pdbmux — merging PuppetDB proxy

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

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.

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

pdbmux proxies GET requests only. The query param (PuppetDB AST JSON, not PQL) is forwarded verbatim.

Path	Behaviour
GET /pdb/query/v4/nodes	Fan out to all backends, dedupe by certname, keep the record with the newer report_timestamp, stamped with the winning backend's name (see provenance). 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), plus a synthetic pdbmux_source fact naming it.
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 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.

Responses carry PuppetDB's X-Records when the query asked for a total, and on the cached paths two headers pdbmux adds itself, X-Cache and Age — see Caching.

Merge semantics

• /nodes — dedupe by certname; the record with the strictly-newer 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 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.

Provenance: the pdbmux_source fact

Once several PuppetDBs sit behind one endpoint, a consumer can no longer tell which backend a node's data came from. pdbmux makes that visible in the response itself, so nothing has to query each backend to find out:

• /facts gains one extra fact record per certname, alongside the node's real facts, in the shape of a real fact record — certname, name, value, environment — with value set to the backend name from backends / PDBMUX_BACKENDS. environment is copied from that node's own facts (all four keys are always present, since clients index them directly).
• /nodes gains a pdbmux_source key on each merged node record. A node record carries no facts, so this is a synthetic field, not a fact — the one key outside PuppetDB's documented node schema. Clients read node fields by name, so an extra key is ignored by anything that doesn't want it.

The value always names the backend whose data won that endpoint's merge, not a backend that merely holds the node. The two endpoints resolve their winner separately, so under merge: static they can legitimately disagree: /facts attributes a shared node to the first backend in configured order, while /nodes always attributes it to the backend holding the newer report_timestamp. Each answer describes the record it is attached to.

While the fact is enabled, any /facts record whose own name field equals the configured name is dropped, on every query shape — including the shapes below, where nothing is injected in its place. The rule reads the record, not the query, so a projection that filters on name without returning it — say ["extract",["certname","value"],["=","name","pdbmux_source"]] — produces rows that no longer identify themselves, and an upstream value of that name comes through. Ask for the name column and the guarantee holds. Each request that drops a record logs it once. Only source_fact_enabled: false restores upstream records of that name; rename the synthetic fact via source_fact if the real one matters more.

Injection is skipped, and no synthetic record is added, when:

• the query contains an extract outside a subquery — it projects a column subset, and with a ["function", ...] column it aggregates. Injecting there would break the row shape or silently inflate a count(), so aggregate results are never changed. The whole query is walked, so an extract nested under and/or/not/from skips injection too; an extract under in, subquery, or select_<entity> projects that subquery rather than the response, so it does not;
• the query is not an AST array — every PQL-syntax query (facts { certname = "web1" }) lands here. pdbmux cannot tell what such a query projects, so it never injects into a PQL response. Use the AST form to get the fact;
• (/facts only) the query constrains name — ["=","name","osfamily"] and friends ask for specific facts, and the synthetic record is not one of them. Only the outer query is inspected: a name filter inside an in/select_facts subquery narrows which nodes match, not which facts come back, so injection still happens;
• injection is turned off (see source_fact_enabled).

Not supported in v1: server-side filtering on the fact. A query that selects it — ["=","name","pdbmux_source"], or an extract naming it — is forwarded to the backends like any other, and they return nothing, because the fact does not exist upstream. pdbmux does not evaluate the AST itself, so it cannot answer such a query correctly for every operator (not, or, subqueries) and does not pretend to for some. Read the fact from an unfiltered (or certname-filtered) /facts response and filter client-side. The same applies to the /pdb/query/v4/facts/<name> route, which is served unmerged pass-through.

Not covered: /factsets and /inventory. Both carry facts, but pdbmux does not merge either today — they take the unmerged pass-through path, where the answer comes from whichever backend replied first rather than from a merge winner, so there is no owner to attribute. Injecting there would state a provenance that isn't true.

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. That fan-out runs on its own context, bounded by timeout, so a client that disconnects can neither cancel nor fail the requests sharing its flight; a waiter whose own client goes away leaves the flight running for the others. The flight is cancelled once its last participant leaves, so a lone client disconnecting releases the upstream connections straight away.
• Response headers — every response on a cached path carries X-Cache (hit served from a fresh entry, miss built by this request, stale the expired-entry fallback) and Age in whole seconds since the served copy was stored (0 on a miss). Uncached paths carry neither.
• 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.
• Provenance is stored, not re-applied — what a cache entry holds is the fully merged body, pdbmux_source already injected and upstream records of that name already dropped. Attribution names the backend that supplied the data, which is a property of that fetch, so it stays correct for as long as the body does and ages out with it — X-Cache and Age say how old both are. Two requests can only share an entry when they share a key, and the key is path plus query, which is exactly what decides whether injection applies; a name-filtered /facts query and a plain one therefore cache separately and neither is ever served the other's shape. source_fact and source_fact_enabled are read once at startup, and the cache lives for the same process, so changing either cannot leave differently-shaped entries behind.

Config

Precedence (lowest → highest): defaults < config file < env vars (PDBMUX_*) < flags.

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.

listen: ":8080"
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
source_fact: pdbmux_source   # name of the synthetic provenance fact
source_fact_enabled: true    # false serves backends' records untouched

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_MERGE	merge
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
PDBMUX_SOURCE_FACT	source_fact (default pdbmux_source)
PDBMUX_SOURCE_FACT_ENABLED	source_fact_enabled (default true); false disables injection

Flags: --config, --listen, --merge.

config init writes to --config/PDBMUX_CONFIG when set, else to $XDG_CONFIG_HOME/pdbmux/config.yaml.

Running

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.

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

make build (static binary into dist/), make test, make lint. Requires Go 1.25+.

Deployment

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.

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.