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