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. Consider two backends being merged during a migration:

• old — the PuppetDB nodes are moving off, e.g. http://puppetdb1.example.com:8080
• new — the PuppetDB nodes are moving on to, e.g. http://puppetdb2.example.com:8080

Nodes move from old to new as they migrate. pdbmux merges both so consumers don't have to know (or query twice) which PuppetDB a node currently lives in. The backend names are arbitrary labels; there is no fixed number of backends.

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.
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/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 neither does.
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.

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.

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 more than one backend, pdbmux keeps all of that node's facts from one backend and drops the other's, 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). Ties/fallbacks use prefer.
  • static — always keep the prefer backend's facts for shared nodes. No extra /nodes query.
  • 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 that migrated legitimately has reports in the old PuppetDB and the new one and both belong in the merged view. Reports dedupe on hash; events, which carry no id of their own, dedupe on the verbatim record (a node briefly reporting to both PuppetDBs 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 a /reports query whose extract carries a ["function", ...] column.
  • The grouping key is the row's non-aggregate fields: for /reports 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.
  • 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.

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 backend precedence). 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.

Config

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

Config file: $XDG_CONFIG_HOME/pdbmux/config.yaml. In a container there is no config file — everything comes from PDBMUX_* env vars.

listen: ":8080"
backends:
  - name: old
    url: http://puppetdb1.example.com:8080
  - name: new
    url: https://puppetdb2.example.com
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[*].url is a base URL (scheme://host[:port]); pdbmux appends the /pdb/query/v4/... path per request.

Env var	Overrides
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_BACKENDS	whole backend list, as name=url,name=url

Flags: --listen, --primary, --merge.

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='old=http://puppetdb1.example.com:8080,new=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, configured entirely via PDBMUX_* env vars; a container needs at minimum PDBMUX_BACKENDS. Stateless, so run as many replicas as you like; use /healthz for liveness/readiness probes.