plus any `group_by` clause — and for the event-count endpoints from
+    the row itself (`subject_type`/`subject`, or `summarize_by`), whose
+    remaining fields are all counts.
+  - On `/nodes` this takes precedence over the `certname` merge: a count row has
+    no `certname`, so deduping would collapse every backend's count into one
+    backend's number. A `/nodes` query with no `function` column — including a
+    plain `extract` projection — still merges by `certname`.
+  - `/resources` has no cross-backend record identity to dedupe on, so only its
+    aggregate queries merge; everything else stays an unmerged pass-through.
+  - Rows sharing a key collapse into one with their numeric columns summed. A key
+    only one backend reported is passed through byte-for-byte. An aggregate column
+    that is absent or non-numeric in a row is skipped, never zeroed, so the
+    backends that did report a number still count.
+  - A `/reports` query with no `function` column is a projection of real reports,
+    not an aggregate, and stays on the union path.
+  - `include_total=true` on a summed endpoint reports the **merged** row count,
+    not the sum of the backends' `X-Records`, since shared keys collapse.
+
+### Metadata and metrics
+
+- **`/pdb/meta/v1/version`** — when the backends agree, that version is served.
+  When they differ, `pdbmux` reports the **lowest**: a client reads this as the
+  feature level it may rely on, and the estate can only be relied on for what its
+  oldest PuppetDB implements. Versions compare segment by segment, numerically
+  where both segments are numbers (`7.9.0` < `7.12.0`), lexically otherwise.
+  A backend whose body is unparseable is skipped rather than treated as lowest.
+- **`/pdb/meta/v1/server-time`** — the clock of whichever PuppetDB answered is
+  not estate state and has no meaningful merge, so the first **reachable**
+  backend in configured order supplies it, the same tie-break used elsewhere.
+- **`/metrics/...`** — the Jolokia envelope's `value` is merged and the rest of
+  the envelope comes from the first backend (with the newest `timestamp`).
+  Values merge recursively:
+  - Objects merge over the **union** of their keys, so an MBean attribute only
+    one backend exposes still survives.
+  - Numbers combine by the attribute's own name. The default is a **sum** —
+    almost everything here is a population count (`num-nodes`, `num-resources`,
+    queue depth, command totals) whose estate-wide value is the total, and rates
+    are additive throughput. The exceptions describe a distribution or a bound,
+    where adding two servers' numbers yields a figure that was never true of
+    either: `Min` takes the minimum; `Max`, `Uptime` and `StartTime` take the
+    maximum; `Mean`, `Median`, `StdDev` and `*Percentile` take the unweighted
+    arithmetic mean (`pdbmux` has no per-backend sample counts to weight by).
+    Matching is case-insensitive.
+  - Strings, booleans, arrays, nulls and mixed kinds keep the first backend's
+    value — there is no sound way to add them.
+  - Jolokia signals a bad MBean as a non-2xx `status` **inside** an HTTP 200.
+    Such a backend is skipped; if every backend does so, the first one's error
+    envelope is replayed verbatim so the client sees the real reason.
+  - MBean names arrive percent-encoded over Jolokia's own `!`-escaping; the raw
+    path is forwarded so neither layer is lost.
+
+### Paging and ordering on the merged endpoints
+
+Each backend applies `order_by`/`limit`/`offset` to its own slice only, so
+`pdbmux` re-does all three over the union:
+
+- `order_by` is parsed and the merged set re-sorted by those fields (ties keep
+  the merged set's existing order). A record missing an ordered field sorts first.
+- Backends are asked for the first `offset + limit` records — never an `offset`
+  — and the requested window is then cut from the merged, re-sorted set.
+- `include_total=true` on a union endpoint makes `pdbmux` sum each backend's
+  `X-Records` header into one merged header. Deduped records are counted once per
+  backend, so the total is an upper bound. Summed endpoints report the merged row
+  count instead.
+- A malformed `limit`, `offset` or `order_by` gets a `400` rather than being
+  forwarded.
+
+## Caching
+
+`pdbmux` caches merged `/nodes` and `/facts` record sets **in memory** so a busy
+Puppetboard does not re-fan-out the same query every few seconds. Everything else
+runs uncached — including `extract`/`count` aggregates on those two paths, and
+the `/pdb/meta/v1/*` and `/metrics/*` endpoints, which are served live on every
+request. The cache is an interface, and `/reports` gets its own (S3-backed)
+backend later without further handler changes.
+
+- **Key** — `<path>?<params>`, where the params are the ones that actually
+  determine the response, URL-encoded with keys sorted ascending and a repeated
+  param's values sorted ascending. Param order in the request is therefore
+  irrelevant: one canonical key per distinct request. A request with no params
+  keys on the bare path.
+- **TTL** — `facts_ttl`, default `30s`, **hard cap `30s`**. A larger configured
+  value is **clamped** down to the cap, not rejected, so a stray env var cannot
+  crash-loop a container; `pdbmux config show` prints
+  `facts_ttl : 30s (clamped from 600s, cap 30s)` when that happens. `facts_ttl: 0`
+  disables the cache entirely and the merged endpoints behave exactly as before.
+- **Stale on failure only** — an expired entry is kept, not dropped. When the TTL
+  has passed `pdbmux` always re-queries the backends; the expired copy is served
+  **only** if every backend fails, which turns a `502` into slightly-old data. A
+  healthy backend is never shadowed by a stale entry.
+- **Bounded** — `facts_cache_bytes` (default 64 MiB) is a byte budget, evicted
+  least-recently-used; reads count as use, so a stale entry that is still being
+  asked for survives. A single response larger than the whole budget is not
+  cached at all. The budget counts stored response bodies only — cache keys and
+  the list/map bookkeeping are not accounted for, so it is a target for body
+  bytes rather than a hard cap on process memory.
+- **Single-flight** — concurrent requests for the same key collapse into one
+  upstream fan-out; the rest wait for it and share the result.
+- **Visibility** — `/healthz` carries a `cache` object: `backend`
+  (`memory`/`none`), `ttl`, `entries`, `stale_entries`, `bytes`, `serving_stale`,
+  `stale_served` and `last_stale_served`. `serving_stale` is `true` from the
+  moment a stale fallback is served until the next response comes from a live
+  fan-out or a fresh entry.
 
 ## Config
 
 Precedence (lowest → highest): **defaults < config file < env vars (`PDBMUX_*`) < flags**.
 
-Config file: `$XDG_CONFIG_HOME/pdbmux/config.yaml`. In Kubernetes, configuration
-is supplied entirely via `PDBMUX_*` env vars (no config file), which is the
-supported deployment path — see [Deployment](#deployment).
+The config file is optional; a file, env vars, or both work equally well,
+including in a container.
+
+Which file is read: `--config <path>`, else `PDBMUX_CONFIG`, else the first that
+exists of `$XDG_CONFIG_HOME/pdbmux/config.yaml` (or `$HOME/.config/pdbmux/config.yaml`),
+then `/etc/pdbmux/config.yaml`. A path given via `--config`/`PDBMUX_CONFIG` **must**
+exist — pdbmux fails rather than silently falling back — while a missing file on
+the default search path is fine. `pdbmux config show` prints the file it loaded,
+or the paths it searched.
 
 ```yaml
-# ~/.config/pdbmux/config.yaml  (local dev; in k8s use PDBMUX_* env instead)
 listen: ":8080"
-backends:
-  - name: old
-    url: http://puppetdbapi.service.consul:8080
-  - name: new
-    url: https://puppetdb.k8s.syd1.au.unkin.net
-primary: new          # backend used for non-merged /pdb/query/v4/* pass-through
-merge: freshness      # freshness | static
-prefer: new           # winner on ties / static merge / fallback
-timeout: 10s          # per-upstream request timeout
-freshness_ttl: 30s    # freshness-map cache TTL (freshness merge only)
+backends:                          # order is a tie-break only, not a ranking
+  - name: pdb-a
+    url: http://puppetdb1.example.com:8080
+  - name: pdb-b
+    url: https://puppetdb2.example.com
+merge: freshness             # freshness | static
+timeout: 10s                 # per-upstream request timeout
+freshness_ttl: 30s           # freshness-map cache TTL (freshness merge only)
+facts_ttl: 30s               # /facts + /nodes response cache TTL; 0 disables, capped at 30s
+facts_cache_bytes: 67108864  # byte budget for that cache (64 MiB), LRU-evicted
 ```
 
-`backends[*].url` is a **base** URL (`scheme://host[:port]`), without the
-`/pdb/query/v4/...` path — `pdbmux` appends the path per request.
+`backends[*].url` is a **base** URL (`scheme://host[:port]`); `pdbmux` appends
+the `/pdb/query/v4/...` path per request.
 
 | Env var | Overrides |
 |---|---|
+| `PDBMUX_CONFIG` | config file path (not a file key) |
 | `PDBMUX_LISTEN` | `listen` |
-| `PDBMUX_PRIMARY` | `primary` |
 | `PDBMUX_MERGE` | `merge` |
-| `PDBMUX_PREFER` | `prefer` |
 | `PDBMUX_TIMEOUT` | `timeout` (Go duration, e.g. `10s`) |
 | `PDBMUX_FRESHNESS_TTL` | `freshness_ttl` |
+| `PDBMUX_FACTS_TTL` | `facts_ttl` (clamped to 30s) |
+| `PDBMUX_FACTS_CACHE_BYTES` | `facts_cache_bytes` (plain integer bytes) |
 | `PDBMUX_BACKENDS` | whole backend list, as `name=url,name=url` |
 
-Flags: `--listen`, `--primary`, `--merge`.
+Flags: `--config`, `--listen`, `--merge`.
+
+`config init` writes to `--config`/`PDBMUX_CONFIG` when set, else to
+`$XDG_CONFIG_HOME/pdbmux/config.yaml`.
 
 ## Running
 
-```bash
-pdbmux                 # start the proxy (serve is the default action)
-pdbmux serve           # explicit
-pdbmux config init     # write a default config file
-pdbmux config show     # print active config after all overrides
-pdbmux version
-```
-
-Point a consumer at it:
+Subcommands: `serve` (default), `config init`, `config show`, `version`. Run
+`pdbmux --help` for details. Any PuppetDB v4 client works against the `pdbmux`
+base URL in place of a PuppetDB one.
 
 ```bash
-node-lookup --url http://localhost:8080/pdb/query/v4/facts -R
-NODE_LOOKUP_URL=http://localhost:8080/pdb/query/v4/facts pblastreport somehost
+PDBMUX_BACKENDS='pdb-a=http://puppetdb1.example.com:8080,pdb-b=http://puppetdb2.example.com:8080' pdbmux
+curl -s --get http://localhost:8080/pdb/query/v4/nodes \
+  --data-urlencode 'query=["=","certname","host1.example.com"]'
 ```
 
 ## Build
 
-```bash
-make build             # -> dist/pdbmux (CGO disabled, static)
-make test              # go test -race ./...
-make lint              # golangci-lint
-```
-
-Requires Go 1.25+. Dependencies: `github.com/spf13/cobra` (CLI),
-`gopkg.in/yaml.v3` (config file).
+`make build` (static binary into `dist/`), `make test`, `make lint`. Requires Go 1.25+.
 
 ## Deployment
 
-`pdbmux` runs **in Kubernetes** as a container, in line with the all-in-k8s
-estate direction — it is not shipped as a per-VM RPM/systemd service. The image
-is built and pushed on every `v*` tag (`.woodpecker/docker.yaml`) to:
+Container image only — no OS package. Every `v*` tag builds and pushes the image
+(`.woodpecker/docker.yaml`); registry and repository are pipeline settings. Tag
+with `make patch` / `minor` / `major`.
 
-```
-git.unkin.net/unkin/pdbmux:<tag>
-```
-
-It is a minimal static (`CGO_ENABLED=0`) binary on a distroless base
-(`Dockerfile`), configured entirely via `PDBMUX_*` env vars, with a single HTTP
-listener and `/healthz` for liveness/readiness probes.
-
-The Deployment/Service/Gateway manifests live in the estate's `argocd-apps` repo
-under `apps/base/pdbmux/` (namespace `pdbmux`, 2 replicas), and it is exposed to
-VM/workstation `node-lookup` consumers over HTTPS at:
-
-```
-https://pdbmux.k8s.syd1.au.unkin.net
-```
-
-Locally you can still run the binary directly for development:
-
-```bash
-PDBMUX_BACKENDS='old=http://puppetdbapi.service.consul:8080,new=http://puppetdb.puppet.svc.cluster.local:8080' \
-  pdbmux serve
-curl -s localhost:8080/healthz
-```
-
-## Version bumps
-
-```bash
-make patch             # tag vX.Y.(Z+1) and push (triggers the docker release)
-make minor             # tag vX.(Y+1).0
-make major             # tag v(X+1).0.0
-```
+A static (`CGO_ENABLED=0`) binary on a distroless base. Configure it with
+`PDBMUX_*` env vars (at minimum `PDBMUX_BACKENDS`), or mount a config file — a
+configmap at `/etc/pdbmux/config.yaml` is picked up with no env var at all, and
+any other mount path works via `PDBMUX_CONFIG`. Env vars still override file
+values, so the two mix. Run as many replicas as you like — the only state is the
+in-memory cache, which is per-replica and bounded by `facts_cache_bytes`, so size
+the memory limit above it. Use `/healthz` for liveness/readiness probes.

aggregate.go

@@ -0,0 +1,270 @@
+package main
+
+import (
+	"encoding/json"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// aggregateSpec names the columns of an extract/group_by result set: keys
+// identify a row across backends, sums are the numeric aggregate columns added
+// together.
+type aggregateSpec struct {
+	keys []string
+	sums []string
+}
+
+// parseAggregate reads a PuppetDB AST query and returns the aggregate shape of
+// its response, or nil when the query is not an aggregate — only a top-level
+// `extract` carrying at least one `["function", ...]` column produces summable
+// rows. Key columns are the plain (non-function) extract fields, unioned with an
+// explicit `group_by` clause when the query has one.
+func parseAggregate(query string) *aggregateSpec {
+	if strings.TrimSpace(query) == "" {
+		return nil
+	}
+	var ast []json.RawMessage
+	if json.Unmarshal([]byte(query), &ast) != nil || len(ast) < 2 {
+		return nil
+	}
+	var op string
+	if json.Unmarshal(ast[0], &op) != nil || op != "extract" {
+		return nil
+	}
+	var cols []json.RawMessage
+	if json.Unmarshal(ast[1], &cols) != nil {
+		return nil
+	}
+
+	spec := &aggregateSpec{}
+	for _, col := range cols {
+		var name string
+		if json.Unmarshal(col, &name) == nil {
+			spec.keys = appendUnique(spec.keys, name)
+			continue
+		}
+		if fn, ok := functionName(col); ok {
+			spec.sums = appendUnique(spec.sums, fn)
+		}
+	}
+	if len(spec.sums) == 0 {
+		return nil
+	}
+	for _, node := range ast[2:] {
+		for _, f := range groupByFields(node) {
+			spec.keys = appendUnique(spec.keys, f)
+		}
+	}
+	return spec
+}
+
+// functionName returns the response column an extract function produces, which
+// PuppetDB names after the function itself: ["function","count","certname"]
+// yields a "count" column.
+func functionName(col json.RawMessage) (string, bool) {
+	var parts []json.RawMessage
+	if json.Unmarshal(col, &parts) != nil || len(parts) < 2 {
+		return "", false
+	}
+	var head, name string
+	if json.Unmarshal(parts[0], &head) != nil || head != "function" {
+		return "", false
+	}
+	if json.Unmarshal(parts[1], &name) != nil || name == "" {
+		return "", false
+	}
+	return name, true
+}
+
+// groupByFields returns the field names of a ["group_by", ...] AST node, or nil
+// for any other node.
+func groupByFields(node json.RawMessage) []string {
+	var parts []json.RawMessage
+	if json.Unmarshal(node, &parts) != nil || len(parts) < 2 {
+		return nil
+	}
+	var head string
+	if json.Unmarshal(parts[0], &head) != nil || head != "group_by" {
+		return nil
+	}
+	var out []string
+	for _, p := range parts[1:] {
+		var name string
+		if json.Unmarshal(p, &name) == nil {
+			out = append(out, name)
+		}
+	}
+	return out
+}
+
+func appendUnique(s []string, v string) []string {
+	if contains(s, v) {
+		return s
+	}
+	return append(s, v)
+}
+
+// columns reports which fields of a row form its grouping key and which are
+// summed. A spec is fixed by the query, so the row is ignored.
+func (a *aggregateSpec) columns(map[string]json.RawMessage) ([]string, []string) {
+	return a.keys, a.sums
+}
+
+// inferredColumns derives an event-counts row's shape from the row itself: the
+// counts to add (successes, failures, noops, skips, total) are its numeric
+// fields, plus any null one — PuppetDB nulls an aggregate column when a backend
+// matched nothing — and everything else, subject_type/subject/summarize_by,
+// identifies the row. Those endpoints have a fixed response shape with no
+// numeric key field, so nothing summable is mistaken for identity.
+func inferredColumns(row map[string]json.RawMessage) ([]string, []string) {
+	var keys, sums []string
+	for name, val := range row {
+		if isJSONNumber(val) || isJSONNull(val) {
+			sums = append(sums, name)
+			continue
+		}
+		keys = append(keys, name)
+	}
+	sort.Strings(keys)
+	sort.Strings(sums)
+	return keys, sums
+}
+
+// isJSONNumber reports whether a raw JSON value is a number.
+func isJSONNumber(raw json.RawMessage) bool {
+	v := strings.TrimSpace(string(raw))
+	if v == "" {
+		return false
+	}
+	return v[0] == '-' || (v[0] >= '0' && v[0] <= '9')
+}
+
+func isJSONNull(raw json.RawMessage) bool {
+	return strings.TrimSpace(string(raw)) == "null"
+}
+
+// sumGroup accumulates the rows sharing one grouping key.
+type sumGroup struct {
+	raw    json.RawMessage            // first contributing row, verbatim
+	row    map[string]json.RawMessage // its decoded fields
+	totals map[string]float64         // running sum per aggregate column
+	merged bool                       // a second row was folded in
+}
+
+// sumRows folds each backend's aggregate rows into one row per grouping key,
+// adding the numeric aggregate columns. columns decides, per row, which fields
+// are the key and which are summed.
+//
+// A row that is not a JSON object passes through untouched, as does the sole row
+// of a key only one backend reported — those keep their upstream bytes. An
+// aggregate column that is absent or non-numeric in a later row is left at the
+// earlier backend's value rather than being coerced to zero. results come in
+// configured backend order and the output keeps first-seen order, a tie-break
+// only.
+func sumRows(results []backendResult, columns func(map[string]json.RawMessage) ([]string, []string)) []json.RawMessage {
+	type slot struct {
+		raw   json.RawMessage // passthrough row, when group is nil
+		group *sumGroup
+	}
+	var order []slot
+	groups := map[string]*sumGroup{}
+
+	for _, res := range results {
+		for _, rec := range res.records {
+			var row map[string]json.RawMessage
+			if json.Unmarshal(rec.Raw, &row) != nil {
+				order = append(order, slot{raw: rec.Raw})
+				continue
+			}
+			keys, sums := columns(row)
+			k := groupKey(row, keys)
+			g, ok := groups[k]
+			if !ok {
+				g = &sumGroup{raw: rec.Raw, row: row, totals: map[string]float64{}}
+				for _, s := range sums {
+					if n, ok := numberOf(row[s]); ok {
+						g.totals[s] = n
+					}
+				}
+				groups[k] = g
+				order = append(order, slot{group: g})
+				continue
+			}
+			for _, s := range sums {
+				n, ok := numberOf(row[s])
+				if !ok {
+					continue
+				}
+				if _, seen := g.totals[s]; !seen {
+					// First numeric value for a column the earlier row lacked.
+					g.totals[s] = 0
+				}
+				g.totals[s] += n
+				g.merged = true
+			}
+		}
+	}
+
+	out := make([]json.RawMessage, 0, len(order))
+	for _, sl := range order {
+		if sl.group == nil {
+			out = append(out, sl.raw)
+			continue
+		}
+		out = append(out, sl.group.encode())
+	}
+	return out
+}
+
+// encode renders a group back to JSON, reusing the first row's bytes when
+// nothing was added to it.
+func (g *sumGroup) encode() json.RawMessage {
+	if !g.merged {
+		return g.raw
+	}
+	row := make(map[string]json.RawMessage, len(g.row))
+	for k, v := range g.row {
+		row[k] = v
+	}
+	for col, total := range g.totals {
+		row[col] = json.RawMessage(strconv.FormatFloat(total, 'f', -1, 64))
+	}
+	raw, err := json.Marshal(row)
+	if err != nil {
+		return g.raw
+	}
+	return raw
+}
+
+// groupKey builds a row's identity from the named fields' verbatim JSON values.
+// Every backend runs the same PuppetDB serialiser, so byte equality is a sound
+// comparison for object-valued keys such as event-counts' subject. An absent
+// field is distinct from any present value.
+func groupKey(row map[string]json.RawMessage, keys []string) string {
+	var b strings.Builder
+	for _, k := range keys {
+		b.WriteString(k)
+		b.WriteByte(0)
+		if v, ok := row[k]; ok {
+			b.Write(v)
+		} else {
+			b.WriteByte(1)
+		}
+		b.WriteByte(0)
+	}
+	return b.String()
+}
+
+// numberOf decodes a raw JSON number, reporting ok=false for anything else so
+// non-numeric aggregate columns are carried through instead of summed.
+func numberOf(raw json.RawMessage) (float64, bool) {
+	if !isJSONNumber(raw) {
+		return 0, false
+	}
+	var n float64
+	if json.Unmarshal(raw, &n) != nil {
+		return 0, false
+	}
+	return n, true
+}

aggregate_test.go

@@ -0,0 +1,210 @@
+package main
+
+import (
+	"encoding/json"
+	"reflect"
+	"slices"
+	"testing"
+)
+
+func rows(raws ...string) []record {
+	out := make([]record, 0, len(raws))
+	for _, r := range raws {
+		out = append(out, record{Raw: json.RawMessage(r)})
+	}
+	return out
+}
+
+// decodeRows turns a merged result set into comparable maps.
+func decodeRows(t *testing.T, raws []json.RawMessage) []map[string]any {
+	t.Helper()
+	out := make([]map[string]any, 0, len(raws))
+	for _, raw := range raws {
+		var m map[string]any
+		if err := json.Unmarshal(raw, &m); err != nil {
+			t.Fatalf("unmarshal %s: %v", raw, err)
+		}
+		out = append(out, m)
+	}
+	return out
+}
+
+func TestParseAggregate_ExtractWithGroupBy(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	if spec == nil {
+		t.Fatal("expected an aggregate spec")
+	}
+	if !slices.Equal(spec.keys, []string{"status"}) {
+		t.Errorf("keys = %v, want [status]", spec.keys)
+	}
+	if !slices.Equal(spec.sums, []string{"count"}) {
+		t.Errorf("sums = %v, want [count]", spec.sums)
+	}
+}
+
+func TestParseAggregate_GroupByAddsUnextractedField(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count","certname"]],["~","certname",".*"],["group_by","status"]]`)
+	if spec == nil {
+		t.Fatal("expected an aggregate spec")
+	}
+	if !slices.Equal(spec.keys, []string{"status"}) {
+		t.Errorf("keys = %v, want [status] from the group_by clause", spec.keys)
+	}
+	if !slices.Equal(spec.sums, []string{"count"}) {
+		t.Errorf("sums = %v, want [count]", spec.sums)
+	}
+}
+
+func TestParseAggregate_NoFunctionIsNotAggregate(t *testing.T) {
+	for _, q := range []string{
+		``,
+		`["=","certname","h1"]`,
+		`["extract",["certname","hash"],["=","certname","h1"]]`, // projection, still real reports
+		`not json`,
+		`["extract"]`,
+		`{"not":"an array"}`,
+	} {
+		if spec := parseAggregate(q); spec != nil {
+			t.Errorf("parseAggregate(%q) = %+v, want nil", q, spec)
+		}
+	}
+}
+
+func TestSumRows_SharedKeysAreAdded(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`{"count":3,"status":"changed"}`, `{"count":1,"status":"failed"}`)},
+		{name: "b", records: rows(`{"count":4,"status":"changed"}`, `{"count":2,"status":"failed"}`)},
+	}, spec.columns)
+
+	want := []map[string]any{
+		{"count": float64(7), "status": "changed"},
+		{"count": float64(3), "status": "failed"},
+	}
+	if got := decodeRows(t, merged); !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want %v", got, want)
+	}
+}
+
+func TestSumRows_DisjointKeysAreKept(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`{"count":3,"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":2,"status":"skipped"}`)},
+	}, spec.columns)
+
+	want := []map[string]any{
+		{"count": float64(3), "status": "changed"},
+		{"count": float64(2), "status": "skipped"},
+	}
+	if got := decodeRows(t, merged); !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want %v", got, want)
+	}
+}
+
+func TestSumRows_SingleBackendRowKeepsUpstreamBytes(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	const raw = `{"count":3,"status":"changed","extra":{"kept":true}}`
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(raw)},
+		{name: "b", records: nil},
+	}, spec.columns)
+
+	if len(merged) != 1 || string(merged[0]) != raw {
+		t.Errorf("merged = %s, want the row verbatim %s", merged, raw)
+	}
+}
+
+func TestSumRows_NonNumericAggregateColumnIsNotZeroed(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`{"count":5,"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":null,"status":"changed"}`)},
+	}, spec.columns)
+
+	want := []map[string]any{{"count": float64(5), "status": "changed"}}
+	if got := decodeRows(t, merged); !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want the numeric value preserved %v", got, want)
+	}
+}
+
+func TestSumRows_MissingAggregateColumnStartsFromTheNumericRow(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`{"status":"changed"}`)},
+		{name: "b", records: rows(`{"count":6,"status":"changed"}`)},
+	}, spec.columns)
+
+	want := []map[string]any{{"count": float64(6), "status": "changed"}}
+	if got := decodeRows(t, merged); !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want %v", got, want)
+	}
+}
+
+func TestSumRows_NonObjectRowsPassThrough(t *testing.T) {
+	spec := parseAggregate(`["extract",[["function","count"],"status"],["=","certname","h1"],["group_by","status"]]`)
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`"surprise"`)},
+		{name: "b", records: rows(`{"count":1,"status":"changed"}`)},
+	}, spec.columns)
+
+	if len(merged) != 2 || string(merged[0]) != `"surprise"` {
+		t.Fatalf("merged = %s, want the non-object row kept as-is", merged)
+	}
+}
+
+func TestSumRows_NoKeyColumnsCollapseToOneRow(t *testing.T) {
+	// ["extract",[["function","count"]],...] is a whole-estate count: one row
+	// per backend, and the merged answer is their sum.
+	spec := parseAggregate(`["extract",[["function","count"]],["=","certname","h1"]]`)
+	if spec == nil {
+		t.Fatal("expected an aggregate spec")
+	}
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(`{"count":10}`)},
+		{name: "b", records: rows(`{"count":32}`)},
+	}, spec.columns)
+
+	want := []map[string]any{{"count": float64(42)}}
+	if got := decodeRows(t, merged); !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want %v", got, want)
+	}
+}
+
+func TestInferredColumns_SplitsCountsFromIdentity(t *testing.T) {
+	var row map[string]json.RawMessage
+	if err := json.Unmarshal([]byte(`{"subject_type":"certname","subject":{"title":"h1"},"failures":1,"successes":2,"skips":null}`), &row); err != nil {
+		t.Fatal(err)
+	}
+	keys, sums := inferredColumns(row)
+	if !slices.Equal(keys, []string{"subject", "subject_type"}) {
+		t.Errorf("keys = %v, want [subject subject_type]", keys)
+	}
+	// A null count is an empty aggregate, not part of the row's identity.
+	if !slices.Equal(sums, []string{"failures", "skips", "successes"}) {
+		t.Errorf("sums = %v, want [failures skips successes]", sums)
+	}
+}
+
+func TestSumRows_EventCountsPerSubject(t *testing.T) {
+	merged := sumRows([]backendResult{
+		{name: "a", records: rows(
+			`{"subject_type":"certname","subject":{"title":"h1"},"failures":1,"successes":2,"noops":0,"skips":0}`,
+			`{"subject_type":"certname","subject":{"title":"h2"},"failures":0,"successes":5,"noops":0,"skips":0}`,
+		)},
+		{name: "b", records: rows(
+			`{"subject_type":"certname","subject":{"title":"h1"},"failures":3,"successes":4,"noops":1,"skips":0}`,
+		)},
+	}, inferredColumns)
+
+	got := decodeRows(t, merged)
+	want := []map[string]any{
+		{"subject_type": "certname", "subject": map[string]any{"title": "h1"},
+			"failures": float64(4), "successes": float64(6), "noops": float64(1), "skips": float64(0)},
+		{"subject_type": "certname", "subject": map[string]any{"title": "h2"},
+			"failures": float64(0), "successes": float64(5), "noops": float64(0), "skips": float64(0)},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("merged = %v, want %v", got, want)
+	}
+}

cache.go

@@ -0,0 +1,267 @@
+package main
+
+import (
+	"container/list"
+	"context"
+	"fmt"
+	"net/url"
+	"runtime/debug"
+	"sort"
+	"sync"
+	"time"
+)
+
+// CacheStatus distinguishes the three outcomes of a lookup: nothing stored, a
+// stored entry within its TTL, and a stored entry past it.
+type CacheStatus int
+
+const (
+	CacheMiss CacheStatus = iota
+	CacheFresh
+	CacheStale
+)
+
+func (s CacheStatus) String() string {
+	switch s {
+	case CacheFresh:
+		return "fresh"
+	case CacheStale:
+		return "stale"
+	default:
+		return "miss"
+	}
+}
+
+// CacheEntry is a stored response body and the time it was stored.
+type CacheEntry struct {
+	Body     []byte
+	StoredAt time.Time
+}
+
+// CacheStats is the cache state reported by /healthz.
+type CacheStats struct {
+	Backend      string `json:"backend"`
+	Entries      int    `json:"entries"`
+	StaleEntries int    `json:"stale_entries"`
+	Bytes        int64  `json:"bytes"`
+}
+
+// Cache stores merged responses keyed by cacheKey. Get reports freshness rather
+// than hiding expired entries so a caller can fall back to a stale body when the
+// upstream fetch fails. The context and error exist for out-of-process backends
+// (the reports cache lands on S3); an in-process backend ignores both.
+//
+// A Body handed back by Get aliases the cache's copy and must not be mutated.
+type Cache interface {
+	Get(ctx context.Context, key string) (CacheEntry, CacheStatus, error)
+	Put(ctx context.Context, key string, body []byte) error
+	Stats() CacheStats
+}
+
+// cacheKey is the request path when there are no params, else "<path>?<params>"
+// where params is url.Values.Encode() over a copy whose repeated values have
+// been sorted. Encode() already sorts keys ascending, so both the order params
+// arrive in and the order of a repeated param's values are irrelevant to the
+// key: one canonical string per distinct request.
+func cacheKey(path string, params url.Values) string {
+	if len(params) == 0 {
+		return path
+	}
+	norm := make(url.Values, len(params))
+	for k, vs := range params {
+		sorted := append([]string(nil), vs...)
+		sort.Strings(sorted)
+		norm[k] = sorted
+	}
+	encoded := norm.Encode()
+	if encoded == "" {
+		return path
+	}
+	return path + "?" + encoded
+}
+
+// noopCache is the default for every path with no cache configured, so wiring a
+// handler through the cache leaves its behaviour unchanged.
+type noopCache struct{}
+
+func (noopCache) Get(context.Context, string) (CacheEntry, CacheStatus, error) {
+	return CacheEntry{}, CacheMiss, nil
+}
+
+func (noopCache) Put(context.Context, string, []byte) error { return nil }
+
+func (noopCache) Stats() CacheStats { return CacheStats{Backend: "none"} }
+
+type memoryEntry struct {
+	key      string
+	body     []byte
+	storedAt time.Time
+}
+
+// memoryCache is a byte-bounded LRU. Expired entries are kept, not dropped, so
+// they remain available as a stale fallback; only the byte budget evicts.
+type memoryCache struct {
+	ttl      time.Duration
+	maxBytes int64
+	now      func() time.Time
+
+	mu    sync.Mutex
+	ll    *list.List // front = most recently used
+	items map[string]*list.Element
+	bytes int64
+}
+
+func newMemoryCache(ttl time.Duration, maxBytes int64) *memoryCache {
+	return &memoryCache{
+		ttl:      ttl,
+		maxBytes: maxBytes,
+		now:      time.Now,
+		ll:       list.New(),
+		items:    make(map[string]*list.Element),
+	}
+}
+
+func (c *memoryCache) Get(_ context.Context, key string) (CacheEntry, CacheStatus, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	el, ok := c.items[key]
+	if !ok {
+		return CacheEntry{}, CacheMiss, nil
+	}
+	c.ll.MoveToFront(el)
+	e := el.Value.(*memoryEntry)
+	status := CacheFresh
+	if c.now().Sub(e.storedAt) >= c.ttl {
+		status = CacheStale
+	}
+	return CacheEntry{Body: e.body, StoredAt: e.storedAt}, status, nil
+}
+
+func (c *memoryCache) Put(_ context.Context, key string, body []byte) error {
+	// A response larger than the whole budget would evict everything else.
+	if int64(len(body)) > c.maxBytes {
+		return nil
+	}
+	stored := append([]byte(nil), body...)
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if el, ok := c.items[key]; ok {
+		e := el.Value.(*memoryEntry)
+		c.bytes += int64(len(stored)) - int64(len(e.body))
+		e.body, e.storedAt = stored, c.now()
+		c.ll.MoveToFront(el)
+	} else {
+		c.items[key] = c.ll.PushFront(&memoryEntry{key: key, body: stored, storedAt: c.now()})
+		c.bytes += int64(len(stored))
+	}
+	for c.bytes > c.maxBytes {
+		back := c.ll.Back()
+		if back == nil {
+			break
+		}
+		e := c.ll.Remove(back).(*memoryEntry)
+		delete(c.items, e.key)
+		c.bytes -= int64(len(e.body))
+	}
+	return nil
+}
+
+func (c *memoryCache) Stats() CacheStats {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	st := CacheStats{Backend: "memory", Entries: len(c.items), Bytes: c.bytes}
+	now := c.now()
+	for el := c.ll.Front(); el != nil; el = el.Next() {
+		if now.Sub(el.Value.(*memoryEntry).storedAt) >= c.ttl {
+			st.StaleEntries++
+		}
+	}
+	return st
+}
+
+// flightGroup collapses concurrent identical builds so N simultaneous requests
+// for one key cause one upstream fan-out.
+type flightGroup struct {
+	mu    sync.Mutex
+	calls map[string]*flightCall
+}
+
+type flightCall struct {
+	wg   sync.WaitGroup
+	resp cachedResponse
+	err  error
+}
+
+// flightPanic is a panic from a flight's fn, reported to the leader and to every
+// waiter as an error so callers keep their error handling (stale fallback, 502)
+// instead of seeing a zero-value success.
+type flightPanic struct {
+	value any
+	stack []byte
+}
+
+func (p *flightPanic) Error() string {
+	return fmt.Sprintf("panic building response: %v\n%s", p.value, p.stack)
+}
+
+// Do returns fn's result and whether this caller shared another's in-flight run.
+func (g *flightGroup) Do(key string, fn func() (cachedResponse, error)) (resp cachedResponse, err error, shared bool) {
+	g.mu.Lock()
+	if g.calls == nil {
+		g.calls = make(map[string]*flightCall)
+	}
+	if c, ok := g.calls[key]; ok {
+		g.mu.Unlock()
+		c.wg.Wait()
+		return c.resp, c.err, true
+	}
+	c := &flightCall{}
+	c.wg.Add(1)
+	g.calls[key] = c
+	g.mu.Unlock()
+
+	defer func() {
+		if r := recover(); r != nil {
+			c.resp, c.err = cachedResponse{}, &flightPanic{value: r, stack: debug.Stack()}
+			resp, err = c.resp, c.err
+		}
+		// Done only after the results are stored, so waiters read them.
+		c.wg.Done()
+		g.mu.Lock()
+		delete(g.calls, key)
+		g.mu.Unlock()
+	}()
+
+	c.resp, c.err = fn()
+	return c.resp, c.err, false
+}
+
+// staleTracker records stale fallbacks for /healthz. serving flips back to false
+// as soon as a response is served from a live fan-out or a fresh entry.
+type staleTracker struct {
+	mu      sync.Mutex
+	serving bool
+	served  uint64
+	last    time.Time
+}
+
+func (t *staleTracker) markStale(now time.Time) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.serving = true
+	t.served++
+	t.last = now
+}
+
+func (t *staleTracker) markFresh() {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.serving = false
+}
+
+func (t *staleTracker) snapshot() (serving bool, served uint64, last time.Time) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.serving, t.served, t.last
+}

cache_test.go

@@ -0,0 +1,821 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// fakeClock drives the cache's TTL without sleeping.
+type fakeClock struct {
+	mu sync.Mutex
+	t  time.Time
+}
+
+func newFakeClock() *fakeClock { return &fakeClock{t: time.Unix(1_800_000_000, 0)} }
+
+func (c *fakeClock) now() time.Time {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.t
+}
+
+func (c *fakeClock) advance(d time.Duration) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.t = c.t.Add(d)
+}
+
+// countingBackend is a PuppetDB stand-in that counts requests per path and can
+// be made to fail or block, so cache hits and single-flight are observable.
+type countingBackend struct {
+	srv *httptest.Server
+
+	mu     sync.Mutex
+	hits   map[string]int
+	bodies map[string]string
+	fail   bool
+	block  chan struct{}
+}
+
+func newCountingBackend(t *testing.T, bodies map[string]string) *countingBackend {
+	t.Helper()
+	cb := &countingBackend{hits: map[string]int{}, bodies: bodies}
+	cb.srv = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cb.mu.Lock()
+		cb.hits[r.URL.Path]++
+		fail, block, body := cb.fail, cb.block, cb.bodies[r.URL.Path]
+		cb.mu.Unlock()
+
+		if block != nil {
+			<-block
+		}
+		if fail {
+			http.Error(w, "boom", http.StatusInternalServerError)
+			return
+		}
+		if body == "" {
+			body = "[]"
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, body)
+	}))
+	t.Cleanup(cb.srv.Close)
+	return cb
+}
+
+func (cb *countingBackend) hitCount(path string) int {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	return cb.hits[path]
+}
+
+func (cb *countingBackend) setFail(v bool) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.fail = v
+}
+
+func (cb *countingBackend) setBody(path, body string) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.bodies[path] = body
+}
+
+func (cb *countingBackend) setBlock(ch chan struct{}) {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	cb.block = ch
+}
+
+// newCachedServer builds a server whose facts cache runs on a fake clock.
+func newCachedServer(t *testing.T, cfg Config) (*Server, *fakeClock) {
+	t.Helper()
+	srv := newTestServer(cfg)
+	mc, ok := srv.factsCache.(*memoryCache)
+	if !ok {
+		t.Fatalf("expected a memory cache, got %T", srv.factsCache)
+	}
+	clk := newFakeClock()
+	mc.now = clk.now
+	return srv, clk
+}
+
+func cacheTestConfig(aURL, bURL string) Config {
+	cfg := testConfig(aURL, bURL, mergeStatic)
+	cfg.FactsTTL = 30 * time.Second
+	cfg.CacheBytes = defaultCacheSize
+	return cfg
+}
+
+func TestCacheKey_CanonicalOrdering(t *testing.T) {
+	a := cacheKey(factsPath, url.Values{
+		"query":  {`["=","name","role"]`},
+		"limit":  {"10"},
+		"expand": {"z", "a"},
+	})
+	b := cacheKey(factsPath, url.Values{
+		"expand": {"a", "z"},
+		"limit":  {"10"},
+		"query":  {`["=","name","role"]`},
+	})
+	if a != b {
+		t.Errorf("param order must not change the key:\n %s\n %s", a, b)
+	}
+	if !strings.HasPrefix(a, factsPath+"?") {
+		t.Errorf("key must start with the path, got %q", a)
+	}
+	if got := cacheKey(nodesPath, nil); got != nodesPath {
+		t.Errorf("no params should give the bare path, got %q", got)
+	}
+	if cacheKey(factsPath, url.Values{"query": {"x"}}) == cacheKey(factsPath, url.Values{"query": {"y"}}) {
+		t.Error("different queries must not share a key")
+	}
+	if cacheKey(factsPath, url.Values{"query": {"x"}}) == cacheKey(nodesPath, url.Values{"query": {"x"}}) {
+		t.Error("different paths must not share a key")
+	}
+}
+
+func TestMemoryCache_FreshThenStale(t *testing.T) {
+	c := newMemoryCache(30*time.Second, 1<<20)
+	clk := newFakeClock()
+	c.now = clk.now
+
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheMiss {
+		t.Fatalf("empty cache should miss, got %s", status)
+	}
+	if err := c.Put(context.Background(), "k", []byte(`{"body":[]}`)); err != nil {
+		t.Fatal(err)
+	}
+
+	ent, status, err := c.Get(context.Background(), "k")
+	if err != nil || status != CacheFresh || string(ent.Body) != `{"body":[]}` {
+		t.Fatalf("want a fresh hit, got %s %q (%v)", status, ent.Body, err)
+	}
+
+	clk.advance(29 * time.Second)
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheFresh {
+		t.Fatalf("inside the TTL should still be fresh, got %s", status)
+	}
+
+	clk.advance(2 * time.Second)
+	ent, status, _ = c.Get(context.Background(), "k")
+	if status != CacheStale {
+		t.Fatalf("past the TTL should be stale, got %s", status)
+	}
+	if string(ent.Body) != `{"body":[]}` {
+		t.Errorf("a stale entry must still carry its body, got %q", ent.Body)
+	}
+	if st := c.Stats(); st.Entries != 1 || st.StaleEntries != 1 {
+		t.Errorf("stats should report 1 entry, 1 stale, got %+v", st)
+	}
+}
+
+func TestMemoryCache_EvictsLeastRecentlyUsed(t *testing.T) {
+	body := []byte("0123456789") // 10 bytes
+	c := newMemoryCache(time.Minute, 25)
+	ctx := context.Background()
+
+	for _, k := range []string{"a", "b"} {
+		if err := c.Put(ctx, k, body); err != nil {
+			t.Fatal(err)
+		}
+	}
+	// Reading "a" makes "b" the eviction candidate.
+	if _, status, _ := c.Get(ctx, "a"); status != CacheFresh {
+		t.Fatalf("a should be cached, got %s", status)
+	}
+	if err := c.Put(ctx, "c", body); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, status, _ := c.Get(ctx, "b"); status != CacheMiss {
+		t.Errorf("b was least recently used and should have been evicted, got %s", status)
+	}
+	for _, k := range []string{"a", "c"} {
+		if _, status, _ := c.Get(ctx, k); status != CacheFresh {
+			t.Errorf("%s should have survived eviction, got %s", k, status)
+		}
+	}
+	if st := c.Stats(); st.Entries != 2 || st.Bytes != 20 {
+		t.Errorf("stats after eviction = %+v, want 2 entries / 20 bytes", st)
+	}
+}
+
+func TestMemoryCache_SkipsOversizedEntry(t *testing.T) {
+	c := newMemoryCache(time.Minute, 8)
+	if err := c.Put(context.Background(), "big", []byte("123456789")); err != nil {
+		t.Fatal(err)
+	}
+	if _, status, _ := c.Get(context.Background(), "big"); status != CacheMiss {
+		t.Error("an entry larger than the whole budget must not be stored")
+	}
+	if st := c.Stats(); st.Bytes != 0 {
+		t.Errorf("bytes = %d, want 0", st.Bytes)
+	}
+}
+
+func TestNoopCache_AlwaysMisses(t *testing.T) {
+	var c Cache = noopCache{}
+	if err := c.Put(context.Background(), "k", []byte("x")); err != nil {
+		t.Fatal(err)
+	}
+	if _, status, _ := c.Get(context.Background(), "k"); status != CacheMiss {
+		t.Errorf("noop cache must always miss, got %s", status)
+	}
+	if st := c.Stats(); st.Backend != "none" {
+		t.Errorf("backend = %q, want none", st.Backend)
+	}
+}
+
+func TestHandler_FactsCacheFreshHit(t *testing.T) {
+	body := `[` + fact("h1", "role", "web", "") + `]`
+	a := newCountingBackend(t, map[string]string{factsPath: body})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	first := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
+	second := doGet(t, srv.Handler(), factsPath, `["=","name","role"]`)
+
+	if first.Code != http.StatusOK || second.Code != http.StatusOK {
+		t.Fatalf("statuses %d/%d", first.Code, second.Code)
+	}
+	if first.Body.String() != second.Body.String() {
+		t.Errorf("cache hit changed the body:\n %s\n %s", first.Body.String(), second.Body.String())
+	}
+	if got := a.hitCount(factsPath); got != 1 {
+		t.Errorf("backend a saw %d requests, want 1 (second served from cache)", got)
+	}
+	if got := b.hitCount(factsPath); got != 1 {
+		t.Errorf("backend b saw %d requests, want 1", got)
+	}
+
+	// A different query is a different key and must go upstream.
+	doGet(t, srv.Handler(), factsPath, `["=","name","osfamily"]`)
+	if got := a.hitCount(factsPath); got != 2 {
+		t.Errorf("a different query should refetch: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_FactsCacheExpires(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), factsPath, "")
+	clk.advance(31 * time.Second)
+	doGet(t, srv.Handler(), factsPath, "")
+
+	if got := a.hitCount(factsPath); got != 2 {
+		t.Errorf("an expired entry should refetch: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_ServesStaleOnlyWhenBackendsFail(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "old", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	warm := doGet(t, srv.Handler(), factsPath, "")
+	if warm.Code != http.StatusOK {
+		t.Fatalf("warm-up status %d", warm.Code)
+	}
+	if !strings.Contains(warm.Body.String(), `"old"`) {
+		t.Fatalf("warm-up body = %s", warm.Body.String())
+	}
+
+	// Backends healthy but the entry expired: fresh data wins, never the stale copy.
+	clk.advance(31 * time.Second)
+	a.setBody(factsPath, `[`+fact("h1", "role", "new", "")+`]`)
+	refetch := doGet(t, srv.Handler(), factsPath, "")
+	if !strings.Contains(refetch.Body.String(), `"new"`) {
+		t.Errorf("a healthy backend must not be shadowed by the stale entry: %s", refetch.Body.String())
+	}
+	if serving, _, _ := srv.stale.snapshot(); serving {
+		t.Error("serving_stale must stay false while backends are healthy")
+	}
+
+	// Every backend down and the entry expired: the stale copy is served.
+	clk.advance(31 * time.Second)
+	a.setFail(true)
+	b.setFail(true)
+	stale := doGet(t, srv.Handler(), factsPath, "")
+	if stale.Code != http.StatusOK {
+		t.Fatalf("stale fallback status %d: %s", stale.Code, stale.Body.String())
+	}
+	if !strings.Contains(stale.Body.String(), `"new"`) {
+		t.Errorf("stale body = %s, want the last cached copy", stale.Body.String())
+	}
+
+	serving, served, last := srv.stale.snapshot()
+	if !serving || served != 1 || last.IsZero() {
+		t.Errorf("stale tracker = %v/%d/%v, want serving=true served=1", serving, served, last)
+	}
+}
+
+func TestHandler_NoCacheEntryMeansBackendFailureIs502(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	a.setFail(true)
+	b.setFail(true)
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	rec := doGet(t, srv.Handler(), factsPath, "")
+	if rec.Code != http.StatusBadGateway {
+		t.Fatalf("status %d, want 502", rec.Code)
+	}
+	if !strings.Contains(rec.Body.String(), "all backends failed") {
+		t.Errorf("body = %q", rec.Body.String())
+	}
+}
+
+func TestHandler_SingleFlightCollapsesConcurrentRequests(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+	h := srv.Handler()
+
+	release := make(chan struct{})
+	a.setBlock(release)
+	b.setBlock(release)
+
+	const callers = 16
+	var wg sync.WaitGroup
+	codes := make([]int, callers)
+	for i := range callers {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			codes[i] = doGet(t, h, factsPath, "").Code
+		}(i)
+	}
+
+	// The leader is parked inside the blocked backend, so every caller that
+	// reaches the handler before the release joins its flight.
+	waitFor(t, func() bool { return a.hitCount(factsPath) >= 1 })
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	for i, code := range codes {
+		if code != http.StatusOK {
+			t.Fatalf("caller %d got %d", i, code)
+		}
+	}
+	if got := a.hitCount(factsPath); got != 1 {
+		t.Errorf("backend a saw %d requests, want 1 for %d concurrent callers", got, callers)
+	}
+	if got := b.hitCount(factsPath); got != 1 {
+		t.Errorf("backend b saw %d requests, want 1 for %d concurrent callers", got, callers)
+	}
+}
+
+func TestFlightGroup_LeaderRunsOnce(t *testing.T) {
+	var g flightGroup
+	var calls, shared atomic.Int64
+	entered := make(chan struct{})
+	release := make(chan struct{})
+
+	run := func() (cachedResponse, error) {
+		if calls.Add(1) == 1 {
+			close(entered)
+		}
+		<-release
+		return cachedResponse{Body: json.RawMessage(`[]`), Records: -1}, nil
+	}
+
+	var wg sync.WaitGroup
+	for range 16 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if _, _, s := g.Do("k", run); s {
+				shared.Add(1)
+			}
+		}()
+	}
+	<-entered
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	if calls.Load() != 1 {
+		t.Errorf("fn ran %d times, want 1", calls.Load())
+	}
+	if shared.Load() != 15 {
+		t.Errorf("%d callers shared the flight, want 15", shared.Load())
+	}
+	// The key is released once the flight finishes.
+	if _, _, s := g.Do("k", func() (cachedResponse, error) { return cachedResponse{Records: -1}, nil }); s {
+		t.Error("a later call must start its own flight")
+	}
+}
+
+func TestFlightGroup_LeaderPanicFailsLeaderAndWaiters(t *testing.T) {
+	var g flightGroup
+	entered := make(chan struct{})
+	release := make(chan struct{})
+
+	type result struct {
+		resp   cachedResponse
+		err    error
+		shared bool
+	}
+	waiters := make([]result, 8)
+	var wg sync.WaitGroup
+
+	var leader result
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		leader.resp, leader.err, leader.shared = g.Do("k", func() (cachedResponse, error) {
+			close(entered)
+			<-release
+			panic("build exploded")
+		})
+	}()
+
+	<-entered
+	for i := range waiters {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			waiters[i].resp, waiters[i].err, waiters[i].shared = g.Do("k", func() (cachedResponse, error) {
+				t.Error("a waiter must not run its own fn")
+				return cachedResponse{}, nil
+			})
+		}(i)
+	}
+	// Park the waiters in Wait() before the leader panics.
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	if leader.err == nil {
+		t.Errorf("leader err = nil, want a panic error (resp %+v)", leader.resp)
+	}
+	for i, w := range waiters {
+		if !w.shared {
+			t.Errorf("waiter %d did not share the flight", i)
+		}
+		if w.err == nil {
+			t.Fatalf("waiter %d unblocked with err = nil and resp %+v, want an error", i, w.resp)
+		}
+		if !strings.Contains(w.err.Error(), "build exploded") {
+			t.Errorf("waiter %d err = %v, want the panic value", i, w.err)
+		}
+	}
+
+	// The key is released on the panic path, so a later call leads its own flight.
+	resp, err, shared := g.Do("k", func() (cachedResponse, error) {
+		return cachedResponse{Body: json.RawMessage(`[]`), Records: 0}, nil
+	})
+	if shared {
+		t.Error("a call after a panicking flight must start its own flight")
+	}
+	if err != nil || resp.Records != 0 {
+		t.Errorf("later call = (%+v, %v), want a clean success", resp, err)
+	}
+}
+
+func TestServeCached_BuildPanicIs502ThenStale(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	panicBuild := func() (cachedResponse, error) { panic("build exploded") }
+	serve := func(build func() (cachedResponse, error)) *httptest.ResponseRecorder {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, factsPath, nil)
+		srv.serveCached(rec, req, factsPath, nil, build)
+		return rec
+	}
+
+	// No entry to fall back to: a panicking build must not look like a success.
+	rec := serve(panicBuild)
+	if rec.Code != http.StatusBadGateway {
+		t.Fatalf("status %d (%s), want 502", rec.Code, rec.Body.String())
+	}
+
+	stored := `[` + fact("h1", "role", "web", "") + `]`
+	rec = serve(func() (cachedResponse, error) {
+		return cachedResponse{Body: json.RawMessage(stored), Records: 1}, nil
+	})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("priming status %d (%s)", rec.Code, rec.Body.String())
+	}
+
+	clk.advance(31 * time.Second)
+	rec = serve(panicBuild)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("stale fallback status %d (%s), want 200", rec.Code, rec.Body.String())
+	}
+	if got := strings.TrimSpace(rec.Body.String()); got != stored {
+		t.Errorf("stale body = %s, want %s", got, stored)
+	}
+	if got := rec.Header().Get(recordsHeader); got != "1" {
+		t.Errorf("%s = %q, want 1", recordsHeader, got)
+	}
+	if hr := health(t, srv); !hr.Cache.ServingStale || hr.Cache.StaleServed != 1 {
+		t.Errorf("panic fallback not counted as stale: %+v", hr.Cache)
+	}
+}
+
+func TestServeCached_BuildPanicFailsConcurrentWaiters(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	entered := make(chan struct{})
+	release := make(chan struct{})
+	serve := func(build func() (cachedResponse, error)) *httptest.ResponseRecorder {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, factsPath, nil)
+		srv.serveCached(rec, req, factsPath, nil, build)
+		return rec
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	var leaderRec *httptest.ResponseRecorder
+	go func() {
+		defer wg.Done()
+		leaderRec = serve(func() (cachedResponse, error) {
+			close(entered)
+			<-release
+			panic("build exploded")
+		})
+	}()
+
+	<-entered
+	const waiters = 8
+	recs := make([]*httptest.ResponseRecorder, waiters)
+	for i := range waiters {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			recs[i] = serve(func() (cachedResponse, error) {
+				t.Error("a waiter must not run its own build")
+				return cachedResponse{}, nil
+			})
+		}(i)
+	}
+	time.Sleep(250 * time.Millisecond)
+	close(release)
+	wg.Wait()
+
+	if leaderRec.Code != http.StatusBadGateway {
+		t.Errorf("leader status %d, want 502", leaderRec.Code)
+	}
+	for i, rec := range recs {
+		if rec.Code != http.StatusBadGateway {
+			t.Fatalf("waiter %d got %d (%s), want 502 rather than an empty success",
+				i, rec.Code, rec.Body.String())
+		}
+	}
+}
+
+func TestHandler_HealthzReportsCacheState(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{factsPath: `[` + fact("h1", "role", "web", "") + `]`})
+	b := newCountingBackend(t, map[string]string{factsPath: `[]`})
+	srv, clk := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	hr := health(t, srv)
+	if hr.Cache.Backend != "memory" || hr.Cache.TTL != "30s" {
+		t.Fatalf("cache health = %+v, want memory/30s", hr.Cache)
+	}
+	if hr.Cache.Entries != 0 || hr.Cache.ServingStale {
+		t.Fatalf("a cold cache should be empty and not stale: %+v", hr.Cache)
+	}
+
+	doGet(t, srv.Handler(), factsPath, "")
+	hr = health(t, srv)
+	if hr.Cache.Entries != 1 || hr.Cache.StaleEntries != 0 || hr.Cache.Bytes == 0 {
+		t.Fatalf("after one request: %+v", hr.Cache)
+	}
+
+	clk.advance(31 * time.Second)
+	if hr = health(t, srv); hr.Cache.StaleEntries != 1 {
+		t.Fatalf("expired entry should count as stale: %+v", hr.Cache)
+	}
+	if hr.Cache.ServingStale {
+		t.Error("an expired entry alone is not serving_stale")
+	}
+
+	a.setFail(true)
+	b.setFail(true)
+	doGet(t, srv.Handler(), factsPath, "")
+	hr = health(t, srv)
+	if !hr.Cache.ServingStale || hr.Cache.StaleServed != 1 || hr.Cache.LastStale == "" {
+		t.Errorf("staleness not surfaced in /healthz: %+v", hr.Cache)
+	}
+	if hr.Status != "down" {
+		t.Errorf("status = %q, want down", hr.Status)
+	}
+}
+
+func TestHandler_HealthzReportsDisabledCache(t *testing.T) {
+	a := newCountingBackend(t, nil)
+	b := newCountingBackend(t, nil)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic)) // FactsTTL zero
+
+	hr := health(t, srv)
+	if hr.Cache.Backend != "none" || hr.Cache.TTL != "0" {
+		t.Errorf("disabled cache health = %+v, want none/0", hr.Cache)
+	}
+}
+
+func TestHandler_UncachedPathsStillRefetch(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{reportsPath: `[{"hash":"h"}]`})
+	b := newCountingBackend(t, map[string]string{reportsPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), reportsPath, "")
+	doGet(t, srv.Handler(), reportsPath, "")
+	if got := a.hitCount(reportsPath); got != 2 {
+		t.Errorf("/reports is uncached: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_NodesRecordSetCached(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{nodesPath: `[{"certname":"h1"}]`})
+	b := newCountingBackend(t, map[string]string{nodesPath: `[]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), nodesPath, "")
+	doGet(t, srv.Handler(), nodesPath, "")
+	if got := a.hitCount(nodesPath); got != 1 {
+		t.Errorf("merged /nodes is cached: %d requests, want 1", got)
+	}
+}
+
+func TestHandler_NodesAggregateNotCached(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{nodesPath: `[{"count":90}]`})
+	b := newCountingBackend(t, map[string]string{nodesPath: `[{"count":53}]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	first := doGet(t, srv.Handler(), nodesPath, nodeCountQuery)
+	second := doGet(t, srv.Handler(), nodesPath, nodeCountQuery)
+	if first.Code != http.StatusOK || second.Code != http.StatusOK {
+		t.Fatalf("statuses %d/%d", first.Code, second.Code)
+	}
+	if got := a.hitCount(nodesPath); got != 2 {
+		t.Errorf("/nodes aggregates are uncached: %d requests, want 2", got)
+	}
+	if got := counts(t, second.Body.Bytes(), "count"); !slices.Equal(got, []float64{143}) {
+		t.Errorf("count = %v, want [143]", got)
+	}
+}
+
+func TestHandler_ResourcesAggregateNotCached(t *testing.T) {
+	a := newCountingBackend(t, map[string]string{resourcesPath: `[{"count":7}]`})
+	b := newCountingBackend(t, map[string]string{resourcesPath: `[{"count":5}]`})
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	doGet(t, srv.Handler(), resourcesPath, nodeCountQuery)
+	doGet(t, srv.Handler(), resourcesPath, nodeCountQuery)
+	if got := a.hitCount(resourcesPath); got != 2 {
+		t.Errorf("/resources aggregates are uncached: %d requests, want 2", got)
+	}
+}
+
+func TestHandler_MetaAndMetricsNotCached(t *testing.T) {
+	bodies := map[string]string{
+		metaVersionPath:      `{"version":"8.4.0"}`,
+		metaServerTimePath:   `{"server_time":"2026-01-01T00:00:00.000Z"}`,
+		"/metrics/v2/read/x": `{"request":{},"value":{"Value":1},"status":200}`,
+	}
+	a := newCountingBackend(t, bodies)
+	b := newCountingBackend(t, bodies)
+	srv, _ := newCachedServer(t, cacheTestConfig(a.srv.URL, b.srv.URL))
+
+	for path := range bodies {
+		doGet(t, srv.Handler(), path, "")
+		doGet(t, srv.Handler(), path, "")
+		if got := a.hitCount(path); got != 2 {
+			t.Errorf("%s is uncached: %d requests, want 2", path, got)
+		}
+	}
+}
+
+func TestConfig_FactsTTLClampedAtCap(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("XDG_CONFIG_HOME", dir)
+	clearEnv(t)
+
+	path := filepath.Join(dir, appName, configFileName)
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte("facts_ttl: 5m\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != maxFactsTTL {
+		t.Errorf("file facts_ttl = %s, want it clamped to %s", cfg.FactsTTL, maxFactsTTL)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "10m")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != maxFactsTTL {
+		t.Errorf("env facts_ttl = %s, want it clamped to %s", cfg.FactsTTL, maxFactsTTL)
+	}
+	if cfg.factsTTLClamped != 10*time.Minute {
+		t.Errorf("pre-clamp value = %s, want 10m0s", cfg.factsTTLClamped)
+	}
+
+	out := captureStdout(t, func() { printConfig(cfg) })
+	if !strings.Contains(out, "facts_ttl    : 30s (clamped from 600s, cap 30s)") {
+		t.Errorf("config show must report the clamp, got:\n%s", out)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "5s")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.FactsTTL != 5*time.Second || cfg.factsTTLClamped != 0 {
+		t.Errorf("a value under the cap must pass through: %s / %s", cfg.FactsTTL, cfg.factsTTLClamped)
+	}
+
+	t.Setenv(envPrefix+"FACTS_TTL", "0s")
+	cfg, err = Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.cacheEnabled() {
+		t.Error("facts_ttl 0 must disable the cache")
+	}
+	out = captureStdout(t, func() { printConfig(cfg) })
+	if !strings.Contains(out, "(cache disabled)") {
+		t.Errorf("config show must say the cache is off, got:\n%s", out)
+	}
+}
+
+func TestNewServer_ClampsFactsTTL(t *testing.T) {
+	cfg := testConfig("http://a.invalid", "http://b.invalid", mergeStatic)
+	cfg.FactsTTL = time.Hour
+	cfg.CacheBytes = 1 << 20
+	srv := newTestServer(cfg)
+
+	mc, ok := srv.factsCache.(*memoryCache)
+	if !ok {
+		t.Fatalf("expected a memory cache, got %T", srv.factsCache)
+	}
+	if mc.ttl != maxFactsTTL {
+		t.Errorf("cache ttl = %s, want %s", mc.ttl, maxFactsTTL)
+	}
+}
+
+func TestConfig_ValidateRejectsNegativeCacheSettings(t *testing.T) {
+	base := testConfig("http://a.invalid", "http://b.invalid", mergeStatic)
+	for name, mutate := range map[string]func(*Config){
+		"negative facts_ttl":         func(c *Config) { c.FactsTTL = -time.Second },
+		"negative facts_cache_bytes": func(c *Config) { c.CacheBytes = -1 },
+	} {
+		cfg := base
+		mutate(&cfg)
+		if err := cfg.Validate(); err == nil {
+			t.Errorf("%s should not validate", name)
+		}
+	}
+}
+
+func health(t *testing.T, srv *Server) healthReport {
+	t.Helper()
+	rec := doGet(t, srv.Handler(), "/healthz", "")
+	var hr healthReport
+	if err := json.Unmarshal(rec.Body.Bytes(), &hr); err != nil {
+		t.Fatalf("decode healthz: %v (%s)", err, rec.Body.String())
+	}
+	return hr
+}
+
+func waitFor(t *testing.T, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for !cond() {
+		if time.Now().After(deadline) {
+			t.Fatal("condition not met within 5s")
+		}
+		time.Sleep(time.Millisecond)
+	}
+}

config.go

@@ -15,130 +15,162 @@ const (
 	appName        = "pdbmux"
 	configFileName = "config.yaml"
 	envPrefix      = "PDBMUX_"
+	envConfigPath  = envPrefix + "CONFIG"
+
+	// systemConfigDir is the last resort in the search order, and the path a
+	// container mount (configmap, secret) is expected to land on.
+	systemConfigDir = "/etc/" + appName
 
-	// defaultListen is the default HTTP listen address.
 	defaultListen = ":8080"
-	// defaultOldURL / defaultNewURL are the two PuppetDBs merged during the
-	// VM -> k8s migration. old = legacy Consul-registered puppetdbapi; new =
-	// the k8s PuppetDB behind the gateway (TLS terminated there).
-	defaultOldURL = "http://puppetdbapi.service.consul:8080"
-	defaultNewURL = "https://puppetdb.k8s.syd1.au.unkin.net"
-	// defaultPrimary is the backend name used for pass-through (non-merged)
-	// /pdb/query/v4/* paths and as static precedence for merge fallback.
-	defaultPrimary = "new"
 
 	defaultTimeout      = 10 * time.Second
 	defaultFreshnessTTL = 30 * time.Second
+
+	// maxFactsTTL is a hard cap, not a default: a larger configured value is
+	// clamped down to it rather than rejected, so a stray env var cannot make a
+	// container crash-loop.
+	maxFactsTTL      = 30 * time.Second
+	defaultFactsTTL  = 30 * time.Second
+	defaultCacheSize = int64(64 << 20)
 )
 
-// Backend is one upstream PuppetDB. URL is the base URL (scheme://host[:port]),
-// without the /pdb/query/v4/... path — that is appended per request.
-type Backend struct {
-	Name string `yaml:"name"`
-	URL  string `yaml:"url"`
+var exampleBackends = []Backend{
+	{Name: "pdb-a", URL: "http://puppetdb1.example.com:8080"},
+	{Name: "pdb-b", URL: "http://puppetdb2.example.com:8080"},
 }
 
-// Config holds every configurable value. Fields map 1:1 to config-file keys and
-// env vars (PDBMUX_*). See Load for precedence.
-type Config struct {
-	// Listen is the HTTP listen address (host:port).
-	Listen string `yaml:"listen"`
-	// Backends is the ordered list of upstream PuppetDBs to fan out to.
-	Backends []Backend `yaml:"backends"`
-	// Primary is the backend Name used for transparent pass-through of
-	// non-merged /pdb/query/v4/* paths.
-	Primary string `yaml:"primary"`
-	// Merge selects how /facts records are attributed to a backend when a
-	// certname appears in both: "freshness" (query /nodes report_timestamp,
-	// newer wins) or "static" (always prefer the Prefer backend).
-	Merge string `yaml:"merge"`
-	// Prefer names the backend that wins under static merge and as the
-	// tie-breaker/fallback under freshness merge.
-	Prefer string `yaml:"prefer"`
-	// Timeout bounds each upstream request.
-	Timeout time.Duration `yaml:"timeout"`
-	// FreshnessTTL is how long a per-certname freshness map (from /nodes) is
-	// cached under the "freshness" merge strategy.
-	FreshnessTTL time.Duration `yaml:"freshness_ttl"`
+type Backend struct {
+	Name string `yaml:"name"`
+	URL  string `yaml:"url"` // base URL only; the query path is appended per request
 }
 
+type Config struct {
+	Listen       string        `yaml:"listen"`
+	Backends     []Backend     `yaml:"backends"` // all equal; order is only a deterministic tie-break
+	Merge        string        `yaml:"merge"`
+	Timeout      time.Duration `yaml:"timeout"`
+	FreshnessTTL time.Duration `yaml:"freshness_ttl"`
+	FactsTTL     time.Duration `yaml:"facts_ttl"`         // 0 disables the /facts+/nodes cache
+	CacheBytes   int64         `yaml:"facts_cache_bytes"` // byte budget for that cache
+
+	sourcePath      string        // file this config was read from, empty if none was found
+	factsTTLClamped time.Duration // pre-clamp facts_ttl, zero when nothing was clamped
+}
+
+// SourcePath returns the config file Load read, or "" when none was loaded.
+func (c Config) SourcePath() string { return c.sourcePath }
+
 const (
 	mergeFreshness = "freshness"
 	mergeStatic    = "static"
 )
 
-// DefaultConfig returns the built-in defaults: both migration PuppetDBs,
-// freshness merge, "new" primary/preferred.
 func DefaultConfig() Config {
 	return Config{
-		Listen: defaultListen,
-		Backends: []Backend{
-			{Name: "old", URL: defaultOldURL},
-			{Name: "new", URL: defaultNewURL},
-		},
-		Primary:      defaultPrimary,
+		Listen:       defaultListen,
 		Merge:        mergeFreshness,
-		Prefer:       defaultPrimary,
 		Timeout:      defaultTimeout,
 		FreshnessTTL: defaultFreshnessTTL,
+		FactsTTL:     defaultFactsTTL,
+		CacheBytes:   defaultCacheSize,
 	}
 }
 
-// ConfigDir returns the XDG_CONFIG_HOME/pdbmux directory.
+func ExampleConfig() Config {
+	cfg := DefaultConfig()
+	cfg.Backends = append([]Backend(nil), exampleBackends...)
+	return cfg
+}
+
 func ConfigDir() string {
 	base := os.Getenv("XDG_CONFIG_HOME")
 	if base == "" {
 		home, _ := os.UserHomeDir()
+		if home == "" {
+			return systemConfigDir
+		}
 		base = filepath.Join(home, ".config")
 	}
 	return filepath.Join(base, appName)
 }
 
-// ConfigPath returns the full path to the config file.
 func ConfigPath() string {
 	return filepath.Join(ConfigDir(), configFileName)
 }
 
-// Load reads the config file (if present), then applies env var overrides.
-// Precedence (lowest -> highest): defaults < config file < env vars < flags
-// (flags are applied by the caller). Backends can be overridden wholesale via
-// PDBMUX_BACKENDS ("name=url,name=url").
-func Load() (Config, error) {
+// configSearchPaths lists the default config locations, highest priority first.
+func configSearchPaths() []string {
+	paths := []string{ConfigPath()}
+	if system := filepath.Join(systemConfigDir, configFileName); system != paths[0] {
+		paths = append(paths, system)
+	}
+	return paths
+}
+
+// explicitConfigPath returns the config path named by --config or PDBMUX_CONFIG,
+// or "" when neither is set.
+func explicitConfigPath(flagPath string) string {
+	if flagPath != "" {
+		return flagPath
+	}
+	return os.Getenv(envConfigPath)
+}
+
+// resolveConfigPath picks the config file to read: --config, else PDBMUX_CONFIG,
+// else the first existing default search path. explicit reports whether the path
+// was named outright, in which case a missing file is an error.
+func resolveConfigPath(flagPath string) (path string, explicit bool) {
+	if p := explicitConfigPath(flagPath); p != "" {
+		return p, true
+	}
+	paths := configSearchPaths()
+	for _, p := range paths {
+		if st, err := os.Stat(p); err == nil && !st.IsDir() {
+			return p, false
+		}
+	}
+	return paths[0], false
+}
+
+// Precedence: defaults < config file < env vars < flags, and flags are applied by the caller.
+func Load(flagPath string) (Config, error) {
 	cfg := DefaultConfig()
 
-	path := ConfigPath()
+	path, explicit := resolveConfigPath(flagPath)
 	data, err := os.ReadFile(path)
-	if err != nil && !os.IsNotExist(err) {
-		return cfg, fmt.Errorf("reading config %s: %w", path, err)
-	}
-	if err == nil {
+	switch {
+	case err == nil:
 		if err := yaml.Unmarshal(data, &cfg); err != nil {
 			return cfg, fmt.Errorf("parsing config %s: %w", path, err)
 		}
+		cfg.sourcePath = path
+	case os.IsNotExist(err) && !explicit:
+		// No config file anywhere on the search path: defaults + env only.
+	default:
+		return cfg, fmt.Errorf("reading config %s: %w", path, err)
 	}
 
 	applyEnv(&cfg, os.Getenv)
-
-	if err := cfg.Validate(); err != nil {
-		return cfg, err
-	}
+	cfg.clampFactsTTL()
 	return cfg, nil
 }
 
-// applyEnv overlays PDBMUX_* env vars onto cfg. getenv is injected for testing.
+// clampFactsTTL pins facts_ttl to maxFactsTTL, remembering the configured value
+// so `config show` can say the cap was applied.
+func (c *Config) clampFactsTTL() {
+	if c.FactsTTL > maxFactsTTL {
+		c.factsTTLClamped = c.FactsTTL
+		c.FactsTTL = maxFactsTTL
+	}
+}
+
 func applyEnv(cfg *Config, getenv func(string) string) {
 	if v := getenv(envPrefix + "LISTEN"); v != "" {
 		cfg.Listen = v
 	}
-	if v := getenv(envPrefix + "PRIMARY"); v != "" {
-		cfg.Primary = v
-	}
 	if v := getenv(envPrefix + "MERGE"); v != "" {
 		cfg.Merge = v
 	}
-	if v := getenv(envPrefix + "PREFER"); v != "" {
-		cfg.Prefer = v
-	}
 	if v := getenv(envPrefix + "TIMEOUT"); v != "" {
 		if d, err := time.ParseDuration(v); err == nil {
 			cfg.Timeout = d
@@ -149,6 +181,16 @@ func applyEnv(cfg *Config, getenv func(string) string) {
 			cfg.FreshnessTTL = d
 		}
 	}
+	if v := getenv(envPrefix + "FACTS_TTL"); v != "" {
+		if d, err := time.ParseDuration(v); err == nil {
+			cfg.FactsTTL = d
+		}
+	}
+	if v := getenv(envPrefix + "FACTS_CACHE_BYTES"); v != "" {
+		if n, err := strconv.ParseInt(v, 10, 64); err == nil {
+			cfg.CacheBytes = n
+		}
+	}
 	if v := getenv(envPrefix + "BACKENDS"); v != "" {
 		if bs := parseBackends(v); len(bs) > 0 {
 			cfg.Backends = bs
@@ -156,8 +198,7 @@ func applyEnv(cfg *Config, getenv func(string) string) {
 	}
 }
 
-// parseBackends parses "name=url,name=url" into Backends. Entries without an
-// "=" are skipped. Used for the PDBMUX_BACKENDS env override.
+// Parses the PDBMUX_BACKENDS form "name=url,name=url"; entries without an "=" are skipped.
 func parseBackends(s string) []Backend {
 	var out []Backend
 	for _, part := range strings.Split(s, ",") {
@@ -175,10 +216,19 @@ func parseBackends(s string) []Backend {
 	return out
 }
 
-// Validate checks the config is internally consistent and usable.
+// configHint names the file a user should edit: the one actually loaded, else
+// the default write target.
+func (c Config) configHint() string {
+	if c.sourcePath != "" {
+		return c.sourcePath
+	}
+	return ConfigPath()
+}
+
 func (c Config) Validate() error {
 	if len(c.Backends) == 0 {
-		return fmt.Errorf("no backends configured")
+		return fmt.Errorf("no backends configured: set %sBACKENDS to \"name=url,name=url\" or add a backends list to %s",
+			envPrefix, c.configHint())
 	}
 	seen := map[string]bool{}
 	for _, b := range c.Backends {
@@ -190,49 +240,43 @@ func (c Config) Validate() error {
 		}
 		seen[b.Name] = true
 	}
-	if !seen[c.Primary] {
-		return fmt.Errorf("primary %q is not a configured backend", c.Primary)
-	}
 	switch c.Merge {
 	case mergeFreshness, mergeStatic:
 	default:
 		return fmt.Errorf("merge must be %q or %q, got %q", mergeFreshness, mergeStatic, c.Merge)
 	}
-	if !seen[c.Prefer] {
-		return fmt.Errorf("prefer %q is not a configured backend", c.Prefer)
-	}
 	if c.Timeout <= 0 {
 		return fmt.Errorf("timeout must be positive")
 	}
+	if c.FactsTTL < 0 {
+		return fmt.Errorf("facts_ttl must not be negative (0 disables the cache)")
+	}
+	if c.CacheBytes < 0 {
+		return fmt.Errorf("facts_cache_bytes must not be negative")
+	}
 	return nil
 }
 
-// PrimaryBackend returns the backend named by Primary (guaranteed present after
-// Validate).
-func (c Config) PrimaryBackend() Backend {
-	for _, b := range c.Backends {
-		if b.Name == c.Primary {
-			return b
-		}
-	}
-	return c.Backends[0]
-}
+// cacheEnabled reports whether a facts/nodes cache should be built: both a TTL
+// and a byte budget are required.
+func (c Config) cacheEnabled() bool { return c.FactsTTL > 0 && c.CacheBytes > 0 }
 
-// writeDefaultConfig creates the config dir and writes a default config file.
-func writeDefaultConfig() error {
-	dir := ConfigDir()
-	if err := os.MkdirAll(dir, 0o755); err != nil {
+func writeDefaultConfig(path string) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return fmt.Errorf("creating config dir: %w", err)
 	}
-	path := ConfigPath()
 	if _, err := os.Stat(path); err == nil {
 		return fmt.Errorf("config already exists at %s", path)
 	}
-	data, _ := yaml.Marshal(DefaultConfig())
+	data, _ := yaml.Marshal(ExampleConfig())
 	header := []byte("# pdbmux configuration\n" +
-		"# A merging proxy over two PuppetDBs (old Consul + new k8s) during migration.\n" +
-		"# Env overrides: PDBMUX_LISTEN, PDBMUX_PRIMARY, PDBMUX_MERGE, PDBMUX_PREFER,\n" +
-		"#   PDBMUX_TIMEOUT, PDBMUX_FRESHNESS_TTL, PDBMUX_BACKENDS (name=url,name=url).\n\n")
+		"# A merging proxy presenting one PuppetDB v4 query surface over several\n" +
+		"# PuppetDB backends. The backend URLs below are placeholders — edit them.\n" +
+		"# Env overrides: PDBMUX_LISTEN, PDBMUX_MERGE, PDBMUX_TIMEOUT,\n" +
+		"#   PDBMUX_FRESHNESS_TTL, PDBMUX_FACTS_TTL, PDBMUX_FACTS_CACHE_BYTES,\n" +
+		"#   PDBMUX_BACKENDS (name=url,name=url).\n" +
+		"# facts_ttl caches merged /facts and /nodes in memory; it is capped at 30s\n" +
+		"# (a larger value is clamped) and 0 disables the cache.\n\n")
 	if err := os.WriteFile(path, append(header, data...), 0o644); err != nil {
 		return fmt.Errorf("writing config: %w", err)
 	}
@@ -240,8 +284,6 @@ func writeDefaultConfig() error {
 	return nil
 }
 
-// durationString renders a duration for `config show` (falls back to a plain
-// seconds count for zero to avoid "0s" ambiguity in logs).
 func durationString(d time.Duration) string {
 	if d == 0 {
 		return "0"

config_test.go

@@ -1,28 +1,73 @@
 package main
 
 import (
+	"bytes"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
 
-func TestLoad_Defaults(t *testing.T) {
-	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
-	clearEnv(t)
-
-	cfg, err := Load()
-	if err != nil {
-		t.Fatal(err)
+// testConfigValid returns a minimal valid config for Validate/merge tests, with
+// neutral placeholder backends.
+func testConfigValid() Config {
+	cfg := DefaultConfig()
+	cfg.Backends = []Backend{
+		{Name: "a", URL: "http://localhost:18080"},
+		{Name: "b", URL: "http://localhost:18081"},
 	}
+	return cfg
+}
+
+func TestDefaultConfig_NoBackends(t *testing.T) {
+	cfg := DefaultConfig()
 	if cfg.Listen != defaultListen {
 		t.Errorf("listen = %q, want %q", cfg.Listen, defaultListen)
 	}
-	if len(cfg.Backends) != 2 || cfg.Backends[0].Name != "old" || cfg.Backends[1].Name != "new" {
-		t.Errorf("unexpected default backends: %+v", cfg.Backends)
+	if len(cfg.Backends) != 0 {
+		t.Errorf("defaults must not ship backends, got %+v", cfg.Backends)
 	}
-	if cfg.Merge != mergeFreshness || cfg.Primary != "new" {
-		t.Errorf("unexpected defaults merge=%s primary=%s", cfg.Merge, cfg.Primary)
+	if cfg.Merge != mergeFreshness {
+		t.Errorf("merge = %q, want %q", cfg.Merge, mergeFreshness)
+	}
+	if err := cfg.Validate(); err == nil {
+		t.Error("defaults alone must not validate: backends are required")
+	}
+}
+
+func TestLoad_NoBackendsLoadsButFailsValidation(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	clearEnv(t)
+
+	// Load itself must succeed so `config init` / `version` work unconfigured.
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	err = cfg.Validate()
+	if err == nil {
+		t.Fatal("expected a validation error when no backends are configured")
+	}
+	if !strings.Contains(err.Error(), envPrefix+"BACKENDS") {
+		t.Errorf("error should name the env var to set, got: %v", err)
+	}
+}
+
+func TestLoad_BackendsKeepConfiguredOrder(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	clearEnv(t)
+	t.Setenv(envPrefix+"BACKENDS", "a=http://localhost:18080,b=http://localhost:18081")
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(cfg.Backends) != 2 || cfg.Backends[0].Name != "a" || cfg.Backends[1].Name != "b" {
+		t.Errorf("backends should keep the configured order, got %+v", cfg.Backends)
+	}
+	if err := cfg.Validate(); err != nil {
+		t.Errorf("a bare backend list must validate: %v", err)
 	}
 }
 
@@ -35,7 +80,8 @@ func TestLoad_FileAndEnvOverride(t *testing.T) {
 	if err := os.MkdirAll(cfgDir, 0o755); err != nil {
 		t.Fatal(err)
 	}
-	body := "listen: :9999\nmerge: static\nprimary: old\nprefer: old\n"
+	body := "listen: :9999\nmerge: static\n" +
+		"backends:\n  - name: a\n    url: http://localhost:18080\n  - name: b\n    url: http://localhost:18081\n"
 	if err := os.WriteFile(filepath.Join(cfgDir, configFileName), []byte(body), 0o644); err != nil {
 		t.Fatal(err)
 	}
@@ -43,24 +89,22 @@ func TestLoad_FileAndEnvOverride(t *testing.T) {
 	// env beats file for listen.
 	t.Setenv(envPrefix+"LISTEN", "127.0.0.1:1234")
 
-	cfg, err := Load()
+	cfg, err := Load("")
 	if err != nil {
 		t.Fatalf("load: %v", err)
 	}
 	if cfg.Listen != "127.0.0.1:1234" {
 		t.Errorf("env should beat file for listen, got %q", cfg.Listen)
 	}
-	if cfg.Merge != mergeStatic || cfg.Primary != "old" {
-		t.Errorf("file override failed: merge=%s primary=%s", cfg.Merge, cfg.Primary)
+	if cfg.Merge != mergeStatic {
+		t.Errorf("file override failed: merge=%s", cfg.Merge)
 	}
 }
 
 func TestApplyEnv_Backends(t *testing.T) {
-	cfg := DefaultConfig()
+	cfg := testConfigValid()
 	env := map[string]string{
 		envPrefix + "BACKENDS":      "a=http://a:8080,b=http://b:8080",
-		envPrefix + "PRIMARY":       "a",
-		envPrefix + "PREFER":        "a",
 		envPrefix + "TIMEOUT":       "3s",
 		envPrefix + "FRESHNESS_TTL": "45s",
 	}
@@ -82,16 +126,14 @@ func TestValidate(t *testing.T) {
 	}{
 		{"ok", func(*Config) {}, false},
 		{"no backends", func(c *Config) { c.Backends = nil }, true},
-		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "old", URL: "x"}) }, true},
+		{"dup name", func(c *Config) { c.Backends = append(c.Backends, Backend{Name: "a", URL: "x"}) }, true},
 		{"missing url", func(c *Config) { c.Backends[0].URL = "" }, true},
-		{"primary not a backend", func(c *Config) { c.Primary = "ghost" }, true},
-		{"prefer not a backend", func(c *Config) { c.Prefer = "ghost" }, true},
 		{"bad merge", func(c *Config) { c.Merge = "wrong" }, true},
 		{"zero timeout", func(c *Config) { c.Timeout = 0 }, true},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			cfg := DefaultConfig()
+			cfg := testConfigValid()
 			tc.mutate(&cfg)
 			err := cfg.Validate()
 			if (err != nil) != tc.wantErr {
@@ -111,16 +153,245 @@ func TestParseBackends(t *testing.T) {
 	}
 }
 
-func TestPrimaryBackend(t *testing.T) {
-	cfg := DefaultConfig()
-	if cfg.PrimaryBackend().URL != defaultNewURL {
-		t.Errorf("primary backend URL = %q, want %q", cfg.PrimaryBackend().URL, defaultNewURL)
+func TestExampleConfig_IsValidAndNeutral(t *testing.T) {
+	cfg := ExampleConfig()
+	if err := cfg.Validate(); err != nil {
+		t.Fatalf("example config must validate: %v", err)
 	}
+	for _, b := range cfg.Backends {
+		if !strings.Contains(b.URL, "example.com") {
+			t.Errorf("example backend %q must use a placeholder host, got %q", b.Name, b.URL)
+		}
+	}
+}
+
+const testConfigBody = "listen: \":9999\"\nmerge: static\n" +
+	"backends:\n  - name: a\n    url: http://localhost:18080\n  - name: b\n    url: http://localhost:18081\n"
+
+func writeConfigFile(t *testing.T, path string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte(testConfigBody), 0o644); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestLoad_FileOnly(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("XDG_CONFIG_HOME", dir)
+	clearEnv(t)
+	path := filepath.Join(dir, appName, configFileName)
+	writeConfigFile(t, path)
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	if cfg.Listen != ":9999" || cfg.Merge != mergeStatic || len(cfg.Backends) != 2 {
+		t.Errorf("file values not applied: %+v", cfg)
+	}
+	if cfg.SourcePath() != path {
+		t.Errorf("source path = %q, want %q", cfg.SourcePath(), path)
+	}
+}
+
+func TestLoad_EnvOnly_DefaultPathMissing(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	clearEnv(t)
+	t.Setenv(envPrefix+"BACKENDS", "a=http://localhost:18080")
+	t.Setenv(envPrefix+"LISTEN", "127.0.0.1:1234")
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("a missing default config file must not be an error: %v", err)
+	}
+	if cfg.SourcePath() != "" {
+		t.Errorf("no file was loaded, source path should be empty, got %q", cfg.SourcePath())
+	}
+	if cfg.Listen != "127.0.0.1:1234" || len(cfg.Backends) != 1 {
+		t.Errorf("env values not applied: %+v", cfg)
+	}
+	if err := cfg.Validate(); err != nil {
+		t.Errorf("env-only config should validate: %v", err)
+	}
+}
+
+// A mounted config file must load with neither HOME nor XDG_CONFIG_HOME set.
+func TestLoad_ExplicitPath(t *testing.T) {
+	mounted := filepath.Join(t.TempDir(), "mounted.yaml")
+	writeConfigFile(t, mounted)
+
+	for _, tc := range []struct {
+		name string
+		flag string
+		env  string
+	}{
+		{"flag", mounted, ""},
+		{"env", "", mounted},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("XDG_CONFIG_HOME", "")
+			t.Setenv("HOME", "")
+			clearEnv(t)
+			t.Setenv(envConfigPath, tc.env)
+
+			cfg, err := Load(tc.flag)
+			if err != nil {
+				t.Fatalf("load: %v", err)
+			}
+			if cfg.SourcePath() != mounted {
+				t.Errorf("source path = %q, want %q", cfg.SourcePath(), mounted)
+			}
+			if cfg.Listen != ":9999" {
+				t.Errorf("listen = %q, want :9999", cfg.Listen)
+			}
+		})
+	}
+}
+
+func TestLoad_ExplicitPathMissingIsError(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "typo.yaml")
+
+	for _, tc := range []struct {
+		name string
+		flag string
+		env  string
+	}{
+		{"flag", missing, ""},
+		{"env", "", missing},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+			clearEnv(t)
+			t.Setenv(envConfigPath, tc.env)
+
+			_, err := Load(tc.flag)
+			if err == nil {
+				t.Fatal("an explicitly named config file that does not exist must fail loudly")
+			}
+			if !strings.Contains(err.Error(), missing) {
+				t.Errorf("error should name the missing path, got: %v", err)
+			}
+		})
+	}
+}
+
+func TestLoad_ExplicitFileStillLosesToEnv(t *testing.T) {
+	mounted := filepath.Join(t.TempDir(), "mounted.yaml")
+	writeConfigFile(t, mounted)
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	clearEnv(t)
+	t.Setenv(envConfigPath, mounted)
+	t.Setenv(envPrefix+"LISTEN", "127.0.0.1:1234")
+	t.Setenv(envPrefix+"MERGE", mergeFreshness)
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	if cfg.Listen != "127.0.0.1:1234" || cfg.Merge != mergeFreshness {
+		t.Errorf("env must beat the file: %+v", cfg)
+	}
+	if len(cfg.Backends) != 2 {
+		t.Errorf("unset env must leave file backends alone: %+v", cfg.Backends)
+	}
+}
+
+func TestResolveConfigPath_Precedence(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("XDG_CONFIG_HOME", dir)
+	clearEnv(t)
+	defaultPath := filepath.Join(dir, appName, configFileName)
+
+	if got, explicit := resolveConfigPath(""); got != defaultPath || explicit {
+		t.Errorf("no file anywhere: got %q explicit=%v, want %q false", got, explicit, defaultPath)
+	}
+
+	writeConfigFile(t, defaultPath)
+	if got, explicit := resolveConfigPath(""); got != defaultPath || explicit {
+		t.Errorf("default search: got %q explicit=%v, want %q false", got, explicit, defaultPath)
+	}
+
+	t.Setenv(envConfigPath, "/from/env.yaml")
+	if got, explicit := resolveConfigPath(""); got != "/from/env.yaml" || !explicit {
+		t.Errorf("env should beat the search path: got %q explicit=%v", got, explicit)
+	}
+	if got, explicit := resolveConfigPath("/from/flag.yaml"); got != "/from/flag.yaml" || !explicit {
+		t.Errorf("flag should beat env: got %q explicit=%v", got, explicit)
+	}
+}
+
+func TestConfigSearchPaths_EndsAtSystemDir(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	paths := configSearchPaths()
+	want := filepath.Join(systemConfigDir, configFileName)
+	if len(paths) != 2 || paths[1] != want {
+		t.Errorf("search paths = %v, want the system path %q last", paths, want)
+	}
+
+	t.Setenv("XDG_CONFIG_HOME", "")
+	t.Setenv("HOME", "")
+	if paths := configSearchPaths(); len(paths) != 1 || paths[0] != want {
+		t.Errorf("without HOME/XDG the search path should be just %q, got %v", want, paths)
+	}
+}
+
+func TestPrintConfig_ReportsSource(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("XDG_CONFIG_HOME", dir)
+	clearEnv(t)
+
+	out := captureStdout(t, func() {
+		cfg, err := Load("")
+		if err != nil {
+			t.Fatal(err)
+		}
+		printConfig(cfg)
+	})
+	if !strings.Contains(out, "none loaded") || !strings.Contains(out, filepath.Join(dir, appName, configFileName)) {
+		t.Errorf("config show should report nothing was loaded and what it searched, got:\n%s", out)
+	}
+
+	path := filepath.Join(dir, appName, configFileName)
+	writeConfigFile(t, path)
+	out = captureStdout(t, func() {
+		cfg, err := Load("")
+		if err != nil {
+			t.Fatal(err)
+		}
+		printConfig(cfg)
+	})
+	if !strings.Contains(out, path+" (loaded)") {
+		t.Errorf("config show should name the loaded file, got:\n%s", out)
+	}
+}
+
+func captureStdout(t *testing.T, f func()) string {
+	t.Helper()
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	orig := os.Stdout
+	os.Stdout = w
+	defer func() { os.Stdout = orig }()
+
+	f()
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	var buf bytes.Buffer
+	if _, err := buf.ReadFrom(r); err != nil {
+		t.Fatal(err)
+	}
+	return buf.String()
 }
 
 func clearEnv(t *testing.T) {
 	t.Helper()
-	for _, k := range []string{"LISTEN", "PRIMARY", "MERGE", "PREFER", "TIMEOUT", "FRESHNESS_TTL", "BACKENDS"} {
+	for _, k := range []string{"CONFIG", "LISTEN", "MERGE", "TIMEOUT", "FRESHNESS_TTL", "FACTS_TTL", "FACTS_CACHE_BYTES", "BACKENDS"} {
 		t.Setenv(envPrefix+k, "")
 	}
 }

main.go

@@ -1,21 +1,4 @@
-// Command pdbmux is a small merging HTTP proxy over two PuppetDB backends.
-//
-// During the VM -> k8s Puppet migration there are two PuppetDBs — the legacy
-// Consul-registered one and the new k8s one — and nodes move between them as
-// they migrate. pdbmux presents a single merged PuppetDB v4 query surface so
-// node-lookup and pblastreport (and anything else) see one consistent view:
-//
-//   - GET /pdb/query/v4/nodes  — fan out to both backends, dedupe by certname,
-//     keep the record with the newer report_timestamp.
-//   - GET /pdb/query/v4/facts  — fan out to both, and for a certname present in
-//     both keep ALL facts from the backend holding that node's newer report
-//     (freshness merge) or a static preferred backend (static merge).
-//   - any other GET /pdb/query/v4/* — transparently proxied to the primary.
-//   - GET /healthz — per-backend reachability.
-//
-// The query param is forwarded verbatim (PuppetDB AST JSON). If one backend
-// errors/times out, the other's results are served; only if both fail does a
-// merged endpoint return 502.
+// Command pdbmux is a small merging HTTP proxy over several PuppetDB backends.
 package main
 
 import (
@@ -26,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
@@ -35,25 +19,30 @@ import (
 var version = "dev"
 
 func main() {
-	cfg, err := Load()
-	if err != nil {
-		fmt.Fprintln(os.Stderr, "config error:", err)
-		os.Exit(1)
-	}
-
 	var (
-		listen  string
-		primary string
-		merge   string
+		cfg        Config
+		configPath string
+		listen     string
+		merge      string
 	)
 
+	// Loaded lazily: --config is only known once cobra has parsed flags.
+	loadConfig := func() error {
+		c, err := Load(configPath)
+		if err != nil {
+			return err
+		}
+		cfg = c
+		return nil
+	}
+
 	serve := func(cmd *cobra.Command) error {
+		if err := loadConfig(); err != nil {
+			return err
+		}
 		if cmd.Flags().Changed("listen") {
 			cfg.Listen = listen
 		}
-		if cmd.Flags().Changed("primary") {
-			cfg.Primary = primary
-		}
 		if cmd.Flags().Changed("merge") {
 			cfg.Merge = merge
 		}
@@ -65,19 +54,19 @@ func main() {
 
 	root := &cobra.Command{
 		Use:   appName,
-		Short: "Merging HTTP proxy over two PuppetDB backends.",
-		Long: "pdbmux presents a single merged PuppetDB v4 query surface over the old\n" +
-			"(Consul) and new (k8s) PuppetDBs during the migration, so node-lookup and\n" +
-			"pblastreport see one consistent view. Running pdbmux with no subcommand\n" +
-			"(or `pdbmux serve`) starts the proxy.",
+		Short: "Merging HTTP proxy over several PuppetDB backends.",
+		Long: "pdbmux presents a single merged PuppetDB v4 query surface over several\n" +
+			"PuppetDB backends, so clients see one consistent view of nodes, facts and\n" +
+			"reports spanning all of them. Running pdbmux with no subcommand (or\n" +
+			"`pdbmux serve`) starts the proxy.",
 		SilenceUsage: true,
 		RunE:         func(cmd *cobra.Command, args []string) error { return serve(cmd) },
 	}
 
 	pf := root.PersistentFlags()
-	pf.StringVar(&listen, "listen", cfg.Listen, "HTTP listen address (overrides config and PDBMUX_LISTEN)")
-	pf.StringVar(&primary, "primary", cfg.Primary, "Primary backend name for non-merged pass-through")
-	pf.StringVar(&merge, "merge", cfg.Merge, "Facts merge strategy: freshness or static")
+	pf.StringVar(&configPath, "config", "", "Config file path (overrides PDBMUX_CONFIG and the default search path)")
+	pf.StringVar(&listen, "listen", defaultListen, "HTTP listen address (overrides config and PDBMUX_LISTEN)")
+	pf.StringVar(&merge, "merge", mergeFreshness, "Facts merge strategy: freshness or static")
 
 	serveCmd := &cobra.Command{
 		Use:          "serve",
@@ -90,15 +79,24 @@ func main() {
 	configCmd.AddCommand(
 		&cobra.Command{
 			Use:          "init",
-			Short:        "Write a default config file to " + ConfigPath(),
+			Short:        "Write a default config file (--config path, else " + ConfigPath() + ")",
 			SilenceUsage: true,
-			RunE:         func(cmd *cobra.Command, args []string) error { return writeDefaultConfig() },
+			RunE: func(cmd *cobra.Command, args []string) error {
+				path := explicitConfigPath(configPath)
+				if path == "" {
+					path = ConfigPath()
+				}
+				return writeDefaultConfig(path)
+			},
 		},
 		&cobra.Command{
 			Use:          "show",
 			Short:        "Print the active configuration",
 			SilenceUsage: true,
 			RunE: func(cmd *cobra.Command, args []string) error {
+				if err := loadConfig(); err != nil {
+					return err
+				}
 				printConfig(cfg)
 				return nil
 			},
@@ -119,8 +117,6 @@ func main() {
 	}
 }
 
-// runServer starts the HTTP server and blocks until SIGINT/SIGTERM, then
-// gracefully shuts down.
 func runServer(cfg Config) error {
 	logger := log.New(os.Stderr, "pdbmux: ", log.LstdFlags)
 	srv := NewServer(cfg, logger)
@@ -131,8 +127,8 @@ func runServer(cfg Config) error {
 		ReadHeaderTimeout: 10 * time.Second,
 	}
 
-	logger.Printf("listening on %s (merge=%s primary=%s backends=%d)",
-		cfg.Listen, cfg.Merge, cfg.Primary, len(cfg.Backends))
+	logger.Printf("listening on %s (merge=%s backends=%d)",
+		cfg.Listen, cfg.Merge, len(cfg.Backends))
 
 	errCh := make(chan error, 1)
 	go func() {
@@ -155,15 +151,30 @@ func runServer(cfg Config) error {
 	}
 }
 
-// printConfig renders the active config for `config show`.
+func factsTTLString(cfg Config) string {
+	s := durationString(cfg.FactsTTL)
+	switch {
+	case cfg.factsTTLClamped > 0:
+		return fmt.Sprintf("%s (clamped from %s, cap %s)",
+			s, durationString(cfg.factsTTLClamped), durationString(maxFactsTTL))
+	case !cfg.cacheEnabled():
+		return s + " (cache disabled)"
+	}
+	return s
+}
+
 func printConfig(cfg Config) {
-	fmt.Printf("config file  : %s\n", ConfigPath())
+	if p := cfg.SourcePath(); p != "" {
+		fmt.Printf("config file  : %s (loaded)\n", p)
+	} else {
+		fmt.Printf("config file  : none loaded (searched %s)\n", strings.Join(configSearchPaths(), ", "))
+	}
 	fmt.Printf("listen       : %s\n", cfg.Listen)
-	fmt.Printf("primary      : %s\n", cfg.Primary)
 	fmt.Printf("merge        : %s\n", cfg.Merge)
-	fmt.Printf("prefer       : %s\n", cfg.Prefer)
 	fmt.Printf("timeout      : %s\n", durationString(cfg.Timeout))
 	fmt.Printf("freshness_ttl: %s\n", durationString(cfg.FreshnessTTL))
+	fmt.Printf("facts_ttl    : %s\n", factsTTLString(cfg))
+	fmt.Printf("facts_cache  : %d bytes\n", cfg.CacheBytes)
 	fmt.Println("backends:")
 	for _, b := range cfg.Backends {
 		fmt.Printf("  - %-8s %s\n", b.Name, b.URL)

merge.go

@@ -5,24 +5,20 @@ import (
 	"time"
 )
 
-// record is a single PuppetDB result element kept as raw JSON so unknown fields
-// survive the merge untouched. certname/report_timestamp are decoded only for
-// merge decisions.
+// Raw is kept verbatim so unknown PuppetDB fields survive the merge.
 type record struct {
 	Raw             json.RawMessage
 	Certname        string
 	ReportTimestamp string // only populated for /nodes records
+	Hash            string // only populated for /reports records
 }
 
-// recordMeta is the subset we decode from any /nodes or /facts element to drive
-// merge decisions.
 type recordMeta struct {
 	Certname        string `json:"certname"`
 	ReportTimestamp string `json:"report_timestamp"`
+	Hash            string `json:"hash"`
 }
 
-// decodeRecords turns a raw PuppetDB JSON array into records, preserving each
-// element verbatim in Raw. A body that is not a JSON array yields (nil, err).
 func decodeRecords(body []byte) ([]record, error) {
 	var raws []json.RawMessage
 	if err := json.Unmarshal(body, &raws); err != nil {
@@ -36,13 +32,13 @@ func decodeRecords(body []byte) ([]record, error) {
 			Raw:             raw,
 			Certname:        m.Certname,
 			ReportTimestamp: m.ReportTimestamp,
+			Hash:            m.Hash,
 		})
 	}
 	return out, nil
 }
 
-// parseTimestamp parses a PuppetDB RFC3339(nano) timestamp. Zero time on
-// failure sorts oldest, so a backend with a well-formed newer timestamp wins.
+// An unparseable timestamp yields the zero time, which sorts oldest.
 func parseTimestamp(s string) time.Time {
 	if s == "" {
 		return time.Time{}
@@ -53,10 +49,7 @@ func parseTimestamp(s string) time.Time {
 	return time.Time{}
 }
 
-// mergeNodes dedupes /nodes records by certname, keeping the one with the newer
-// report_timestamp. backends is the ordered list of (name, records) results;
-// when timestamps tie (or both are zero), the earlier backend in the slice
-// wins, so callers should order by precedence.
+// Ties keep the earlier backend's record — a deterministic tie-break, not a preference.
 func mergeNodes(results []backendResult) []json.RawMessage {
 	type pick struct {
 		raw json.RawMessage
@@ -73,7 +66,6 @@ func mergeNodes(results []backendResult) []json.RawMessage {
 				order = append(order, rec.Certname)
 				continue
 			}
-			// Strictly-newer wins; ties keep the existing (earlier-backend) pick.
 			if ts.After(cur.ts) {
 				best[rec.Certname] = pick{raw: rec.Raw, ts: ts}
 			}
@@ -86,12 +78,10 @@ func mergeNodes(results []backendResult) []json.RawMessage {
 	return out
 }
 
-// freshness maps certname -> backend name that holds that node's newest report.
+// certname -> name of the backend holding that node's newest report.
 type freshness map[string]string
 
-// buildFreshness computes, per certname, which backend has the newer
-// report_timestamp. results must be ordered by precedence; on a tie the
-// earlier backend wins.
+// Ties keep the earlier backend — a deterministic tie-break, not a preference.
 func buildFreshness(results []backendResult) freshness {
 	type pick struct {
 		backend string
@@ -114,19 +104,9 @@ func buildFreshness(results []backendResult) freshness {
 	return f
 }
 
-// mergeFacts merges /facts records at node granularity: for each certname, all
-// facts from the winning backend are kept and the other backend's facts for
-// that certname are dropped.
-//
-// The winner is chosen per certname by `owner(certname)`. Callers supply owner
-// from either a freshness map (freshness merge) or a constant preferred backend
-// (static merge). When owner returns a backend that has no facts for a certname
-// (or a name not in results), records fall back to precedence order so a node
-// present in only one backend still appears.
+// owner names the winning backend per certname; a nil owner (static merge), or one holding no facts for that certname, falls back to configured order.
 func mergeFacts(results []backendResult, owner func(certname string) string) []json.RawMessage {
-	// Which backends actually returned facts for each certname, in precedence
-	// order, so we can fall back if the chosen owner has none.
-	present := map[string][]string{} // certname -> ordered backend names
+	present := map[string][]string{} // certname -> backend names, in configured order
 	byKey := map[string][]json.RawMessage{}
 	for _, res := range results {
 		for _, rec := range res.records {
@@ -153,8 +133,10 @@ func mergeFacts(results []backendResult, owner func(certname string) string) []j
 	out := []json.RawMessage{}
 	for _, cn := range order {
 		backends := present[cn]
-		chosen := owner(cn)
-		// Fall back to precedence order if the chosen backend has no facts here.
+		chosen := ""
+		if owner != nil {
+			chosen = owner(cn)
+		}
 		if !contains(backends, chosen) {
 			chosen = backends[0]
 		}

merge_test.go

@@ -74,11 +74,25 @@ func fact(cn, name, val, ts string) string {
 	return `{"certname":"` + cn + `","name":"` + name + `","value":"` + val + `","report_timestamp":"` + ts + `"}`
 }
 
-func TestMergeNodes_NewerWins(t *testing.T) {
-	old := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-10T00:00:00Z"))
-	nw := recs(t, "new", node("h1", "2026-07-20T00:00:00Z"), node("h3", "2026-07-05T00:00:00Z"))
+// report builds a /reports record with the fields the merge and ordering paths
+// care about.
+func report(cn, hash, receive string) string {
+	return `{"certname":"` + cn + `","hash":"` + hash + `","receive_time":"` + receive +
+		`","end_time":"` + receive + `","status":"changed","environment":"production"}`
+}
 
-	merged := mergeNodes([]backendResult{old, nw})
+// event builds an /events record, which carries its report's hash but no id of
+// its own.
+func event(cn, reportHash, resource string) string {
+	return `{"certname":"` + cn + `","report":"` + reportHash + `","resource_title":"` + resource +
+		`","status":"success","timestamp":"2026-07-01T00:00:00Z"}`
+}
+
+func TestMergeNodes_NewerWins(t *testing.T) {
+	a := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-10T00:00:00Z"))
+	b := recs(t, "b", node("h1", "2026-07-20T00:00:00Z"), node("h3", "2026-07-05T00:00:00Z"))
+
+	merged := mergeNodes([]backendResult{a, b})
 	got := map[string]string{}
 	for _, r := range merged {
 		var m recordMeta
@@ -86,13 +100,13 @@ func TestMergeNodes_NewerWins(t *testing.T) {
 		got[m.Certname] = m.ReportTimestamp
 	}
 	if got["h1"] != "2026-07-20T00:00:00Z" {
-		t.Errorf("h1: newer (new) should win, got %s", got["h1"])
+		t.Errorf("h1: newer (b) should win, got %s", got["h1"])
 	}
 	if got["h2"] != "2026-07-10T00:00:00Z" {
-		t.Errorf("h2: only in old, got %s", got["h2"])
+		t.Errorf("h2: only in a, got %s", got["h2"])
 	}
 	if got["h3"] != "2026-07-05T00:00:00Z" {
-		t.Errorf("h3: only in new, got %s", got["h3"])
+		t.Errorf("h3: only in b, got %s", got["h3"])
 	}
 	if len(merged) != 3 {
 		t.Errorf("expected 3 deduped nodes, got %d", len(merged))
@@ -100,20 +114,20 @@ func TestMergeNodes_NewerWins(t *testing.T) {
 }
 
 func TestMergeNodes_OneBackendOnly(t *testing.T) {
-	old := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"))
-	// new returned nothing (e.g. empty result).
-	nw := backendResult{name: "new"}
-	merged := mergeNodes([]backendResult{old, nw})
+	a := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"))
+	// b returned nothing (e.g. empty result).
+	b := backendResult{name: "b"}
+	merged := mergeNodes([]backendResult{a, b})
 	if len(merged) != 1 || certnames(t, merged)[0] != "h1" {
 		t.Fatalf("expected only h1, got %v", certnames(t, merged))
 	}
 }
 
 func TestMergeNodes_TieKeepsEarlierBackend(t *testing.T) {
-	// Equal timestamps: the backend listed first (precedence) wins.
-	prefer := recs(t, "new", node("h1", "2026-07-01T00:00:00Z"))
-	other := recs(t, "old", node("h1", "2026-07-01T00:00:00Z"))
-	merged := mergeNodes([]backendResult{prefer, other})
+	// Equal timestamps: the backend listed first wins, as a tie-break.
+	first := recs(t, "b", node("h1", "2026-07-01T00:00:00Z"))
+	second := recs(t, "a", node("h1", "2026-07-01T00:00:00Z"))
+	merged := mergeNodes([]backendResult{first, second})
 	if len(merged) != 1 {
 		t.Fatalf("expected 1 record, got %d", len(merged))
 	}
@@ -124,8 +138,8 @@ func TestMergeNodes_TieKeepsEarlierBackend(t *testing.T) {
 }
 
 func TestMergeNodes_PreservesUnknownFields(t *testing.T) {
-	old := recs(t, "old", `{"certname":"h1","report_timestamp":"2026-07-01T00:00:00Z","extra":{"deep":42}}`)
-	merged := mergeNodes([]backendResult{old})
+	a := recs(t, "a", `{"certname":"h1","report_timestamp":"2026-07-01T00:00:00Z","extra":{"deep":42}}`)
+	merged := mergeNodes([]backendResult{a})
 	if len(merged) != 1 {
 		t.Fatalf("expected 1 record")
 	}
@@ -136,69 +150,70 @@ func TestMergeNodes_PreservesUnknownFields(t *testing.T) {
 	}
 }
 
-func TestMergeFacts_Static_PreferWins(t *testing.T) {
-	// h1 in both; static prefer=new -> new's facts kept, old's dropped.
-	old := recs(t, "old", fact("h1", "role", "web-old", ""), fact("h2", "role", "db-old", ""))
-	nw := recs(t, "new", fact("h1", "role", "web-new", ""))
+func TestMergeFacts_NilOwnerUsesConfiguredOrder(t *testing.T) {
+	// Static merge passes no owner: h1 is in both, so the first backend in the
+	// slice supplies its facts.
+	a := recs(t, "a", fact("h1", "role", "web-a", ""), fact("h2", "role", "db-a", ""))
+	b := recs(t, "b", fact("h1", "role", "web-b", ""))
 
-	merged := mergeFacts([]backendResult{nw, old}, func(string) string { return "new" })
+	merged := mergeFacts([]backendResult{b, a}, nil)
 	got := factValues(t, merged)
-	assertContains(t, got, "h1:role=web-new")
-	assertNotContains(t, got, "h1:role=web-old")
-	// h2 only in old -> falls back to old.
-	assertContains(t, got, "h2:role=db-old")
+	assertContains(t, got, "h1:role=web-b")
+	assertNotContains(t, got, "h1:role=web-a")
+	// h2 only in a -> still served from a.
+	assertContains(t, got, "h2:role=db-a")
 }
 
 func TestMergeFacts_Freshness_NewerBackendWins(t *testing.T) {
-	// owner map says h1 belongs to old (older backend has the newer report),
-	// h2 belongs to new. Multiple facts per node must all come from the winner.
-	old := recs(t, "old",
-		fact("h1", "role", "web-old", ""), fact("h1", "ip", "10.0.0.1", ""),
-		fact("h2", "role", "db-old", ""))
-	nw := recs(t, "new",
-		fact("h1", "role", "web-new", ""), fact("h1", "ip", "10.9.9.9", ""),
-		fact("h2", "role", "db-new", ""), fact("h2", "ip", "10.0.0.2", ""))
+	// owner map says h1 belongs to a and h2 to b. Multiple facts per node must
+	// all come from the winner.
+	a := recs(t, "a",
+		fact("h1", "role", "web-a", ""), fact("h1", "ip", "10.0.0.1", ""),
+		fact("h2", "role", "db-a", ""))
+	b := recs(t, "b",
+		fact("h1", "role", "web-b", ""), fact("h1", "ip", "10.9.9.9", ""),
+		fact("h2", "role", "db-b", ""), fact("h2", "ip", "10.0.0.2", ""))
 
 	owner := func(cn string) string {
 		if cn == "h1" {
-			return "old"
+			return "a"
 		}
-		return "new"
+		return "b"
 	}
-	merged := mergeFacts([]backendResult{nw, old}, owner)
+	merged := mergeFacts([]backendResult{b, a}, owner)
 	got := factValues(t, merged)
-	// h1 -> all old facts, no new facts.
-	assertContains(t, got, "h1:role=web-old")
+	// h1 -> all a facts, no b facts.
+	assertContains(t, got, "h1:role=web-a")
 	assertContains(t, got, "h1:ip=10.0.0.1")
-	assertNotContains(t, got, "h1:role=web-new")
+	assertNotContains(t, got, "h1:role=web-b")
 	assertNotContains(t, got, "h1:ip=10.9.9.9")
-	// h2 -> all new facts.
-	assertContains(t, got, "h2:role=db-new")
+	// h2 -> all b facts.
+	assertContains(t, got, "h2:role=db-b")
 	assertContains(t, got, "h2:ip=10.0.0.2")
-	assertNotContains(t, got, "h2:role=db-old")
+	assertNotContains(t, got, "h2:role=db-a")
 }
 
-func TestMergeFacts_OwnerMissingFallsBackToPrecedence(t *testing.T) {
-	// owner returns a backend with no facts for h1 -> fall back to first
-	// backend present (precedence order of the slice).
-	prefer := recs(t, "new", fact("h1", "role", "web-new", ""))
-	other := recs(t, "old", fact("h1", "role", "web-old", ""))
-	merged := mergeFacts([]backendResult{prefer, other}, func(string) string { return "ghost" })
+func TestMergeFacts_OwnerMissingFallsBackToConfiguredOrder(t *testing.T) {
+	// owner returns a backend with no facts for h1 -> fall back to the first
+	// backend in the slice that has some.
+	first := recs(t, "b", fact("h1", "role", "web-b", ""))
+	second := recs(t, "a", fact("h1", "role", "web-a", ""))
+	merged := mergeFacts([]backendResult{first, second}, func(string) string { return "ghost" })
 	got := factValues(t, merged)
-	assertContains(t, got, "h1:role=web-new") // new is first in slice
-	assertNotContains(t, got, "h1:role=web-old")
+	assertContains(t, got, "h1:role=web-b") // b is first in slice
+	assertNotContains(t, got, "h1:role=web-a")
 }
 
 func TestBuildFreshness(t *testing.T) {
-	// old has newer report for h1; new has newer for h2.
-	old := recs(t, "old", node("h1", "2026-07-20T00:00:00Z"), node("h2", "2026-07-01T00:00:00Z"))
-	nw := recs(t, "new", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-20T00:00:00Z"))
-	f := buildFreshness([]backendResult{old, nw})
-	if f["h1"] != "old" {
-		t.Errorf("h1 should belong to old, got %q", f["h1"])
+	// a holds the newer report for h1; b holds the newer one for h2.
+	a := recs(t, "a", node("h1", "2026-07-20T00:00:00Z"), node("h2", "2026-07-01T00:00:00Z"))
+	b := recs(t, "b", node("h1", "2026-07-01T00:00:00Z"), node("h2", "2026-07-20T00:00:00Z"))
+	f := buildFreshness([]backendResult{a, b})
+	if f["h1"] != "a" {
+		t.Errorf("h1 should belong to a, got %q", f["h1"])
 	}
-	if f["h2"] != "new" {
-		t.Errorf("h2 should belong to new, got %q", f["h2"])
+	if f["h2"] != "b" {
+		t.Errorf("h2 should belong to b, got %q", f["h2"])
 	}
 }
 

meta.go

@@ -0,0 +1,195 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+const (
+	metaVersionPath    = "/pdb/meta/v1/version"
+	metaServerTimePath = "/pdb/meta/v1/server-time"
+)
+
+// rawResult is one backend's verbatim response, for endpoints whose payload is
+// not a PuppetDB record array.
+type rawResult struct {
+	name        string
+	status      int
+	contentType string
+	body        []byte
+	err         error
+}
+
+// ok reports whether the backend answered 2xx.
+func (r rawResult) ok() bool {
+	return r.err == nil && r.status >= 200 && r.status < 300
+}
+
+// fanOutRaw asks every backend for path concurrently and returns one result per
+// backend, in configured order, without interpreting the bodies.
+func (s *Server) fanOutRaw(ctx context.Context, path, rawQuery string) []rawResult {
+	results := make([]rawResult, len(s.cfg.Backends))
+	var wg sync.WaitGroup
+	for i, b := range s.cfg.Backends {
+		wg.Add(1)
+		go func(i int, b Backend) {
+			defer wg.Done()
+			results[i] = s.rawBackend(ctx, b, path, rawQuery)
+		}(i, b)
+	}
+	wg.Wait()
+	return results
+}
+
+func (s *Server) rawBackend(ctx context.Context, b Backend, path, rawQuery string) rawResult {
+	target := strings.TrimRight(b.URL, "/") + path
+	if rawQuery != "" {
+		target += "?" + rawQuery
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
+	if err != nil {
+		return rawResult{name: b.Name, err: err}
+	}
+	resp, err := s.client.Do(req)
+	if err != nil {
+		return rawResult{name: b.Name, err: err}
+	}
+	defer func() { _ = resp.Body.Close() }()
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return rawResult{name: b.Name, err: err}
+	}
+	return rawResult{
+		name:        b.Name,
+		status:      resp.StatusCode,
+		contentType: resp.Header.Get("Content-Type"),
+		body:        body,
+	}
+}
+
+// aliveRaw drops backends that errored or answered non-2xx, writing a 502 and
+// returning ok=false only when none is left.
+func (s *Server) aliveRaw(w http.ResponseWriter, results []rawResult, path string) ([]rawResult, bool) {
+	var alive []rawResult
+	for _, res := range results {
+		if !res.ok() {
+			s.log.Printf("warning: backend %q failed for %s: %s", res.name, path, res.reason())
+			continue
+		}
+		alive = append(alive, res)
+	}
+	if len(alive) == 0 {
+		http.Error(w, "all backends failed", http.StatusBadGateway)
+		return nil, false
+	}
+	return alive, true
+}
+
+func (r rawResult) reason() string {
+	if r.err != nil {
+		return r.err.Error()
+	}
+	return "HTTP " + strconv.Itoa(r.status) + ": " + strings.TrimSpace(string(r.body))
+}
+
+// handleMetaVersion serves /pdb/meta/v1/version. Clients (pypuppetdb, and so
+// Puppetboard's startup check) treat the answer as the feature level they may
+// rely on, so the merged answer is the *lowest* version any backend reports:
+// the estate can only be counted on for what its oldest member implements.
+func (s *Server) handleMetaVersion(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "only GET is supported", http.StatusMethodNotAllowed)
+		return
+	}
+	alive, ok := s.aliveRaw(w, s.fanOutRaw(r.Context(), metaVersionPath, r.URL.RawQuery), metaVersionPath)
+	if !ok {
+		return
+	}
+
+	lowest := alive[0]
+	lowestVer, hasVer := metaField(lowest.body, "version")
+	for _, res := range alive[1:] {
+		v, ok := metaField(res.body, "version")
+		if !ok {
+			continue
+		}
+		if !hasVer || compareVersions(v, lowestVer) < 0 {
+			lowest, lowestVer, hasVer = res, v, true
+		}
+	}
+	writeRaw(w, lowest)
+}
+
+// handleMetaServerTime serves /pdb/meta/v1/server-time. The clock of whichever
+// PuppetDB answered is not estate state and does not merge, so the first
+// reachable backend in configured order supplies it — the same tie-break rule
+// used everywhere else.
+func (s *Server) handleMetaServerTime(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "only GET is supported", http.StatusMethodNotAllowed)
+		return
+	}
+	alive, ok := s.aliveRaw(w, s.fanOutRaw(r.Context(), metaServerTimePath, r.URL.RawQuery), metaServerTimePath)
+	if !ok {
+		return
+	}
+	writeRaw(w, alive[0])
+}
+
+// metaField pulls a string field out of a `{"version": "..."}`-shaped body.
+func metaField(body []byte, field string) (string, bool) {
+	var obj map[string]json.RawMessage
+	if json.Unmarshal(body, &obj) != nil {
+		return "", false
+	}
+	var s string
+	if json.Unmarshal(obj[field], &s) != nil || s == "" {
+		return "", false
+	}
+	return s, true
+}
+
+// compareVersions orders dotted version strings segment by segment, comparing
+// numerically where both segments are numbers and lexically otherwise, so
+// "7.12.1" sorts below "8.4.0" and below "7.12.2". A prefix is lower than a
+// longer string sharing it ("7.12" < "7.12.1"), and a pre-release suffix is
+// compared as text within its segment ("8.0.0" < "8.0.0-SNAPSHOT").
+func compareVersions(a, b string) int {
+	as, bs := strings.Split(a, "."), strings.Split(b, ".")
+	for i := 0; i < len(as) && i < len(bs); i++ {
+		an, aok := strconv.Atoi(as[i])
+		bn, bok := strconv.Atoi(bs[i])
+		if aok == nil && bok == nil {
+			if an != bn {
+				return sign(an - bn)
+			}
+			continue
+		}
+		if c := strings.Compare(as[i], bs[i]); c != 0 {
+			return c
+		}
+	}
+	return sign(len(as) - len(bs))
+}
+
+func sign(n int) int {
+	switch {
+	case n < 0:
+		return -1
+	case n > 0:
+		return 1
+	default:
+		return 0
+	}
+}
+
+func writeRaw(w http.ResponseWriter, res rawResult) {
+	setContentType(w, res.contentType)
+	w.WriteHeader(res.status)
+	_, _ = w.Write(res.body)
+}

meta_test.go

@@ -0,0 +1,170 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func metaGet(t *testing.T, h http.Handler, path string) *httptest.ResponseRecorder {
+	t.Helper()
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, path, nil))
+	return rec
+}
+
+func metaString(t *testing.T, body []byte, field string) string {
+	t.Helper()
+	var obj map[string]string
+	if err := json.Unmarshal(body, &obj); err != nil {
+		t.Fatalf("unmarshal %s: %v", body, err)
+	}
+	return obj[field]
+}
+
+func TestMetaVersion_BackendsAgree(t *testing.T) {
+	// Puppetboard's check_db_version() calls this at import and exits 2 on any
+	// non-200, so a 404 here is the difference between a running dashboard and
+	// CrashLoopBackOff.
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[metaVersionPath] = `{"version":"7.12.1"}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaVersionPath] = `{"version":"7.12.1"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), metaVersionPath)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
+	}
+	if got := metaString(t, rec.Body.Bytes(), "version"); got != "7.12.1" {
+		t.Errorf("version = %q, want 7.12.1", got)
+	}
+}
+
+func TestMetaVersion_DisagreementReportsLowest(t *testing.T) {
+	// The estate can only be relied on for what its oldest PuppetDB implements.
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[metaVersionPath] = `{"version":"8.4.0"}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaVersionPath] = `{"version":"7.12.1"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if got := metaString(t, metaGet(t, srv.Handler(), metaVersionPath).Body.Bytes(), "version"); got != "7.12.1" {
+		t.Errorf("version = %q, want the lower 7.12.1", got)
+	}
+}
+
+func TestMetaVersion_LowestIsIndependentOfBackendOrder(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[metaVersionPath] = `{"version":"7.12.1"}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaVersionPath] = `{"version":"8.4.0"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if got := metaString(t, metaGet(t, srv.Handler(), metaVersionPath).Body.Bytes(), "version"); got != "7.12.1" {
+		t.Errorf("version = %q, want the lower 7.12.1", got)
+	}
+}
+
+func TestMetaVersion_OneBackendDown(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaVersionPath] = `{"version":"8.4.0"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), metaVersionPath)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200 serving the survivor, got %d", rec.Code)
+	}
+	if got := metaString(t, rec.Body.Bytes(), "version"); got != "8.4.0" {
+		t.Errorf("version = %q, want 8.4.0", got)
+	}
+}
+
+func TestMetaVersion_AllBackendsDown(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.fail = true
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if rec := metaGet(t, srv.Handler(), metaVersionPath); rec.Code != http.StatusBadGateway {
+		t.Errorf("status %d, want 502", rec.Code)
+	}
+}
+
+func TestMetaServerTime_FirstReachableBackend(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[metaServerTimePath] = `{"server_time":"2026-08-29T01:00:00.000Z"}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaServerTimePath] = `{"server_time":"2026-08-29T02:00:00.000Z"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	got := metaString(t, metaGet(t, srv.Handler(), metaServerTimePath).Body.Bytes(), "server_time")
+	if got != "2026-08-29T01:00:00.000Z" {
+		t.Errorf("server_time = %q, want the first backend's", got)
+	}
+}
+
+func TestMetaServerTime_SkipsDeadBackend(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaServerTimePath] = `{"server_time":"2026-08-29T02:00:00.000Z"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), metaServerTimePath)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200 serving the survivor, got %d", rec.Code)
+	}
+	if got := metaString(t, rec.Body.Bytes(), "server_time"); got != "2026-08-29T02:00:00.000Z" {
+		t.Errorf("server_time = %q, want the survivor's", got)
+	}
+}
+
+func TestMetaVersion_RejectsNonGET(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := httptest.NewRecorder()
+	srv.Handler().ServeHTTP(rec, httptest.NewRequest(http.MethodPost, metaVersionPath, nil))
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Errorf("status %d, want 405", rec.Code)
+	}
+}
+
+func TestCompareVersions(t *testing.T) {
+	cases := []struct {
+		a, b string
+		want int
+	}{
+		{"7.12.1", "7.12.1", 0},
+		{"7.12.1", "8.4.0", -1},
+		{"8.4.0", "7.12.1", 1},
+		{"7.9.0", "7.12.0", -1}, // numeric, not lexical: 9 < 12
+		{"7.12", "7.12.1", -1},
+		{"8.0.0", "8.0.0-SNAPSHOT", -1},
+		{"8.0.0-SNAPSHOT", "8.0.0", 1},
+	}
+	for _, c := range cases {
+		if got := compareVersions(c.a, c.b); got != c.want {
+			t.Errorf("compareVersions(%q, %q) = %d, want %d", c.a, c.b, got, c.want)
+		}
+	}
+}
+
+func TestMetaField_MalformedBodyIgnored(t *testing.T) {
+	// A backend serving junk must not become the "lowest" version.
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[metaVersionPath] = `not json`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[metaVersionPath] = `{"version":"8.4.0"}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if got := metaString(t, metaGet(t, srv.Handler(), metaVersionPath).Body.Bytes(), "version"); got != "8.4.0" {
+		t.Errorf("version = %q, want 8.4.0 from the only parseable backend", got)
+	}
+}

metrics.go

@@ -0,0 +1,272 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// metricsPrefix covers PuppetDB's Jolokia surface, which sits at the server root
+// rather than under /pdb: pypuppetdb's metric() reads /metrics/v2/read/<mbean>,
+// lists via /metrics/v2/list, and falls back to /metrics/v1/mbeans/<mbean>.
+const metricsPrefix = "/metrics/"
+
+type mergeRule int
+
+const (
+	ruleSum mergeRule = iota
+	ruleMin
+	ruleMax
+	ruleMean
+)
+
+// mergeRuleFor picks how one numeric MBean attribute combines across backends.
+// The default is a sum: the metrics Puppetboard renders are population counts
+// (num-nodes, num-resources, queue depth, command totals) whose estate-wide
+// value is the total. The exceptions are attributes describing a distribution or
+// a bound rather than a quantity, where adding two servers' numbers yields a
+// figure that was never true of either.
+func mergeRuleFor(attr string) mergeRule {
+	a := strings.ToLower(attr)
+	switch a {
+	case "min":
+		return ruleMin
+	case "max", "uptime", "starttime":
+		return ruleMax
+	case "mean", "median", "stddev":
+		return ruleMean
+	}
+	if strings.HasSuffix(a, "percentile") {
+		return ruleMean
+	}
+	return ruleSum
+}
+
+func (s *Server) handleMetrics(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "only GET is supported", http.StatusMethodNotAllowed)
+		return
+	}
+	// MBean names carry Jolokia's !-escapes under percent-encoding; the raw path
+	// is forwarded so neither layer is lost.
+	path := r.URL.EscapedPath()
+	alive, ok := s.aliveRaw(w, s.fanOutRaw(r.Context(), path, r.URL.RawQuery), path)
+	if !ok {
+		return
+	}
+
+	body, ok := mergeMetrics(alive, metricAttribute(r.URL.Path))
+	if !ok {
+		// Every backend answered 2xx but none carried a mergeable payload — a
+		// Jolokia error envelope, or a body that is not a JSON object. Replaying
+		// the first keeps the upstream error text the client expects.
+		writeRaw(w, alive[0])
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	_, _ = w.Write(body)
+}
+
+// metricAttribute names the single attribute a read asked for, when the URL
+// carries one (/metrics/v2/read/<mbean>/<attribute>), so a scalar response body
+// still gets the right numeric rule. Empty means the response is an object whose
+// own keys name its attributes.
+func metricAttribute(path string) string {
+	rest, ok := strings.CutPrefix(path, metricsPrefix)
+	if !ok {
+		return ""
+	}
+	parts := strings.Split(rest, "/") // v2/read/<mbean>[/<attribute>]
+	if len(parts) < 4 {
+		return ""
+	}
+	return parts[len(parts)-1]
+}
+
+// mergeMetrics folds the backends' Jolokia responses into one. A response is
+// either a Jolokia envelope ({"request":…,"value":…,"status":200}), where only
+// "value" merges and the rest comes from the first backend, or a bare attribute
+// object (metrics/v1), which merges whole. ok=false means nothing was mergeable.
+func mergeMetrics(alive []rawResult, attr string) ([]byte, bool) {
+	var objs []map[string]json.RawMessage
+	for _, res := range alive {
+		var obj map[string]json.RawMessage
+		if decodeJSON(res.body, &obj) != nil || obj == nil {
+			continue
+		}
+		// Jolokia reports a per-request failure inside an HTTP 200.
+		if n, ok := numberOf(obj["status"]); ok && (n < 200 || n >= 300) {
+			continue
+		}
+		objs = append(objs, obj)
+	}
+	if len(objs) == 0 {
+		return nil, false
+	}
+
+	_, hasValue := objs[0]["value"]
+	_, hasStatus := objs[0]["status"]
+	if !hasValue || !hasStatus {
+		vals := make([]any, 0, len(objs))
+		for _, obj := range objs {
+			vals = append(vals, decodedObject(obj))
+		}
+		return marshal(mergeMetricValue(vals, attr))
+	}
+
+	vals := make([]any, 0, len(objs))
+	for _, obj := range objs {
+		var v any
+		if decodeJSON(obj["value"], &v) == nil {
+			vals = append(vals, v)
+		}
+	}
+	if len(vals) == 0 {
+		return nil, false
+	}
+	merged, ok := marshal(mergeMetricValue(vals, attr))
+	if !ok {
+		return nil, false
+	}
+
+	out := make(map[string]json.RawMessage, len(objs[0]))
+	for k, v := range objs[0] {
+		out[k] = v
+	}
+	out["value"] = merged
+	// The envelope timestamp says when the answer was produced; the newest one
+	// describes the merged answer.
+	if ts, ok := maxField(objs, "timestamp"); ok {
+		out["timestamp"] = ts
+	}
+	return marshal(out)
+}
+
+// maxField returns the largest numeric value of a field across the responses.
+func maxField(objs []map[string]json.RawMessage, field string) (json.RawMessage, bool) {
+	var best json.RawMessage
+	var bestN float64
+	for _, obj := range objs {
+		n, ok := numberOf(obj[field])
+		if !ok {
+			continue
+		}
+		if best == nil || n > bestN {
+			best, bestN = obj[field], n
+		}
+	}
+	return best, best != nil
+}
+
+// mergeMetricValue folds one attribute's value from every backend into one.
+// Objects merge key by key over the union of keys, so a backend missing an
+// attribute still contributes the rest. Numbers combine by the attribute's rule.
+// Anything else — strings, booleans, arrays, nulls, or a mix of kinds — keeps
+// the first backend's value, there being no sound way to add them.
+func mergeMetricValue(vals []any, attr string) any {
+	if len(vals) == 0 {
+		return nil
+	}
+	if len(vals) == 1 {
+		return vals[0]
+	}
+
+	objs := make([]map[string]any, 0, len(vals))
+	for _, v := range vals {
+		if m, ok := v.(map[string]any); ok {
+			objs = append(objs, m)
+		}
+	}
+	if len(objs) == len(vals) {
+		out := map[string]any{}
+		for _, m := range objs {
+			for k := range m {
+				if _, done := out[k]; done {
+					continue
+				}
+				sub := make([]any, 0, len(objs))
+				for _, o := range objs {
+					if v, ok := o[k]; ok {
+						sub = append(sub, v)
+					}
+				}
+				out[k] = mergeMetricValue(sub, k)
+			}
+		}
+		return out
+	}
+
+	nums := make([]float64, 0, len(vals))
+	for _, v := range vals {
+		n, ok := v.(json.Number)
+		if !ok {
+			break
+		}
+		f, err := n.Float64()
+		if err != nil {
+			break
+		}
+		nums = append(nums, f)
+	}
+	if len(nums) != len(vals) {
+		return vals[0]
+	}
+	return combineNumbers(nums, attr)
+}
+
+func combineNumbers(nums []float64, attr string) json.RawMessage {
+	acc := nums[0]
+	switch mergeRuleFor(attr) {
+	case ruleMin:
+		for _, n := range nums[1:] {
+			if n < acc {
+				acc = n
+			}
+		}
+	case ruleMax:
+		for _, n := range nums[1:] {
+			if n > acc {
+				acc = n
+			}
+		}
+	case ruleMean:
+		for _, n := range nums[1:] {
+			acc += n
+		}
+		acc /= float64(len(nums))
+	default:
+		for _, n := range nums[1:] {
+			acc += n
+		}
+	}
+	return json.RawMessage(strconv.FormatFloat(acc, 'f', -1, 64))
+}
+
+// decodedObject re-reads an object's fields as generic values so the whole thing
+// can go through mergeMetricValue.
+func decodedObject(obj map[string]json.RawMessage) any {
+	out := make(map[string]any, len(obj))
+	for k, raw := range obj {
+		var v any
+		if decodeJSON(raw, &v) == nil {
+			out[k] = v
+		}
+	}
+	return out
+}
+
+// decodeJSON keeps integers exact by decoding numbers as json.Number.
+func decodeJSON(data []byte, v any) error {
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.UseNumber()
+	return dec.Decode(v)
+}
+
+// marshal reports ok=false rather than an error: an unmarshalable merge result
+// has only one recovery, replaying a backend's body verbatim.
+func marshal(v any) ([]byte, bool) {
+	b, err := json.Marshal(v)
+	return b, err == nil
+}

metrics_test.go

@@ -0,0 +1,285 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
+	"strconv"
+	"testing"
+)
+
+const (
+	numNodesMBean = "puppetlabs.puppetdb.population:name=num-nodes"
+	numNodesPath  = metricsPrefix + "v2/read/" + numNodesMBean
+	// What pypuppetdb actually sends: quote() percent-encodes ':' and '='.
+	numNodesEscaped = metricsPrefix + "v2/read/puppetlabs.puppetdb.population%3Aname%3Dnum-nodes"
+)
+
+// jolokiaRead wraps an MBean value in the envelope PuppetDB's Jolokia returns.
+func jolokiaRead(mbean, value string, timestamp int) string {
+	return `{"request":{"mbean":"` + mbean + `","type":"read"},` +
+		`"value":` + value + `,"timestamp":` + strconv.Itoa(timestamp) + `,"status":200}`
+}
+
+func metricValue(t *testing.T, body []byte) map[string]any {
+	t.Helper()
+	var env map[string]json.RawMessage
+	if err := json.Unmarshal(body, &env); err != nil {
+		t.Fatalf("unmarshal envelope %s: %v", body, err)
+	}
+	var val map[string]any
+	if err := json.Unmarshal(env["value"], &val); err != nil {
+		t.Fatalf("unmarshal value %s: %v", env["value"], err)
+	}
+	return val
+}
+
+func TestMetrics_ReadSumsPopulationCounts(t *testing.T) {
+	// Puppetboard's landing page and radiator read num-nodes when
+	// DEFAULT_ENVIRONMENT is '*'; each backend only knows its own nodes.
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":90}`, 1000)
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":53}`, 2000)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), numNodesEscaped)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status %d: %s", rec.Code, rec.Body.String())
+	}
+	if got := metricValue(t, rec.Body.Bytes())["Value"]; got != float64(143) {
+		t.Errorf("Value = %v, want 143", got)
+	}
+}
+
+func TestMetrics_EscapedMBeanNameSurvives(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":1}`, 1)
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":1}`, 1)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	metaGet(t, srv.Handler(), numNodesEscaped)
+	if !a.sawRawPath(numNodesEscaped) {
+		t.Errorf("backend saw %v, want the percent-encoded path %q", a.rawPaths, numNodesEscaped)
+	}
+}
+
+func TestMetrics_EnvelopeKeepsNewestTimestamp(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":1}`, 1000)
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":1}`, 2000)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	var env map[string]any
+	if err := json.Unmarshal(metaGet(t, srv.Handler(), numNodesEscaped).Body.Bytes(), &env); err != nil {
+		t.Fatal(err)
+	}
+	if env["timestamp"] != float64(2000) {
+		t.Errorf("timestamp = %v, want 2000", env["timestamp"])
+	}
+	if env["status"] != float64(200) {
+		t.Errorf("status = %v, want 200", env["status"])
+	}
+}
+
+func TestMetrics_PerAttributeRules(t *testing.T) {
+	const mbean = "puppetlabs.puppetdb.mq:name=global.processing-time"
+	path := metricsPrefix + "v2/read/" + mbean
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = jolokiaRead(mbean,
+		`{"Count":10,"Min":2,"Max":9,"Mean":4,"StdDev":1,"50thPercentile":3,"MeanRate":1.5}`, 1)
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[path] = jolokiaRead(mbean,
+		`{"Count":6,"Min":1,"Max":20,"Mean":6,"StdDev":3,"50thPercentile":5,"MeanRate":0.5}`, 1)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	got := metricValue(t, metaGet(t, srv.Handler(), path).Body.Bytes())
+	want := map[string]any{
+		"Count":          float64(16), // counts add
+		"Min":            float64(1),  // a bound stays a bound
+		"Max":            float64(20),
+		"Mean":           float64(5), // distribution stats average
+		"StdDev":         float64(2),
+		"50thPercentile": float64(4),
+		"MeanRate":       float64(2), // throughput adds
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("merged value = %v, want %v", got, want)
+	}
+}
+
+func TestMetrics_ScalarReadUsesURLAttributeName(t *testing.T) {
+	// /metrics/v2/read/<mbean>/<attribute> answers with a bare number, so the
+	// rule has to come from the URL rather than an object key.
+	const mbean = "puppetlabs.puppetdb.population:name=num-resources"
+	sumPath := metricsPrefix + "v2/read/" + mbean + "/Value"
+	maxPath := metricsPrefix + "v2/read/" + mbean + "/Max"
+
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[sumPath] = jolokiaRead(mbean, `1000`, 1)
+	a.bodies[maxPath] = jolokiaRead(mbean, `1000`, 1)
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[sumPath] = jolokiaRead(mbean, `234`, 1)
+	b.bodies[maxPath] = jolokiaRead(mbean, `234`, 1)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	for _, c := range []struct {
+		path string
+		want float64
+	}{{sumPath, 1234}, {maxPath, 1000}} {
+		var env map[string]any
+		if err := json.Unmarshal(metaGet(t, srv.Handler(), c.path).Body.Bytes(), &env); err != nil {
+			t.Fatal(err)
+		}
+		if env["value"] != c.want {
+			t.Errorf("%s value = %v, want %v", c.path, env["value"], c.want)
+		}
+	}
+}
+
+func TestMetrics_ListUnionsDomains(t *testing.T) {
+	// Puppetboard's /metrics page calls metric() with no name, which is a
+	// Jolokia list; a backend-local MBean must not vanish from the browse tree.
+	const path = metricsPrefix + "v2/list"
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = `{"value":{"java.lang":{"type=Memory":{"attr":{"HeapMemoryUsage":{"rw":false}}}}},"status":200,"timestamp":1}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[path] = `{"value":{"puppetlabs.puppetdb.population":{"name=num-nodes":{"attr":{"Value":{"rw":false}}}}},"status":200,"timestamp":1}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	got := metricValue(t, metaGet(t, srv.Handler(), path).Body.Bytes())
+	if _, ok := got["java.lang"]; !ok {
+		t.Errorf("java.lang missing from merged list: %v", got)
+	}
+	if _, ok := got["puppetlabs.puppetdb.population"]; !ok {
+		t.Errorf("puppetlabs.puppetdb.population missing from merged list: %v", got)
+	}
+}
+
+func TestMetrics_V1BareObjectMerged(t *testing.T) {
+	// metrics/v1/mbeans has no Jolokia envelope; the whole body is the value.
+	const path = metricsPrefix + "v1/mbeans/" + numNodesMBean
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = `{"Value":90}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[path] = `{"Value":53}`
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	var got map[string]any
+	if err := json.Unmarshal(metaGet(t, srv.Handler(), path).Body.Bytes(), &got); err != nil {
+		t.Fatal(err)
+	}
+	if got["Value"] != float64(143) {
+		t.Errorf("Value = %v, want 143", got["Value"])
+	}
+}
+
+func TestMetrics_MissingMBeanKeepsUpstreamError(t *testing.T) {
+	// Jolokia reports a bad MBean as a 200 with an error envelope, which
+	// pypuppetdb turns into DoesNotComputeError; the client must still see it.
+	const path = metricsPrefix + "v2/read/nope:name=nothing"
+	errEnv := `{"request":{"mbean":"nope:name=nothing"},"error_type":"javax.management.InstanceNotFoundException","error":"nope:name=nothing is not registered","status":404}`
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = errEnv
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[path] = errEnv
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), path)
+	var env map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &env); err != nil {
+		t.Fatal(err)
+	}
+	if env["error"] == nil {
+		t.Errorf("expected the upstream Jolokia error to be replayed, got %s", rec.Body.String())
+	}
+}
+
+func TestMetrics_ErroringBackendIgnoredWhenAnotherAnswers(t *testing.T) {
+	const path = metricsPrefix + "v2/read/" + numNodesMBean
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.bodies[path] = `{"request":{},"error":"boom","status":500}`
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[path] = jolokiaRead(numNodesMBean, `{"Value":53}`, 1)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if got := metricValue(t, metaGet(t, srv.Handler(), path).Body.Bytes())["Value"]; got != float64(53) {
+		t.Errorf("Value = %v, want 53 from the backend that answered", got)
+	}
+}
+
+func TestMetrics_OneBackendDown(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.bodies[numNodesPath] = jolokiaRead(numNodesMBean, `{"Value":53}`, 1)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := metaGet(t, srv.Handler(), numNodesEscaped)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("expected 200 serving the survivor, got %d", rec.Code)
+	}
+	if got := metricValue(t, rec.Body.Bytes())["Value"]; got != float64(53) {
+		t.Errorf("Value = %v, want 53", got)
+	}
+}
+
+func TestMetrics_AllBackendsDown(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	a.fail = true
+	b := newFakeBackend(t, `[]`, `[]`)
+	b.fail = true
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	if rec := metaGet(t, srv.Handler(), numNodesEscaped); rec.Code != http.StatusBadGateway {
+		t.Errorf("status %d, want 502", rec.Code)
+	}
+}
+
+func TestMetrics_RejectsNonGET(t *testing.T) {
+	a := newFakeBackend(t, `[]`, `[]`)
+	b := newFakeBackend(t, `[]`, `[]`)
+	srv := newTestServer(testConfig(a.srv.URL, b.srv.URL, mergeStatic))
+
+	rec := httptest.NewRecorder()
+	srv.Handler().ServeHTTP(rec, httptest.NewRequest(http.MethodPost, numNodesEscaped, nil))
+	if rec.Code != http.StatusMethodNotAllowed {
+		t.Errorf("status %d, want 405", rec.Code)
+	}
+}
+
+func TestMergeMetricValue_NonNumericKeepsFirst(t *testing.T) {
+	a := map[string]any{"Name": "pdb-a", "Enabled": true}
+	b := map[string]any{"Name": "pdb-b", "Enabled": false}
+	got, ok := mergeMetricValue([]any{a, b}, "").(map[string]any)
+	if !ok {
+		t.Fatalf("expected an object, got %T", got)
+	}
+	if got["Name"] != "pdb-a" || got["Enabled"] != true {
+		t.Errorf("merged = %v, want the first backend's strings and booleans", got)
+	}
+}
+
+func TestMergeRuleFor(t *testing.T) {
+	cases := map[string]mergeRule{
+		"Count":          ruleSum,
+		"Value":          ruleSum,
+		"MeanRate":       ruleSum,
+		"queue-depth":    ruleSum,
+		"min":            ruleMin,
+		"Max":            ruleMax,
+		"Uptime":         ruleMax,
+		"StartTime":      ruleMax,
+		"Mean":           ruleMean,
+		"StdDev":         ruleMean,
+		"99thPercentile": ruleMean,
+	}
+	for attr, want := range cases {
+		if got := mergeRuleFor(attr); got != want {
+			t.Errorf("mergeRuleFor(%q) = %v, want %v", attr, got, want)
+		}
+	}
+}

reports.go

@@ -0,0 +1,222 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/url"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// The first backend in results holding a key supplies the record; a key func returning ok=false means the record has no identity and is always kept.
+func mergeUnion(results []backendResult, key func(record) (string, bool)) []json.RawMessage {
+	seen := make(map[string]bool)
+	out := []json.RawMessage{}
+	for _, res := range results {
+		for _, rec := range res.records {
+			if k, ok := key(rec); ok {
+				if seen[k] {
+					continue
+				}
+				seen[k] = true
+			}
+			out = append(out, rec.Raw)
+		}
+	}
+	return out
+}
+
+// extract/group_by rows are synthetic and carry no hash, so two backends can legitimately emit identical ones.
+func reportKey(rec record) (string, bool) {
+	if rec.Hash == "" {
+		return "", false
+	}
+	return "hash\x00" + rec.Hash, true
+}
+
+// Events carry no id, but byte-identical events from the same PuppetDB serialiser are the same change.
+func rawKey(rec record) (string, bool) { return "raw\x00" + string(rec.Raw), true }
+
+type orderField struct {
+	Field string
+	Desc  bool
+}
+
+// order_by is a JSON array of {"field": ..., "order": "asc"|"desc"} objects.
+func parseOrderBy(s string) ([]orderField, error) {
+	if strings.TrimSpace(s) == "" {
+		return nil, nil
+	}
+	var raw []struct {
+		Field string `json:"field"`
+		Order string `json:"order"`
+	}
+	if err := json.Unmarshal([]byte(s), &raw); err != nil {
+		return nil, fmt.Errorf("order_by is not a JSON array: %w", err)
+	}
+	out := make([]orderField, 0, len(raw))
+	for _, r := range raw {
+		if r.Field == "" {
+			return nil, fmt.Errorf("order_by entry is missing a field")
+		}
+		out = append(out, orderField{Field: r.Field, Desc: strings.EqualFold(r.Order, "desc")})
+	}
+	return out, nil
+}
+
+// Each backend ordered only its own slice, so the union is re-sorted here; stable, so ties keep the merged set's existing order.
+func sortRecords(recs []json.RawMessage, order []orderField) {
+	if len(order) == 0 || len(recs) < 2 {
+		return
+	}
+	objs := make([]map[string]any, len(recs))
+	for i, raw := range recs {
+		_ = json.Unmarshal(raw, &objs[i]) // non-objects sort as all-missing fields
+	}
+	idx := make([]int, len(recs))
+	for i := range idx {
+		idx[i] = i
+	}
+	sort.SliceStable(idx, func(a, b int) bool {
+		oa, ob := objs[idx[a]], objs[idx[b]]
+		for _, f := range order {
+			c := compareValues(oa[f.Field], ob[f.Field])
+			if c == 0 {
+				continue
+			}
+			if f.Desc {
+				return c > 0
+			}
+			return c < 0
+		}
+		return false
+	})
+	sorted := make([]json.RawMessage, len(recs))
+	for i, j := range idx {
+		sorted[i] = recs[j]
+	}
+	copy(recs, sorted)
+}
+
+// Unlike types order by kind (null < bool < number < string), so a missing field sorts first.
+func compareValues(a, b any) int {
+	ra, rb := valueRank(a), valueRank(b)
+	if ra != rb {
+		if ra < rb {
+			return -1
+		}
+		return 1
+	}
+	switch av := a.(type) {
+	case bool:
+		bv := b.(bool)
+		switch {
+		case av == bv:
+			return 0
+		case bv:
+			return -1
+		default:
+			return 1
+		}
+	case float64:
+		bv := b.(float64)
+		switch {
+		case av < bv:
+			return -1
+		case av > bv:
+			return 1
+		default:
+			return 0
+		}
+	case string:
+		return strings.Compare(av, b.(string))
+	}
+	return 0
+}
+
+func valueRank(v any) int {
+	switch v.(type) {
+	case nil:
+		return 0
+	case bool:
+		return 1
+	case float64:
+		return 2
+	case string:
+		return 3
+	default:
+		return 4
+	}
+}
+
+type paging struct {
+	limit     int // -1 when unset
+	offset    int
+	order     []orderField
+	wantTotal bool
+}
+
+func parsePaging(v url.Values) (paging, error) {
+	p := paging{limit: -1}
+	if s := v.Get("limit"); s != "" {
+		n, err := strconv.Atoi(s)
+		if err != nil || n < 0 {
+			return p, fmt.Errorf("limit must be a non-negative integer, got %q", s)
+		}
+		p.limit = n
+	}
+	if s := v.Get("offset"); s != "" {
+		n, err := strconv.Atoi(s)
+		if err != nil || n < 0 {
+			return p, fmt.Errorf("offset must be a non-negative integer, got %q", s)
+		}
+		p.offset = n
+	}
+	order, err := parseOrderBy(v.Get("order_by"))
+	if err != nil {
+		return p, err
+	}
+	p.order = order
+	p.wantTotal = v.Get("include_total") == "true"
+	return p, nil
+}
+
+// Backends are asked for the first offset+limit records with no offset; the offset is applied to the union instead.
+func (p paging) upstreamParams(in url.Values) url.Values {
+	out := url.Values{}
+	for k, vs := range in {
+		out[k] = append([]string(nil), vs...)
+	}
+	out.Del("offset")
+	if p.limit >= 0 {
+		out.Set("limit", strconv.Itoa(p.limit+p.offset))
+	}
+	return out
+}
+
+func (p paging) apply(recs []json.RawMessage) []json.RawMessage {
+	if p.offset >= len(recs) {
+		return []json.RawMessage{}
+	}
+	recs = recs[p.offset:]
+	if p.limit >= 0 && p.limit < len(recs) {
+		recs = recs[:p.limit]
+	}
+	return recs
+}
+
+// Returns -1 when no backend reported a count; duplicates count once per backend, so the sum is an upper bound.
+func sumTotals(results []backendResult) int {
+	total := -1
+	for _, res := range results {
+		if res.total < 0 {
+			continue
+		}
+		if total < 0 {
+			total = 0
+		}
+		total += res.total
+	}
+	return total
+}

reports_test.go

@@ -0,0 +1,254 @@
+package main
+
+import (
+	"encoding/json"
+	"net/url"
+	"slices"
+	"testing"
+)
+
+func TestMergeUnion_KeepsBothBackendsHistory(t *testing.T) {
+	a := recs(t, "a", report("h1", "r1", "2026-07-01T00:00:00Z"))
+	b := recs(t, "b", report("h1", "r2", "2026-07-02T00:00:00Z"))
+	merged := mergeUnion([]backendResult{b, a}, reportKey)
+	if got := hashesOf(t, merged); !slices.Equal(got, []string{"r2", "r1"}) {
+		t.Errorf("union = %v, want both reports in configured order", got)
+	}
+}
+
+func TestMergeUnion_DedupesSharedHash(t *testing.T) {
+	dup := report("h1", "r1", "2026-07-01T00:00:00Z")
+	merged := mergeUnion([]backendResult{recs(t, "b", dup), recs(t, "a", dup)}, reportKey)
+	if got := hashesOf(t, merged); !slices.Equal(got, []string{"r1"}) {
+		t.Errorf("union = %v, want a single r1", got)
+	}
+}
+
+func TestMergeUnion_HashlessRowsAreAllKept(t *testing.T) {
+	// extract/group_by queries return synthetic rows with no hash; dropping the
+	// second backend's rows as "duplicates" would silently lose half the data.
+	a := recs(t, "a", `{"status":"changed","count":3}`)
+	b := recs(t, "b", `{"status":"changed","count":5}`)
+	merged := mergeUnion([]backendResult{a, b}, reportKey)
+	if len(merged) != 2 {
+		t.Errorf("expected both aggregate rows, got %d: %v", len(merged), merged)
+	}
+}
+
+func TestMergeUnion_IdenticalHashlessRowsAreNotCollapsed(t *testing.T) {
+	// Two backends can legitimately produce the same aggregate row; collapsing
+	// them as duplicates undercounts the merged result.
+	same := `{"status":"changed","count":1}`
+	merged := mergeUnion([]backendResult{recs(t, "a", same), recs(t, "b", same)}, reportKey)
+	if len(merged) != 2 {
+		t.Errorf("expected both backends' aggregate rows, got %d: %v", len(merged), merged)
+	}
+}
+
+func TestMergeUnion_EventsDedupeOnRawIdentity(t *testing.T) {
+	same := event("h1", "r1", "Package[nginx]")
+	other := event("h1", "r1", "Service[nginx]")
+	merged := mergeUnion([]backendResult{recs(t, "b", same, other), recs(t, "a", same)}, rawKey)
+	if len(merged) != 2 {
+		t.Errorf("expected 2 distinct events, got %d: %v", len(merged), merged)
+	}
+}
+
+func TestParseOrderBy(t *testing.T) {
+	got, err := parseOrderBy(`[{"field":"receive_time","order":"desc"},{"field":"certname"}]`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []orderField{{Field: "receive_time", Desc: true}, {Field: "certname"}}
+	if !slices.Equal(got, want) {
+		t.Errorf("parseOrderBy = %v, want %v", got, want)
+	}
+
+	if got, err := parseOrderBy("  "); err != nil || got != nil {
+		t.Errorf("empty order_by = %v, %v; want nil, nil", got, err)
+	}
+	for _, bad := range []string{`receive_time`, `[{"order":"desc"}]`} {
+		if _, err := parseOrderBy(bad); err == nil {
+			t.Errorf("parseOrderBy(%q) should have failed", bad)
+		}
+	}
+}
+
+func TestSortRecords_MultipleFieldsAndStability(t *testing.T) {
+	raws := rawsOf(t,
+		`{"certname":"b","status":"failed","hash":"r1"}`,
+		`{"certname":"a","status":"changed","hash":"r2"}`,
+		`{"certname":"a","status":"changed","hash":"r3"}`,
+		`{"certname":"a","status":"failed","hash":"r4"}`,
+	)
+	sortRecords(raws, []orderField{{Field: "certname"}, {Field: "status", Desc: true}})
+	// certname asc, then status desc; r2/r3 tie fully and keep input order.
+	if got := hashesOf(t, raws); !slices.Equal(got, []string{"r4", "r2", "r3", "r1"}) {
+		t.Errorf("sorted = %v, want [r4 r2 r3 r1]", got)
+	}
+}
+
+func TestSortRecords_MissingFieldSortsFirst(t *testing.T) {
+	raws := rawsOf(t,
+		`{"hash":"r1","receive_time":"2026-07-01T00:00:00Z"}`,
+		`{"hash":"r2"}`,
+	)
+	sortRecords(raws, []orderField{{Field: "receive_time"}})
+	if got := hashesOf(t, raws); !slices.Equal(got, []string{"r2", "r1"}) {
+		t.Errorf("sorted = %v, want the record missing the field first", got)
+	}
+}
+
+func TestSortRecords_NoOrderLeavesInputOrder(t *testing.T) {
+	raws := rawsOf(t, `{"hash":"r1"}`, `{"hash":"r2"}`)
+	sortRecords(raws, nil)
+	if got := hashesOf(t, raws); !slices.Equal(got, []string{"r1", "r2"}) {
+		t.Errorf("sorted = %v, want unchanged", got)
+	}
+}
+
+func TestCompareValues_AcrossKinds(t *testing.T) {
+	cases := []struct {
+		a, b any
+		want int
+	}{
+		{nil, false, -1},
+		{false, true, -1},
+		{true, 1.0, -1},
+		{1.0, 2.0, -1},
+		{2.0, 2.0, 0},
+		{2.0, "x", -1},
+		{"a", "b", -1},
+		{"b", "a", 1},
+	}
+	for _, c := range cases {
+		if got := compareValues(c.a, c.b); got != c.want {
+			t.Errorf("compareValues(%v, %v) = %d, want %d", c.a, c.b, got, c.want)
+		}
+		if got := compareValues(c.b, c.a); got != -c.want {
+			t.Errorf("compareValues(%v, %v) = %d, want %d (antisymmetry)", c.b, c.a, got, -c.want)
+		}
+	}
+}
+
+func TestParsePaging(t *testing.T) {
+	p, err := parsePaging(url.Values{
+		"limit":         {"25"},
+		"offset":        {"50"},
+		"include_total": {"true"},
+		"order_by":      {`[{"field":"receive_time","order":"desc"}]`},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.limit != 25 || p.offset != 50 || !p.wantTotal || len(p.order) != 1 {
+		t.Fatalf("parsePaging = %+v", p)
+	}
+
+	if p, err := parsePaging(nil); err != nil || p.limit != -1 || p.offset != 0 || p.wantTotal {
+		t.Errorf("empty params = %+v, %v; want limit=-1 and no paging", p, err)
+	}
+	for _, bad := range []url.Values{{"limit": {"-1"}}, {"limit": {"x"}}, {"offset": {"x"}}} {
+		if _, err := parsePaging(bad); err == nil {
+			t.Errorf("parsePaging(%v) should have failed", bad)
+		}
+	}
+}
+
+func TestPagingUpstreamParams(t *testing.T) {
+	in := url.Values{
+		"query":  {`["=","certname","h1"]`},
+		"limit":  {"25"},
+		"offset": {"50"},
+	}
+	p, err := parsePaging(in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := p.upstreamParams(in)
+	if out.Get("limit") != "75" {
+		t.Errorf("upstream limit = %q, want 75 (offset+limit)", out.Get("limit"))
+	}
+	if out.Has("offset") {
+		t.Errorf("upstream offset = %q, want it dropped", out.Get("offset"))
+	}
+	if out.Get("query") != in.Get("query") {
+		t.Errorf("query should pass through verbatim, got %q", out.Get("query"))
+	}
+	if in.Get("limit") != "25" {
+		t.Errorf("upstreamParams must not mutate the caller's params, limit is now %q", in.Get("limit"))
+	}
+}
+
+func TestPagingUpstreamParams_NoLimitLeavesQueryUnbounded(t *testing.T) {
+	in := url.Values{"offset": {"5"}}
+	p, err := parsePaging(in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := p.upstreamParams(in)
+	if out.Has("limit") || out.Has("offset") {
+		t.Errorf("upstream params = %v, want neither limit nor offset", out)
+	}
+}
+
+func TestPagingApply(t *testing.T) {
+	raws := rawsOf(t, `{"hash":"r1"}`, `{"hash":"r2"}`, `{"hash":"r3"}`)
+	cases := []struct {
+		name string
+		page paging
+		want []string
+	}{
+		{name: "window", page: paging{limit: 1, offset: 1}, want: []string{"r2"}},
+		{name: "limit past end", page: paging{limit: 10}, want: []string{"r1", "r2", "r3"}},
+		{name: "offset past end", page: paging{limit: 2, offset: 9}, want: nil},
+		{name: "unset limit", page: paging{limit: -1, offset: 2}, want: []string{"r3"}},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := hashesOf(t, c.page.apply(raws))
+			if len(got) == 0 && len(c.want) == 0 {
+				return
+			}
+			if !slices.Equal(got, c.want) {
+				t.Errorf("apply = %v, want %v", got, c.want)
+			}
+		})
+	}
+}
+
+func TestSumTotals(t *testing.T) {
+	if got := sumTotals([]backendResult{{total: 40}, {total: 60}}); got != 100 {
+		t.Errorf("sumTotals = %d, want 100", got)
+	}
+	if got := sumTotals([]backendResult{{total: -1}, {total: 7}}); got != 7 {
+		t.Errorf("sumTotals should skip backends without a count, got %d", got)
+	}
+	if got := sumTotals([]backendResult{{total: -1}, {total: -1}}); got != -1 {
+		t.Errorf("sumTotals with no counts = %d, want -1", got)
+	}
+}
+
+// rawsOf builds a raw record slice from literal JSON elements.
+func rawsOf(t *testing.T, elems ...string) []json.RawMessage {
+	t.Helper()
+	out := make([]json.RawMessage, 0, len(elems))
+	for _, e := range elems {
+		out = append(out, json.RawMessage(e))
+	}
+	return out
+}
+
+// hashesOf extracts the hash field from raw records, in order.
+func hashesOf(t *testing.T, raws []json.RawMessage) []string {
+	t.Helper()
+	out := make([]string, 0, len(raws))
+	for _, r := range raws {
+		var m recordMeta
+		if err := json.Unmarshal(r, &m); err != nil {
+			t.Fatalf("unmarshal %s: %v", r, err)
+		}
+		out = append(out, m.Hash)
+	}
+	return out
+}

server.go

@@ -3,63 +3,99 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log"
 	"net/http"
 	"net/url"
-	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
 )
 
 const (
-	factsPath = "/pdb/query/v4/facts"
-	nodesPath = "/pdb/query/v4/nodes"
-	queryV4   = "/pdb/query/v4/"
+	factsPath                = "/pdb/query/v4/facts"
+	nodesPath                = "/pdb/query/v4/nodes"
+	resourcesPath            = "/pdb/query/v4/resources"
+	reportsPath              = "/pdb/query/v4/reports"
+	eventsPath               = "/pdb/query/v4/events"
+	eventCountsPath          = "/pdb/query/v4/event-counts"
+	aggregateEventCountsPath = "/pdb/query/v4/aggregate-event-counts"
+	queryV4                  = "/pdb/query/v4/"
+
+	// PuppetDB only sends this when the request carries include_total=true.
+	recordsHeader = "X-Records"
 )
 
-// backendResult is one backend's decoded response for a query. err is non-nil
-// when the backend failed (network/timeout/non-2xx); such results carry no
-// records and are excluded from the merge but logged.
 type backendResult struct {
 	name    string
 	records []record
+	total   int // upstream X-Records count, or -1 when the backend sent none
 	err     error
 }
 
-// Server proxies and merges PuppetDB queries across the configured backends.
+var errAllBackendsFailed = errors.New("all backends failed")
+
 type Server struct {
 	cfg    Config
 	client *http.Client
 	log    *log.Logger
 
+	// factsCache is nil when caching is disabled; cacheFor hands out a noop then.
+	factsCache Cache
+	flights    flightGroup
+	stale      staleTracker
+
 	// freshness cache (freshness merge only).
 	mu        sync.Mutex
 	freshData freshness
 	freshAt   time.Time
 }
 
-// NewServer builds a Server with an HTTP client bounded by cfg.Timeout.
 func NewServer(cfg Config, logger *log.Logger) *Server {
-	return &Server{
+	cfg.clampFactsTTL()
+	s := &Server{
 		cfg:    cfg,
 		client: &http.Client{Timeout: cfg.Timeout},
 		log:    logger,
 	}
+	if cfg.cacheEnabled() {
+		s.factsCache = newMemoryCache(cfg.FactsTTL, cfg.CacheBytes)
+	}
+	return s
+}
+
+// cacheFor picks the cache backing a request. Merged /facts and /nodes record
+// sets share the in-memory cache; every other path is uncached until the reports
+// cache lands, and a new backend is a case here rather than a change to any
+// handler.
+func (s *Server) cacheFor(path string, params url.Values) (Cache, bool) {
+	switch path {
+	case factsPath, nodesPath:
+		// An aggregate row is a summed count, not the merged record set the
+		// cache was built for, so it stays on the live path.
+		if parseAggregate(params.Get("query")) != nil {
+			return noopCache{}, false
+		}
+		if s.factsCache != nil {
+			return s.factsCache, true
+		}
+	}
+	return noopCache{}, false
 }
 
-// Handler returns the HTTP mux for the proxy.
 func (s *Server) Handler() http.Handler {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/healthz", s.handleHealth)
 	mux.HandleFunc("/pdb/query/v4/", s.handleQuery)
+	mux.HandleFunc(metaVersionPath, s.handleMetaVersion)
+	mux.HandleFunc(metaServerTimePath, s.handleMetaServerTime)
+	mux.HandleFunc(metricsPrefix, s.handleMetrics)
 	return mux
 }
 
-// handleQuery dispatches /pdb/query/v4/* requests: /facts and /nodes are merged
-// across backends; every other v4 path is transparently proxied to the primary.
 func (s *Server) handleQuery(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodGet {
 		http.Error(w, "only GET is supported", http.StatusMethodNotAllowed)
@@ -67,20 +103,161 @@ func (s *Server) handleQuery(w http.ResponseWriter, r *http.Request) {
 	}
 	switch r.URL.Path {
 	case nodesPath:
-		s.serveMerged(w, r, nodesPath, s.mergeNodesResponse)
+		s.serveNodes(w, r)
+	case resourcesPath:
+		s.serveResources(w, r)
 	case factsPath:
 		s.serveMerged(w, r, factsPath, s.mergeFactsResponse)
+	case reportsPath:
+		s.serveReports(w, r)
+	case eventsPath:
+		s.serveUnion(w, r, eventsPath, rawKey)
+	case eventCountsPath, aggregateEventCountsPath:
+		s.serveSummed(w, r, r.URL.Path, inferredColumns)
 	default:
-		s.proxyPrimary(w, r)
+		if isReportSubResource(r.URL.Path) {
+			s.serveFirstHolder(w, r)
+			return
+		}
+		s.proxyUnmerged(w, r)
 	}
 }
 
-// serveMerged fans out the request to all backends, then hands the per-backend
-// results to merge to produce the response body. If every backend fails it
-// returns 502; if some fail it serves the survivors and logs a warning.
+// Matches /pdb/query/v4/reports/<hash>/{events,logs,metrics}, whose data lives in exactly one backend.
+func isReportSubResource(path string) bool {
+	rest, ok := strings.CutPrefix(path, reportsPath+"/")
+	if !ok {
+		return false
+	}
+	hash, sub, ok := strings.Cut(rest, "/")
+	if !ok || hash == "" {
+		return false
+	}
+	switch sub {
+	case "events", "logs", "metrics":
+		return true
+	}
+	return false
+}
+
 func (s *Server) serveMerged(w http.ResponseWriter, r *http.Request, path string, merge func([]backendResult) []json.RawMessage) {
-	query := r.URL.Query().Get("query")
-	results := s.fanOut(r.Context(), path, query)
+	params := queryParams(r.URL.Query().Get("query"))
+	s.serveCached(w, r, path, params, func() (cachedResponse, error) {
+		alive, err := s.aliveResults(r.Context(), path, params)
+		if err != nil {
+			return cachedResponse{}, err
+		}
+		return cachedResponse{Body: encodeRecords(merge(alive)), Records: -1}, nil
+	})
+}
+
+// Reports and events are immutable history, so both backends' records belong in the merged view.
+func (s *Server) serveUnion(w http.ResponseWriter, r *http.Request, path string, key func(record) (string, bool)) {
+	in := r.URL.Query()
+	page, err := parsePaging(in)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	// Keyed on the request's own params, not the upstream ones: upstreamParams
+	// folds offset into limit, so different windows would collide.
+	s.serveCached(w, r, path, in, func() (cachedResponse, error) {
+		alive, err := s.aliveResults(r.Context(), path, page.upstreamParams(in))
+		if err != nil {
+			return cachedResponse{}, err
+		}
+		merged := mergeUnion(alive, key)
+		sortRecords(merged, page.order)
+		resp := cachedResponse{Body: encodeRecords(page.apply(merged)), Records: -1}
+		if page.wantTotal {
+			if total := sumTotals(alive); total >= 0 {
+				resp.Records = total
+			}
+		}
+		return resp, nil
+	})
+}
+
+// A count row carries no certname, so the certname-keyed merge would collapse every backend's count into one backend's; aggregates take the summing path instead.
+func (s *Server) serveNodes(w http.ResponseWriter, r *http.Request) {
+	if spec := parseAggregate(r.URL.Query().Get("query")); spec != nil {
+		s.serveSummed(w, r, nodesPath, spec.columns)
+		return
+	}
+	s.serveMerged(w, r, nodesPath, s.mergeNodesResponse)
+}
+
+// Only aggregates merge: a resource record has no cross-backend identity to dedupe on, so a plain query stays on the pass-through path.
+func (s *Server) serveResources(w http.ResponseWriter, r *http.Request) {
+	if spec := parseAggregate(r.URL.Query().Get("query")); spec != nil {
+		s.serveSummed(w, r, resourcesPath, spec.columns)
+		return
+	}
+	s.proxyUnmerged(w, r)
+}
+
+// An `extract` query with a `function` column returns synthetic aggregate rows that carry no identity, so they are summed rather than unioned.
+func (s *Server) serveReports(w http.ResponseWriter, r *http.Request) {
+	if spec := parseAggregate(r.URL.Query().Get("query")); spec != nil {
+		s.serveSummed(w, r, reportsPath, spec.columns)
+		return
+	}
+	s.serveUnion(w, r, reportsPath, reportKey)
+}
+
+// Merged rows are fewer than the backends' combined records, so include_total reports the merged count rather than a sum of X-Records.
+func (s *Server) serveSummed(w http.ResponseWriter, r *http.Request, path string, columns func(map[string]json.RawMessage) ([]string, []string)) {
+	in := r.URL.Query()
+	page, err := parsePaging(in)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	s.serveCached(w, r, path, in, func() (cachedResponse, error) {
+		alive, err := s.aliveResults(r.Context(), path, page.upstreamParams(in))
+		if err != nil {
+			return cachedResponse{}, err
+		}
+		merged := sumRows(alive, columns)
+		sortRecords(merged, page.order)
+		resp := cachedResponse{Body: encodeRecords(page.apply(merged)), Records: -1}
+		if page.wantTotal {
+			resp.Records = len(merged)
+		}
+		return resp, nil
+	})
+}
+
+// A backend without the report answers 404, indistinguishable from a failure, so every backend is consulted before serving empty.
+func (s *Server) serveFirstHolder(w http.ResponseWriter, r *http.Request) {
+	results := s.fanOut(r.Context(), r.URL.Path, r.URL.Query())
+
+	var alive []backendResult
+	for _, res := range results {
+		if res.err != nil {
+			s.log.Printf("info: backend %q has no %s: %v", res.name, r.URL.Path, res.err)
+			continue
+		}
+		alive = append(alive, res)
+	}
+	if len(alive) == 0 {
+		http.Error(w, "no backend holds this report", http.StatusNotFound)
+		return
+	}
+	for _, res := range alive {
+		if len(res.records) > 0 {
+			writeJSON(w, rawRecords(res.records))
+			return
+		}
+	}
+	writeJSON(w, nil)
+}
+
+// Returns errAllBackendsFailed only when every backend failed.
+func (s *Server) aliveResults(ctx context.Context, path string, params url.Values) ([]backendResult, error) {
+	results := s.fanOut(ctx, path, params)
 
 	var alive []backendResult
 	for _, res := range results {
@@ -91,59 +268,144 @@ func (s *Server) serveMerged(w http.ResponseWriter, r *http.Request, path string
 		alive = append(alive, res)
 	}
 	if len(alive) == 0 {
-		http.Error(w, "all backends failed", http.StatusBadGateway)
+		return nil, errAllBackendsFailed
+	}
+	return alive, nil
+}
+
+// cachedResponse is the stored form of a merged response: the JSON body plus the
+// X-Records value it carried, so a cache hit reproduces both.
+type cachedResponse struct {
+	Body    json.RawMessage `json:"body"`
+	Records int             `json:"records"` // -1 when the response sets no X-Records
+}
+
+// serveCached answers from the cache when the entry is fresh, otherwise runs
+// build — single-flighted, so N concurrent identical requests cause one upstream
+// fan-out — and stores the result. A build failure falls back to a stale entry
+// when one exists; that is the only path on which stale data is served. Paths
+// with no cache configured run build directly, unchanged.
+func (s *Server) serveCached(w http.ResponseWriter, r *http.Request, path string, params url.Values, build func() (cachedResponse, error)) {
+	cache, enabled := s.cacheFor(path, params)
+	if !enabled {
+		resp, err := build()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadGateway)
+			return
+		}
+		writeCached(w, resp)
 		return
 	}
 
-	merged := merge(alive)
-	writeJSON(w, merged)
-}
-
-// mergeNodesResponse merges /nodes results (dedupe by certname, newer wins).
-func (s *Server) mergeNodesResponse(results []backendResult) []json.RawMessage {
-	return mergeNodes(s.byPrecedence(results))
-}
-
-// mergeFactsResponse merges /facts results at node granularity, choosing each
-// certname's owner via the configured merge strategy.
-func (s *Server) mergeFactsResponse(results []backendResult) []json.RawMessage {
-	ordered := s.byPrecedence(results)
-	if s.cfg.Merge == mergeStatic {
-		prefer := s.cfg.Prefer
-		return mergeFacts(ordered, func(string) string { return prefer })
+	key := cacheKey(path, params)
+	var stale *CacheEntry
+	ent, status, err := cache.Get(r.Context(), key)
+	switch {
+	case err != nil:
+		s.log.Printf("warning: cache lookup for %s failed: %v", key, err)
+	case status == CacheFresh:
+		s.stale.markFresh()
+		s.writeStored(w, ent.Body)
+		return
+	case status == CacheStale:
+		stale = &ent
 	}
-	// freshness merge: attribute each certname to the backend with the newer
-	// report_timestamp, taken from a short-TTL /nodes freshness map.
-	fresh := s.freshnessMap(context.Background(), ordered)
-	prefer := s.cfg.Prefer
-	return mergeFacts(ordered, func(cn string) string {
-		if b, ok := fresh[cn]; ok {
-			return b
+
+	resp, err, _ := s.flights.Do(key, func() (cachedResponse, error) {
+		built, buildErr := build()
+		if buildErr != nil {
+			return cachedResponse{}, buildErr
 		}
-		return prefer
+		body, marshalErr := json.Marshal(built)
+		if marshalErr != nil {
+			s.log.Printf("warning: encoding cache entry for %s failed: %v", key, marshalErr)
+			return built, nil
+		}
+		// The leader's request may be cancelled while followers still wait.
+		if putErr := cache.Put(context.WithoutCancel(r.Context()), key, body); putErr != nil {
+			s.log.Printf("warning: cache store for %s failed: %v", key, putErr)
+		}
+		return built, nil
 	})
+	if err != nil {
+		if stale != nil {
+			s.stale.markStale(time.Now())
+			s.log.Printf("warning: serving stale %s from cache (stored %s): %v",
+				path, stale.StoredAt.UTC().Format(time.RFC3339), err)
+			s.writeStored(w, stale.Body)
+			return
+		}
+		http.Error(w, err.Error(), http.StatusBadGateway)
+		return
+	}
+	s.stale.markFresh()
+	writeCached(w, resp)
 }
 
-// byPrecedence orders results so the Prefer backend comes first, giving it the
-// tie-break on equal timestamps. Remaining backends keep config order.
-func (s *Server) byPrecedence(results []backendResult) []backendResult {
-	ordered := make([]backendResult, len(results))
-	copy(ordered, results)
-	sort.SliceStable(ordered, func(i, j int) bool {
-		return ordered[i].name == s.cfg.Prefer && ordered[j].name != s.cfg.Prefer
-	})
-	return ordered
+func (s *Server) writeStored(w http.ResponseWriter, body []byte) {
+	var resp cachedResponse
+	if err := json.Unmarshal(body, &resp); err != nil {
+		s.log.Printf("warning: unreadable cache entry: %v", err)
+		http.Error(w, "unreadable cache entry", http.StatusBadGateway)
+		return
+	}
+	writeCached(w, resp)
 }
 
-// freshnessMap returns a per-certname owner map derived from each backend's
-// /nodes report_timestamp, cached for cfg.FreshnessTTL. On cache miss it queries
-// /nodes from all backends; a backend that fails is simply absent from the map,
-// so its certnames fall back to precedence/Prefer.
-//
-// When the incoming request already carries /nodes data (results has records),
-// we still query /nodes broadly here because a /facts query's certname set can
-// differ from what the request's query filter returned. The cache keeps this
-// cheap under load.
+func writeCached(w http.ResponseWriter, resp cachedResponse) {
+	w.Header().Set("Content-Type", "application/json")
+	if resp.Records >= 0 {
+		w.Header().Set(recordsHeader, strconv.Itoa(resp.Records))
+	}
+	// resp.Body is shared with the cache and with every caller of a single
+	// flight, so it is written, never appended to.
+	body := []byte(resp.Body)
+	if len(body) == 0 {
+		body = []byte("[]")
+	}
+	_, _ = w.Write(body)
+	_, _ = w.Write([]byte("\n"))
+}
+
+func encodeRecords(recs []json.RawMessage) json.RawMessage {
+	if recs == nil {
+		recs = []json.RawMessage{}
+	}
+	b, err := json.Marshal(recs)
+	if err != nil {
+		return json.RawMessage("[]")
+	}
+	return b
+}
+
+func queryParams(query string) url.Values {
+	if query == "" {
+		return nil
+	}
+	return url.Values{"query": []string{query}}
+}
+
+func rawRecords(recs []record) []json.RawMessage {
+	out := make([]json.RawMessage, 0, len(recs))
+	for _, rec := range recs {
+		out = append(out, rec.Raw)
+	}
+	return out
+}
+
+func (s *Server) mergeNodesResponse(results []backendResult) []json.RawMessage {
+	return mergeNodes(results)
+}
+
+func (s *Server) mergeFactsResponse(results []backendResult) []json.RawMessage {
+	if s.cfg.Merge == mergeStatic {
+		return mergeFacts(results, nil)
+	}
+	fresh := s.freshnessMap(context.Background(), results)
+	return mergeFacts(results, func(cn string) string { return fresh[cn] })
+}
+
+// Queries /nodes unfiltered rather than reusing the request's results, because a /facts query's certname set can differ.
 func (s *Server) freshnessMap(ctx context.Context, _ []backendResult) freshness {
 	s.mu.Lock()
 	if s.freshData != nil && time.Since(s.freshAt) < s.cfg.FreshnessTTL {
@@ -153,8 +415,7 @@ func (s *Server) freshnessMap(ctx context.Context, _ []backendResult) freshness
 	}
 	s.mu.Unlock()
 
-	// Empty query = all nodes; cheap enough for a short-TTL cache.
-	nodeResults := s.fanOut(ctx, nodesPath, "")
+	nodeResults := s.fanOut(ctx, nodesPath, nil)
 	var alive []backendResult
 	for _, res := range nodeResults {
 		if res.err != nil {
@@ -163,7 +424,7 @@ func (s *Server) freshnessMap(ctx context.Context, _ []backendResult) freshness
 		}
 		alive = append(alive, res)
 	}
-	f := buildFreshness(s.byPrecedence(alive))
+	f := buildFreshness(alive)
 
 	s.mu.Lock()
 	s.freshData = f
@@ -172,91 +433,156 @@ func (s *Server) freshnessMap(ctx context.Context, _ []backendResult) freshness
 	return f
 }
 
-// fanOut queries every backend concurrently for path?query=... and returns one
-// backendResult per backend, in config order.
-func (s *Server) fanOut(ctx context.Context, path, query string) []backendResult {
+// Returns one result per backend, in config order.
+func (s *Server) fanOut(ctx context.Context, path string, params url.Values) []backendResult {
 	results := make([]backendResult, len(s.cfg.Backends))
 	var wg sync.WaitGroup
 	for i, b := range s.cfg.Backends {
 		wg.Add(1)
 		go func(i int, b Backend) {
 			defer wg.Done()
-			recs, err := s.queryBackend(ctx, b, path, query)
-			results[i] = backendResult{name: b.Name, records: recs, err: err}
+			recs, total, err := s.queryBackend(ctx, b, path, params)
+			results[i] = backendResult{name: b.Name, records: recs, total: total, err: err}
 		}(i, b)
 	}
 	wg.Wait()
 	return results
 }
 
-// queryBackend performs one GET b.URL+path?query=... and decodes the JSON array.
-func (s *Server) queryBackend(ctx context.Context, b Backend, path, query string) ([]record, error) {
+// The returned count is the upstream X-Records value, or -1 when the backend sent none.
+func (s *Server) queryBackend(ctx context.Context, b Backend, path string, params url.Values) ([]record, int, error) {
 	target := strings.TrimRight(b.URL, "/") + path
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, target, nil)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
-	if query != "" {
-		q := url.Values{}
-		q.Set("query", query)
-		req.URL.RawQuery = q.Encode()
+	if len(params) > 0 {
+		req.URL.RawQuery = params.Encode()
 	}
 	resp, err := s.client.Do(req)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
 	defer func() { _ = resp.Body.Close() }()
 	body, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return nil, err
+		return nil, -1, err
 	}
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("HTTP %d: %s", resp.StatusCode, strings.TrimSpace(string(body)))
+		return nil, -1, fmt.Errorf("HTTP %d: %s", resp.StatusCode, strings.TrimSpace(string(body)))
 	}
-	return decodeRecords(body)
+	total := -1
+	if n, err := strconv.Atoi(resp.Header.Get(recordsHeader)); err == nil && n >= 0 {
+		total = n
+	}
+	recs, err := decodeRecords(body)
+	return recs, total, err
 }
 
-// proxyPrimary transparently forwards a non-merged /pdb/query/v4/* request to
-// the primary backend and streams the response back verbatim.
-func (s *Server) proxyPrimary(w http.ResponseWriter, r *http.Request) {
-	b := s.cfg.PrimaryBackend()
+// The record shape is unknown, so a union would be guesswork: the first 2xx wins and the first error response is replayed when none succeeds.
+func (s *Server) proxyUnmerged(w http.ResponseWriter, r *http.Request) {
+	var fallback *bufferedResponse
+	for _, b := range s.cfg.Backends {
+		resp, err := s.passThrough(r, b)
+		if err != nil {
+			s.log.Printf("warning: backend %q pass-through failed for %s: %v", b.Name, r.URL.Path, err)
+			continue
+		}
+		if resp.StatusCode >= 200 && resp.StatusCode < 300 {
+			setContentType(w, resp.Header.Get("Content-Type"))
+			w.WriteHeader(resp.StatusCode)
+			_, _ = io.Copy(w, resp.Body)
+			_ = resp.Body.Close()
+			return
+		}
+		body, _ := io.ReadAll(resp.Body)
+		_ = resp.Body.Close()
+		if fallback == nil {
+			fallback = &bufferedResponse{
+				status:      resp.StatusCode,
+				contentType: resp.Header.Get("Content-Type"),
+				body:        body,
+			}
+		}
+	}
+	if fallback == nil {
+		http.Error(w, "all backends failed", http.StatusBadGateway)
+		return
+	}
+	setContentType(w, fallback.contentType)
+	w.WriteHeader(fallback.status)
+	_, _ = w.Write(fallback.body)
+}
+
+type bufferedResponse struct {
+	status      int
+	contentType string
+	body        []byte
+}
+
+func (s *Server) passThrough(r *http.Request, b Backend) (*http.Response, error) {
 	target := strings.TrimRight(b.URL, "/") + r.URL.Path
 	if r.URL.RawQuery != "" {
 		target += "?" + r.URL.RawQuery
 	}
 	req, err := http.NewRequestWithContext(r.Context(), http.MethodGet, target, nil)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusBadGateway)
-		return
+		return nil, err
+	}
+	return s.client.Do(req)
+}
+
+func setContentType(w http.ResponseWriter, contentType string) {
+	if contentType != "" {
+		w.Header().Set("Content-Type", contentType)
 	}
-	resp, err := s.client.Do(req)
-	if err != nil {
-		s.log.Printf("warning: primary %q pass-through failed for %s: %v", b.Name, r.URL.Path, err)
-		http.Error(w, "primary backend failed", http.StatusBadGateway)
-		return
-	}
-	defer func() { _ = resp.Body.Close() }()
-	if ct := resp.Header.Get("Content-Type"); ct != "" {
-		w.Header().Set("Content-Type", ct)
-	}
-	w.WriteHeader(resp.StatusCode)
-	_, _ = io.Copy(w, resp.Body)
 }
 
-// healthReport is the /healthz JSON body.
 type healthReport struct {
 	Status   string            `json:"status"`
 	Backends map[string]string `json:"backends"` // name -> "ok" | error text
+	Cache    cacheHealth       `json:"cache"`
+}
+
+type cacheHealth struct {
+	Backend      string `json:"backend"` // "memory" | "none"
+	TTL          string `json:"ttl"`
+	Entries      int    `json:"entries"`
+	StaleEntries int    `json:"stale_entries"` // cached entries past their TTL
+	Bytes        int64  `json:"bytes"`
+	ServingStale bool   `json:"serving_stale"` // last cached response came from a stale entry
+	StaleServed  uint64 `json:"stale_served"`
+	LastStale    string `json:"last_stale_served,omitempty"`
+}
+
+func (s *Server) cacheHealth() cacheHealth {
+	stats := CacheStats{Backend: "none"}
+	ttl := time.Duration(0)
+	if s.factsCache != nil {
+		stats = s.factsCache.Stats()
+		ttl = s.cfg.FactsTTL
+	}
+	serving, served, last := s.stale.snapshot()
+	h := cacheHealth{
+		Backend:      stats.Backend,
+		TTL:          durationString(ttl),
+		Entries:      stats.Entries,
+		StaleEntries: stats.StaleEntries,
+		Bytes:        stats.Bytes,
+		ServingStale: serving,
+		StaleServed:  served,
+	}
+	if !last.IsZero() {
+		h.LastStale = last.UTC().Format(time.RFC3339)
+	}
+	return h
 }
 
-// handleHealth probes every backend's /nodes endpoint with a trivial query and
-// reports per-backend reachability. Overall status is "ok" if any backend is
-// reachable, "degraded" if some fail, "down" if all fail (503 in that case).
 func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	probe := `["=","certname","pdbmux-healthz-probe"]`
-	results := s.fanOut(r.Context(), nodesPath, probe)
+	results := s.fanOut(r.Context(), nodesPath, queryParams(probe))
 
-	report := healthReport{Backends: map[string]string{}}
+	report := healthReport{Backends: map[string]string{}, Cache: s.cacheHealth()}
 	healthy := 0
 	for _, res := range results {
 		if res.err != nil {
@@ -284,7 +610,6 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	_ = enc.Encode(report)
 }
 
-// writeJSON writes a JSON array of raw records as a PuppetDB-style response.
 func writeJSON(w http.ResponseWriter, recs []json.RawMessage) {
 	w.Header().Set("Content-Type", "application/json")
 	if recs == nil {