alkstore/tasks/pg-engine-queues.md

id, name, status, depends_on, scope, risk, impact, level, tags

id	name	status	depends_on	scope	risk	impact	level	tags
pg-engine-queues	Postgres engine — queues (`Queue`, `FOR UPDATE SKIP LOCKED` claims, job handles, backoff curve)	pending	pg-engine-seam-tx pg-engine-notify-listen	moderate	medium	component	implementation	wave-4 postgres-engine

Description

Implement the queue mechanism's pg arm — the re-derived queue machinery (ADR-004/005's greenfield posture, pg-boss schema family as design reference; ADR-010's depth) over the schema task's job/dead tables and the forwarder's wake substrate:

• Store::queue(name, opts) — validated constructor (shared- namespace kinds; construction stays free, queues are names not registered objects). The handle carries its QueueOpts as the stamp source.
• enqueue (auto-commit): resolves EnqueueOpts over the handle's stamps via the seam task's resolve_enqueue_opts + stamps_with_override (the pg engine's own resolution.rs — one clock read per enqueue, delay-over-run_at, relative- expires, max_attempts the one per-job override), encode_payload at the seam, INSERT on a pool connection. Then wake: pg_notify on the queue's wake channel (the mechanism-name-is- the-channel realization) — the LISTEN-driven claim path's push half.
• claim_one / claim_batch — exactly-once handout under concurrency: the FOR UPDATE SKIP LOCKED claim (POC-pinned under 4×4 concurrency) on a pool connection, in one statement or a CTE-scoped transaction — implementer's choice, but the exactly-once property is what the tests pin (two workers, one row; N×M concurrent drain, zero double-claims). Semantics per ADR-010: ordering priority DESC, run_at ASC, enqueue order; attempts += 1 per claim (a reclaim consumes an attempt); the claim sets the row's deadline from the job's own stamped visibility_timeout_s; the pre-claim dead-letter sweep for exhausted reclaimables (the substrate's D-12 shape — a worker dying right after its last allowed claim dead-letters, not strands); worker_id stamps the claimant column (non-empty validated, consumer-local — ADR-019 §2). The extent guard (n <= 0 → empty Vec at trait-impl entry, ADR-023 §2). Claims return boxed JobHandles (ops + row value).
• JobHandle impl (ADR-019 §3, ADR-010 §2's uniform validity predicate — processing + unexpired claim deadline; refusal = Ok(false), never error):
  • one-shot ack/retry/fail as 'static boxed futures over the consumed handle (mirroring TxHandle::commit's shape); repeatable heartbeat(&self, extend) — absolute reset of the claim deadline (not additive), late heartbeat refused.
  • ack deletes the row; retry(err, None) computes the queue's equal-jitter exponential curve — the pg engine's own implementation of ADR-010 §3's pinned form (base * 2^(attempt-1), jitter over [half, cap] integerized inclusive, capped 3600 s; attempt index = the row's post-claim count). The jitter's randomness source is this task's flagged choice: the SQLite engine used a std-only RandomState-hash draw (no rand dep, ADR-005's posture) — reuse that approach unless a pg-native source is materially better, and flag the choice in Notes. retry(err, Some(d)) overrides; fail(None) pins "failed"; retry's exhaust string pins "max attempts exceeded" (the engine-default strings land identically on both engines — the backlog row's wording).
  • Dead-letter moves (retry-at-budget, fail) write the dead table with last_error/died_at; dead rows are get_job-visible.
• ack_batch(ids) — the batch ack applies the uniform predicate per id (lapsed or non-claimed ids silently not counted; count returned; no worker filter — the wave-3 D-31 delta's shape is the contract's, not a substrate artifact).
• cancel(job_id) — unconditional delete (pending or processing, not an interrupt); Ok(false) when row-less.
• get_job(job_id) — dead-visible pure read (pool connection; Option<Job> with last_error/died_at on dead rows; the resolved values — not raw opts — are what the row shows, ADR-020).
• sweep_expired() — both-states no-stranded-rows move (any past-expiry row → dead with last_error='expired') + retention enforcement (dead_letter_retention_s, None = forever) — ADR-010 §5; the queue-scoped handle form.
• LISTEN-driven claim with re-poll safety net (the engine's default consumption posture, ADR-004): the contract claim ops are pull-shaped (callers claim in their own loops); the wake substrate makes those loops reactive — a consumer that listen(queue_name)s and claims on wake gets the measured 3–6 ms path; the re-poll safety net (interval-tunable, PgOpts-carried if wired — implementer's choice, documented either way) covers LISTEN loss (the no-replay hole). The engine's own tests exercise the wake-driven claim loop (commit → notify → wake → claim, the POC's property) to pin the posture; the contract surface itself stays pull-shaped.
• Closed-store posture on all queue ops (the wave-3 shape).

Acceptance Criteria

• enqueue resolves opts over the handle's stamps (the shared resolution arithmetic; max_attempts the one override); payload bytes exact; wake notify fires on the queue's channel
• claim_one/claim_batch: exactly-once under concurrency (two-workers-one-row; 6×40 concurrent drain, zero double-claims); ordering row pinned; reclaim consumes an attempt; deadline from the row's stamp; extent guard (n <= 0 → empty Vec); pre-claim exhausted-reclaimable sweep
• JobHandle: validity predicate uniform (lapse refuses all ops, Ok(false)); heartbeat absolute-reset + late-refused; ack deletes; retry curve matches ADR-010 §3's range/cap (statistical + cap tests, the SQLite task's shape); dead-letter defaults ("max attempts exceeded", "failed", caller strings verbatim); dead rows get_job-visible
• ack_batch per-id predicate (mixed lapsed/valid ids); cancel unconditional both states; sweep_expired both-states + retention (incl. forever-none)
• Wake-driven claim loop pinned end-to-end (enqueue → wake → claim, cross-connection)
• Validation (empty/reserved queue names, empty worker_id) + closed-store fail-closed on all queue ops
• cargo test -p alkstore-postgres (harness server), clippy -D warnings, fmt clean; gates green server-less

References

• docs/architecture/core-contract.md (queues section)
• docs/architecture/decisions/010-queue-semantics-depth.md (§1–§5, the depth)
• docs/architecture/decisions/019-mechanism-handle-surfaces.md §1–§3
• docs/architecture/decisions/020-enqueue-opt-semantics-and-bridges.md
• docs/architecture/decisions/023-fourth-review-round.md §2 (extent guard)
• docs/architecture/decisions/004-postgres-driver.md (re-derive posture, LISTEN-driven default)
• docs/architecture/queues.md (lifecycle, consumption posture)
• docs/research/reference-pgboss-rs-semantics.md (design reference)
• alkstore-sqlite/src/queue.rs (the structural twin — same semantics, own SQL)
• alkstore/src/queue.rs, job_handle.rs (the traits)

Notes

To be filled by implementation agent

Summary

To be filled on completion