---
status: draft
last_updated: 2026-10-08 (ADR-023 — fourth review round: numeric-argument domains pinned by kind, encode_payload typed, plain-path SQLite open, watcher cadence posture)
---

# Core contract

The unified, engine-agnostic surface a `Store` exposes. This document
specifies WHAT the contract is; the per-engine specs map it onto their
machinery; ADRs carry the WHY. The starting artifact is the honker-rs
surface (the Phase 0 §Interface finding) scoped to the
inventory-confirmed features — [ADR-002](decisions/002-feature-scope.md)
— and pinned against it. The base surface is pinned by
[ADR-008](decisions/008-contract-v1-pinning.md) (contract v1 partition,
`TxHandle` representation, wake type, reserved strings, error
taxonomy, config split — amended in place, pre-implementation, by
[ADR-014](decisions/014-outbox-tx-enqueue.md): `outbox_enqueue_tx`
joins the `TxHandle` trait, and by
[ADR-015](decisions/015-streams-depth.md): streams depth — key
semantics, `StreamEvent` shape, the ordering row, `trim_to`,
and `publish_with_key_tx`); ADR-009/ADR-010 add the first extensions
beyond ADR-008's original v1 partition (scheduler collapse surface,
`QueueOpts` depth — factually pre-release additions per ADR-017's
class 1, shipped inside the initial contract v1 text; versioning
discipline for the surface's changes is
[ADR-017](decisions/017-contract-versioning.md)'s, resolved: the
core crate's semver is the contract version);
[ADR-016](decisions/016-deployment-honesty.md) decides the parked
capability-surface question: **none, by default ever** — engine
differences surface at compile time (engine-crate identity) and in
the [deployment matrix](deployment.md), never as a runtime
descriptor; the
*obligations* are this document.

## Concepts

- **Store** — what a consumer opens from a connection string or file
  path ([ADR-001](decisions/001-crate-split.md)): one handle, the engine
  behind it chosen at open time. Consumer code never branches on
  engine type (guiding principle 4).
- **Mechanism** — one of the surface's coordination families: notify,
  streams, queues (+outbox), locks, scheduler. Delivery guarantees are
  pinned per mechanism in [ADR-006](decisions/006-wake-and-delivery-contract.md)'s
  table (notify/streams/queues), extended with the locks row by
  [ADR-008](decisions/008-contract-v1-pinning.md), the scheduler row
  and collapse by [ADR-009](decisions/009-scheduler-collapse.md).
  *(Guiding principles are defined in `docs/research/phase-0.md`
  §Vision and cited by number throughout this directory.)*
- **Wake** — the opaque "something changed, re-read" signal the wake
  contract delivers ([ADR-006](decisions/006-wake-and-delivery-contract.md)).
  The contract's central abstraction.
- **TxHandle** — the caller-held transaction object whose `*_tx`
  methods make side effects commit-atomic with a business write
  ([ADR-007](decisions/007-transactional-seam.md)). Contract shape:
  the `*_tx` methods live on the `TxHandle` trait itself — engines
  implement it for their concrete handle, no downcast
  ([ADR-008](decisions/008-contract-v1-pinning.md) §2).

## The seam

Per [ADR-008](decisions/008-contract-v1-pinning.md) §6, the trait is
constructed by engine crates (durability knobs, pool sizing, watcher
cadence are engine options — [deployment.md](deployment.md) carries
the facts); the contract is the trait surface it returns:

```text
Store::begin_tx() -> Box<dyn TxHandle + Send>

trait TxHandle {
    enqueue_tx(name, opts, payload) -> job_id
    publish_tx(stream, payload) -> offset
    publish_with_key_tx(stream, key, payload) -> offset
    notify_tx(channel, payload)
    save_offset_tx(stream, consumer, offset)
    get_job_tx(queue, job_id) -> Option<Job>              // ADR-021
    get_offset_tx(stream, consumer) -> i64                // ADR-021
    read_since_tx(stream, offset, limit) -> Vec<StreamEvent>            // ADR-021
    read_from_consumer_tx(stream, consumer, limit) -> Vec<StreamEvent>  // ADR-021
    outbox_enqueue_tx(outbox, opts, payload) -> job_id
    commit(self: Box<Self>) -> Result<()>   // or rollback
}
```

The tx read methods (`get_job_tx` / `get_offset_tx` / `read_since_tx`
/ `read_from_consumer_tx`,
[ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §1) make
read-your-own-writes possible inside the business transaction — they
take the mechanism name (mirroring `enqueue_tx`/`publish_tx`'s
name-taking shape), read the caller's own transaction's rows, and are
pure reads (no ops ride them). Reads and saves are tx-shaped;
claim/maintenance ops never are (no `claim_tx` — a claim's visibility
deadline must not be tied to a business transaction's lifetime;
deliberate absence, not an omission). **Dropping a handle without
`commit`/`rollback` rolls it back**
([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §4) — the
no-ghosts property holds through
RAII paths, not only the explicit ones.

`outbox_enqueue_tx` takes the **outbox name** and derives the backing
queue engine-side (the reserved prefix makes the derived name
unreachable by `enqueue_tx` by design —
[ADR-014](decisions/014-outbox-tx-enqueue.md)).
`publish_with_key_tx` validates the key like the non-tx form (a
present key must be non-empty → `InvalidName`) and is
`publish_tx`'s keyed twin — commit-atomic keyed publishes are the
point ([ADR-015](decisions/015-streams-depth.md) §2).

Mechanism handles come off the store (or, for transactional variants,
off the handle). **Payload encoding** ([ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md)
§4, signature typed by [ADR-023](decisions/023-fourth-review-round.md)
§1): the trait crosses `serde_json::Value`; engines serialize it with
serde_json via core's `encode_payload` (fallible — `Codec` on
serialization failure, typed at the helper; never a silent fallback
form) and store exactly those bytes — `payload_as<T>` decodes
them, `Codec` on failure; round-trip (publish → read → decode →
`Value` equality) is what the contract suite pins, on both engines.

**Numeric-argument domains** ([ADR-023](decisions/023-fourth-review-round.md)
§2) — classified by argument kind, uniform on both engines (no engine
may reinterpret a caller-supplied numeric argument via SQL-dialect
semantics; validation happens at the trait-impl entry point, before
the engine's storage layer):

- **Extents** ("at most this many": `claim_batch(n)`,
  `read_since(limit)`, `read_from_consumer(limit)`) — non-positive
  values yield the **empty value result** (a value, not an error —
  the no-work vocabulary); the SQLite `LIMIT -1` dialect artifact
  (a negative extent reaching the whole queue/stream) is dead by
  this rule.
- **Durations** (`try_lock(ttl)`, `renew(ttl)`) — non-positive values
  are rejections (`Err`, opaque `Database`, detail in the source
  chain); an exclusion mechanism must not silently hand back a lease
  the caller believes it holds.
- **Boundaries/offsets** (`trim_to(horizon)`, enqueue `delay`/
  `run_at`/`expires`) — total: any `i64` is well-defined (a negative
  horizon deletes nothing; a negative delay resolves to a ready
  time in the past, i.e. ready now).

```text
store.notify(channel, payload)          handle.notify_tx(channel, payload)
store.stream(name) -> StreamHandle      handle.publish_tx / publish_with_key_tx
                                        handle.save_offset_tx
                                        (the StreamHandle also carries
                                        trim_to — stream-side, outside the
                                        tx seam)
store.queue(name, opts) -> Queue        handle.enqueue_tx
store.outbox(name) -> Outbox            handle.outbox_enqueue_tx(outbox, opts, payload)
store.try_lock(name, owner, ttl) -> Option<Lock>
store.listen(channel) -> Box<dyn WakeReceiver>
store.schedule(name, spec, queue, payload, opts) -> Result<Schedule>
store.unschedule(name) -> bool
store.run_schedules(stop: StopToken) -> Result<()>
```

The mechanism handles' full trait surfaces (`Queue { enqueue,
claim_one, claim_batch, ack_batch, cancel, get_job, sweep_expired }`,
`StreamHandle { publish, publish_with_key, read_since,
read_from_consumer, save_offset, get_offset, trim_to, subscribe }`,
`Outbox { enqueue, run_once }`, `Lock { renew, release }`, plus the
boxed `JobHandle` a claim returns) are pinned by
[ADR-019](decisions/019-mechanism-handle-surfaces.md); the
`StopToken` parameter is a core-crate type (same ADR, §4).

Payloads cross the trait as core value types (non-generic trait
methods — object safety of the boxed handles,
[ADR-008](decisions/008-contract-v1-pinning.md) §2); `payload_as<T>`
decodes on concrete returned values (`Job`, `StreamEvent`). The
`with_tx` closure wrapper ships over the handle shape (the POC-verified
composition direction, [ADR-007](decisions/007-transactional-seam.md);
signature pinned there 2026-10-07: `with_tx(f) -> Result<()>` with
`f: FnOnce(&mut dyn TxHandle) -> BoxedFuture<Result<()>>` — `Ok` ⇒
commit, `Err`/drop/panic ⇒ rollback; non-generic for object safety,
values return through captured state, no handle escapes the closure).
Mechanism handles (`Queue`, `StreamHandle`, `Outbox`, `Lock`) are
core-owned trait objects like the tx handle — engine types never appear
in consumer signatures. Their trait surfaces are pinned by
[ADR-019](decisions/019-mechanism-handle-surfaces.md) (placement
mirroring the honker-rs starting artifact: claim/maintenance ops on the
`Queue` handle, publish/read/trim on the `StreamHandle`, the claim ops
on the boxed `JobHandle`, `run_once` on the `Outbox`); trait-method
additions on these traits post-release ride
[ADR-017](decisions/017-contract-versioning.md) class 2 like any
other trait surface.

## Mechanism contracts

### notify / listen

Fire-and-forget signals, commit-atomic when sent in a transaction
([ADR-007](decisions/007-transactional-seam.md)). No durability, no
replay, no per-listener retry; a listener attached after a commit
never sees it.

- `notify(channel, payload)` — payload ≤ 8000 bytes on Postgres
  (client-side checked, typed `PayloadTooLarge` before the round
  trip, verified by POC #2; the measured quantity is the serde_json
  serialization of the payload `Value` — ADR-020 §4's stored-bytes
  form, so check and storage measure one thing); no limit on SQLite.
  The error variant is
  contract-wide (callers match it identically on both engines), the
  *occurrence* is the documented engine asymmetry
  ([ADR-008](decisions/008-contract-v1-pinning.md) §5) — with no
  capability surface
  ([ADR-016](decisions/016-deployment-honesty.md)), this variant is
  the one runtime carriage of an engine asymmetry; its occurrence
  asymmetry is pinned in the verification backlog below.
- `listen(channel) -> Box<dyn WakeReceiver>` — starts from "now";
  delivers opaque `Wake { channel }` signals
  ([ADR-006](decisions/006-wake-and-delivery-contract.md)), never
  payloads or ids ([ADR-008](decisions/008-contract-v1-pinning.md) §3).
  Wakes are **best-effort hints, not per-commit delivery promises**:
  possibly coalesced (SQLite) or per-notify (Postgres), possibly
  repeated after reconnect, possibly absent entirely (SQLite burst
  coalescing; the pg no-replay hole) — recovery is the consumer's
  re-read plus the engine's reconnect surface, never a guaranteed
  deliver. **Consumers must be idempotent on wake.**
- Failure surfaces as channel events, never silence: watcher death
  closes the receiver (`recv() -> None`, SQLite); the synthetic
  reconnect-wake (a reserved channel
  ([ADR-004](decisions/004-postgres-driver.md))) covers Postgres
  connection gaps.
- Channel name is the one piece of semantic content a wake carries —
  the invalidation key for caching subscribers.

### streams

Durable pub/sub with per-consumer offsets
([ADR-006](decisions/006-wake-and-delivery-contract.md)). The durable
cousin of notify: publish is commit-atomic; every committed event is
readable by every consumer whose offset hasn't passed it;
replay-on-attach is the default; offsets are explicit and
transaction-aware (`save_offset_tx` gives exactly-once-within-a-
business-tx shape). Depth pinned by
[ADR-015](decisions/015-streams-depth.md).

- `publish` / `publish_tx` / `publish_with_key` / `publish_with_key_tx`
  — append to the stream's durable log; return the assigned offset.
  **Key semantics ([ADR-015](decisions/015-streams-depth.md) §1): the
  key is carried metadata** — stored on the event row, round-tripped
  on every read, `Option<String>`-shaped on `StreamEvent` — with no
  engine-enforced behavioral role: **the ordering guarantee stays
  global FIFO by offset within the stream**; per-key in-order reading
  is the documented emergent pattern (filter by key, read in offset
  order), not a server promise. A present key must be non-empty
  (`InvalidName`). Server-enforced per-key ordering re-enters only
  via a consumer-inventory row naming it.
- **`StreamEvent` shape** ([ADR-015](decisions/015-streams-depth.md)
  §3): `{ offset: i64, stream: String, key: Option<String>, payload:
  Vec<u8>, created_at: i64 }` — `stream` renames honker's `topic`
  (one term everywhere, the ADR-008 §4 kinds list); `offset` is
  engine-assigned, monotone per stream, **immutable, never renumbered**
  (gaps after a trim are legal); `created_at` is unix-seconds-at-
  publish, informational, **not an ordering field** (offset is the
  only one); `payload_as<T>` decodes (the error is `Codec`).
- **Ordering guarantee row** ([ADR-015](decisions/015-streams-depth.md)
  §4, extending [ADR-006](decisions/006-wake-and-delivery-contract.md)
  §2's streams row): `read_since`/`subscribe` yield `offset ASC` —
  global FIFO per stream; committed-event visibility and
  replay-on-attach as already pinned; offsets immutable. The
  contract suite pins cross-engine equivalence against this row
  (same publish sequence → same offset sequence → same read order).
- `read_since` / `read_from_consumer(offset)` — cursor-based reads;
  `get_offset(consumer)` — checkpoint inspection. A read from a
  trimmed-away offset region (below) resumes at the trim horizon's
  first remaining row.
- `save_offset` / `save_offset_tx` — consumer checkpoint, explicit;
  **the contract's save is always explicit** (no auto-checkpoint
  cadence — honker's per-binding ambiguity is not inherited). The
  subscription handle exposes no `save_every` / auto-save-on-drop
  ([ADR-008](decisions/008-contract-v1-pinning.md) §8). Saves are
  monotone — a save below the stored checkpoint is a silent no-op —
  so the direct form and the receiver's no-arg form
  (`EventReceiver::save_offset(&mut self)`, which saves the
  last-yielded event's offset through the same op,
  [ADR-019](decisions/019-mechanism-handle-surfaces.md) §6)
   interleave freely and cannot regress one another. Both save
   forms' `Err` arm carries `Database` only
   ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §5) —
   regression refusal is a silent no-op, so a save result errs only
   on storage failure. A saved offset
  below the trim horizon
  ([ADR-015](decisions/015-streams-depth.md) §5) stays a valid
  position marker.
- `trim_to(horizon) -> count` — delete events with `offset <= horizon`
  (exact-boundary; the horizon is a boundary argument, total per
  [ADR-023](decisions/023-fourth-review-round.md) §2 — a negative
  horizon deletes nothing, idempotently)
  ([ADR-015](decisions/015-streams-depth.md) §5; the return is the
  deleted-row count, [ADR-019](decisions/019-mechanism-handle-surfaces.md)). The stream-side
  bounded-growth op, consumer-invoked, no engine-default
  retention and no ambient sweeper (the ADR-010 §6 posture; the
  replay-forever default is the mechanism's purpose — growth is
  documented with the tool in-contract to bound it). Trim emits no
  dedicated wake and no notify — on SQLite the `data_version`
  watcher may still fire (any committed write bumps it; a spurious
  hint, contract-legal); surviving events keep their offsets.
- `subscribe(consumer) -> Box<dyn EventReceiver>` — durable
  consumption: attach, read to current tail, resume after restart
  from the stored offset; explicit `save_offset` on the receiver
  (shape in [ADR-008](decisions/008-contract-v1-pinning.md) §8).
  **Error and close arms pinned**
  ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §5):
  `recv()`'s `Err` arm carries `Database` only (payloads cross raw;
  `Codec` lives at `payload_as<T>`; `Closed` is the `None` arm);
  close is terminal and its *cause* is the documented engine
  asymmetry — SQLite's receiver closes on watcher death (POC-pinned),
  the pg receiver stays open across forwarder reconnects (reconnect
  wakes arrive on it) and closes only at engine shutdown. A closed
  receiver never reopens; recovery is a fresh `subscribe(consumer)`
  resuming from the saved offset.
  Consumption is wake-driven with table re-read — the same
  mechanism split as queues (durable row, LISTEN/watcher wake,
  [ADR-006](decisions/006-wake-and-delivery-contract.md)); the
  receiver's engine-side trigger is engine-internal, but events
  never require polling to become visible (the queues row's pg
  re-poll safety net is an admission of LISTEN loss, not the
  posture).

### queues

Durable at-least-once work
([ADR-002](decisions/002-feature-scope.md)). Depth pinned by
[ADR-010](decisions/010-queue-semantics-depth.md); contract-level
obligations:

- `enqueue` / `enqueue_tx` — commit-atomic; `EnqueueOpts { delay,
  run_at, priority, max_attempts, expires }`; **field types pinned
  (2026-10-07, third review round follow-through)**: `delay`,
  `run_at`, `expires` are `Option<i64>` (honker-rs parity,
  `packages/honker-rs/src/lib.rs:439-447` — unset is distinguishable
  from zero, and the resolution rules below are the unset behavior),
  `priority` is a plain `i64` with default **0** (the claim-order
  key; the row schema's `DEFAULT 0`), and `max_attempts` is
  `Option<i64>` with `None` = the queue-level stamp decides (the
  §3a override; no honker counterpart — our field). Every field has
  a default-resolved meaning, so all are constructor-settable with
  `..Default::default()`. Queue-level
  `QueueOpts { visibility_timeout_s, max_attempts, backoff_base_s,
  dead_letter_retention_s }` ([ADR-010](decisions/010-queue-semantics-depth.md)
  §3/§3a): plain `i64` fields (`dead_letter_retention_s:
  Option<i64>`, `None` = forever) with a `Default` providing the
  pinned defaults 300/3/5/none (honker parities except retention,
  which is ours); `ScheduleOpts { priority, max_attempts, expires }`
  types identical to their `EnqueueOpts` twins. **Option semantics
  ([ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md))**:
  ready time resolves `delay` → `now + delay` (wins) over `run_at` →
  literal absolute over neither → now; `expires` is relative seconds
  from enqueue resolved to an absolute row `expires_at` (`None` =
  never); the resolved values — not the raw fields — are what
  `get_job` shows. The `QueueOpts` stamps land on each job row at
  enqueue (§3a), resolved per
  [ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md) §3 by
  call shape: the auto-commit `Queue::enqueue` (the one handle-carrying
  shape) resolves over that handle's opts; `enqueue_tx` (a
  `TxHandle` carries no queue opts) and every other no-handle-open
  path — outbox backing queues, scheduler boundary fires — stamp the
  engine's derived defaults (plain 300/3/5/none; the outbox's
  60/5/5 set).
- `claim_one` / `claim_batch` — exactly-once handout under concurrency
  (POC-pinned on both engines); take a caller-supplied `worker_id`
  (claimant identity — consumer-local, no reserved rule, non-empty;
  [ADR-019](decisions/019-mechanism-handle-surfaces.md) §2); claim
  ordering: priority DESC, then
  ready-time, then enqueue order (FIFO under equal priority). Claims
  return boxed `JobHandle`s — ops + row value (same ADR, §3).
  `claim_batch(n)`'s `n` is an extent
  ([ADR-023](decisions/023-fourth-review-round.md) §2): `n <= 0`
  claims nothing (the empty `Vec`, a value — never an error, never
  the whole queue).
- Job handle: `ack / retry / fail / heartbeat` on the boxed
  `JobHandle` a claim returns ([ADR-019](decisions/019-mechanism-handle-surfaces.md)
  §3 — `job(&self) -> &Job` beside the ops; one-shot `self: Box<Self>`
  ops like the tx commit). The one-shot/repeatable split is
  deliberate: `ack`/`retry`/`fail` consume the handle (each is a
  terminal transition of the claim); **`heartbeat(self, extend)`
  takes `&self` and is repeatable** — renewal is inherently repeated
  within one claim; do not "fix" this asymmetry at implementation
  ([ADR-019](decisions/019-mechanism-handle-surfaces.md) §3). `ack`
  deletes the row;
  `heartbeat(extend)` is *renewal* — an absolute reset of the claim
  deadline (extend is the new full deadline from now, **not additive**
  to elapsed time); late heartbeat refused; **a reclaim consumes an
  attempt**; `retry(err, None)` computes the queue's equal-jitter
  exponential delay (range pinned by
  [ADR-010](decisions/010-queue-semantics-depth.md) §3), `retry(err,
  Some(d))` overrides; `fail` = immediate dead-letter. Handle-op
  validity (uniform predicate): an op succeeds only while the row is
  `processing` and the caller's claim deadline is unexpired — deadline
  lapse refuses all handle ops (heartbeats, acks included) and a
  reclaim does too; the dual-execution window's worker may still
  complete its work but its ack will not land (at-least-once, as
  documented); false, not error
  ([ADR-010](decisions/010-queue-semantics-depth.md) §2). Dead letters:
  move-to-dead storage, `get_job` sees dead rows (with
  `last_error`/`died_at`), retention via `dead_letter_retention_s`
  (default forever), no redrive API
  ([ADR-010](decisions/010-queue-semantics-depth.md) §1–§4).
  `QueueOpts` stamp onto the job row at enqueue (§3a) — queues are
  names, not config owners. The remaining v1-skeleton ops (`ack_batch`
  — the batch ack applies the uniform predicate per id, lapsed or
  non-claimed ids silently not counted; `cancel` — unconditional
  delete, not an interrupt) pin in
  [ADR-010](decisions/010-queue-semantics-depth.md) §1.
- `sweep_expired()` — moves *every* past-expiry row (any state) to
  dead + enforces dead-letter retention — the no-stranded-rows
  property ([ADR-010](decisions/010-queue-semantics-depth.md) §5);
  the queue-scoped handle form ([ADR-019](decisions/019-mechanism-handle-surfaces.md)
  §1 — the name rides the handle); cadence recipe in
  [queues.md](queues.md) (no ambient sweeper).

### named locks

TTL-bounded coordination locks. Lock rows live in the same database
as the caller's business data — coordination and data co-locate —
but lock ops are **not** part of the tx seam: there is no `lock_tx`;
a lock held across a business transaction is held by explicit
acquisition before/inside it, not transaction-scoped (acquisition is
a separate auto-commit op; rollback of the business tx does not
release it — the TTL discipline governs).

- `try_lock(name, owner, ttl)` — acquire or fail (`Option<Lock>` —
  no-work is a value, not an error); `renew` and `release` on the
  lock handle. Release on explicit unlock or TTL expiry.
  Re-acquirable after expiry (POC-pinned on Postgres; on SQLite it
  rests on the forked substrate's lock machinery — the SQLite-side pin
  is in the verification backlog below). `ttl <= 0` is a rejection
  (duration kind, [ADR-023](decisions/023-fourth-review-round.md) §2)
  — opaque `Err`, never a silent immediately-expired lease.
- **Guarantee row** ([ADR-008](decisions/008-contract-v1-pinning.md)
  §7): mutual exclusion bounded by TTL + renewal — after TTL expiry
  exclusion lapses *silently* (no revocation event); holders must
  `renew` within TTL; expiry is a loss of exclusivity, not an error.
- Lock names are shared-namespace (reserved-prefix rules below,
  [ADR-008](decisions/008-contract-v1-pinning.md) §4).

### outbox

A helper over queues, not a separate mechanism: enqueue inside the
business transaction + the delivery/consumption worker entry points.
Same evidence base and guarantee as queues
([ADR-002](decisions/002-feature-scope.md)). Worker semantics
(honker's `run_once` shape, inherited as the pinned posture; the
delivery callable's shape pinned by
[ADR-019](decisions/019-mechanism-handle-surfaces.md) §5):
`run_once(worker_id, delivery) -> bool` is a *pull* op — claim one
job, run
the delivery closure, `ack` on `Ok`, `retry(err, None)` (the queue's
curve) on `Err`; the consumer calls it in its own loop. **No
engine-issued heartbeat inside delivery** (honker parity, inherited
deliberately — the helper does not auto-heartbeat on the caller's
behalf; the boxed `JobHandle`'s own `heartbeat` remains available to
the delivery closure, and renewal inside the deadline is the
consumer's obligation):
delivery slower than the backing queue's stamped visibility timeout
(default 60 s for outbox-backed queues) can have its claim expire
mid-delivery and be redelivered — the dual-execution window;
idempotent delivery is the consumer's obligation, same as queues.

The transactional enqueue side is `outbox_enqueue_tx(outbox, opts,
payload)` on the `TxHandle` trait
([ADR-014](decisions/014-outbox-tx-enqueue.md)): it takes the outbox
name (validated like `store.outbox(name)` — empty → `InvalidName`,
reserved-prefixed → `ReservedName`), derives the backing queue name
engine-side, and lands the job row in the caller's transaction with
`EnqueueOpts` stamped per
[ADR-010](decisions/010-queue-semantics-depth.md) §3a over the backing
queue's derived `QueueOpts`. Commit
makes the job visible to `run_once` exactly when the business write
commits; rollback drops both (the no-ghosts property,
[ADR-007](decisions/007-transactional-seam.md)). The derived backing
queue name is not expressible through `enqueue_tx` (the reserved
prefix is rejected on directly-supplied names) —
`outbox_enqueue_tx` is the only transactional path into it, which is
itself the guarantee that the derivation cannot be collided with.

### scheduler

Collapsed into queues per [ADR-009](decisions/009-scheduler-collapse.md):
`schedule(name, spec, queue, payload, opts)` (upsert by name),
`unschedule(name)`, and the opt-in `run_schedules(stop: StopToken)`
runner (`StopToken` is a core-crate cloneable cancel handle —
[ADR-019](decisions/019-mechanism-handle-surfaces.md) §4; the
leader-elected leadership lock is the reserved name
`__alkstore_scheduler`). Both name-bearing arguments validate at the
entry point: the schedule *name* like a queue name (non-empty,
reserved-prefix rejected —
[ADR-009](decisions/009-scheduler-collapse.md) §1) and, since
[ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §3, the
**queue argument** identically (`InvalidName`/`ReservedName`) — a
schedule cannot target a reserved name, so the outbox backing queue
stays reachable only through the outbox surface
([ADR-014](decisions/014-outbox-tx-enqueue.md)). Schedules never fire
without a runner (no ambient timers). v1 spec grammar:
`@every <n><unit>` only (`s|m|h|d`); cron strings are rejected
(`InvalidSpec`) pending a consumer-inventory row naming wall-clock
cron. Each boundary fire enqueues ordinary work stamped with
`ScheduleOpts { priority, max_attempts, expires }`
([ADR-009](decisions/009-scheduler-collapse.md) §3) — resolved over
the target queue's *derived engine defaults* (visibility/backoff/
retention; no handle is open), per
[ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md) §3.
Boundary
guarantee: [ADR-009](decisions/009-scheduler-collapse.md) §4
(at-least-once per elapsed boundary, bounded catch-up with
skip-forward past the cap).

## Cross-cutting contracts

### Delivery-guarantee table

The per-mechanism table in
[ADR-006](decisions/006-wake-and-delivery-contract.md) is the contract
of record for notify/streams/queues; the locks row is
[ADR-008](decisions/008-contract-v1-pinning.md) §7's and the scheduler
row is [ADR-009](decisions/009-scheduler-collapse.md) §4's. This spec
inherits them and adds the consumer obligations:

- wake idempotence (notify's best-effort, coalescing behavior);
- explicit offset saves (streams);
- visibility-timeout budgeting — heartbeat inside the deadline for
  long work; the dual-execution window and reclaim-eats-attempt rules
  are documented consumer obligations
  ([ADR-010](decisions/010-queue-semantics-depth.md) §2);
- running `run_schedules` for schedules to fire at all, and scheduling
  sweep cadences via the collapse recipe — timeliness is the
  consumer's, correctness-of-transition the engine's
  ([ADR-010](decisions/010-queue-semantics-depth.md) §6);
- `*_tx` operations are *only* durable after the caller's commit —
  rollback drops job rows, event rows (keyed and unkeyed alike),
  notifications, and offset saves together (the no-ghosts property,
  POC-pinned on both engines).

### Errors

`thiserror`-typed, per the family standard. Pinned by
[ADR-008](decisions/008-contract-v1-pinning.md) §5: one top-level
`Error`, with v1 variants `PayloadTooLarge` (universal; produced
pg-side, contract-wide matchable), `ReservedName`, `InvalidName`,
`Closed`, `Codec`, and the opaque `Database` fallback (engine detail
preserved via the source chain). `InvalidSpec` (schedule spec grammar)
and `LeadershipLost` (`run_schedules` return on leadership loss) —
both from [ADR-009](decisions/009-scheduler-collapse.md) §6 — are
part of contract v1's initial text (pre-release additions per
[ADR-017](decisions/017-contract-versioning.md) class 1; the
"post-v1 additions" framing an earlier draft used is retired). Pinning rule: a variant exists only when
callers can act differently on it. Queue claim "no work" and job/lock
boolean results are values, not errors; dead-letter is observable
state (`get_job`), not an error.

### Capability surface

None — decided by
[ADR-016](decisions/016-deployment-honesty.md) (2026-10-06): `Store`
carries no capabilities accessor, in v1 and by default ever. The
honest single-host/multi-host boundary lives at compile time (the
engine-crate dependency *is* the deployment statement —
[ADR-001](decisions/001-crate-split.md)'s single-driver binaries) and
in [deployment.md](deployment.md)'s matrix. Runtime carriage of the
one caller-actionable engine asymmetry is the contract-wide
matchable `PayloadTooLarge` variant (Errors above); re-entry requires
a consumer-inventory row naming a runtime-adapt need.

### Naming / reserved namespace

Consumer-visible names (channels, streams, queues, outboxes, locks,
and schedule names) share engine-visible namespaces on Postgres (LISTEN
channel names are server-global per database). The canonical kinds
list is exactly these six (notify/stream/queue/outbox/lock/schedule;
pinned by [ADR-008](decisions/008-contract-v1-pinning.md) §4, outbox
added by [ADR-014](decisions/014-outbox-tx-enqueue.md), schedule names
by [ADR-009](decisions/009-scheduler-collapse.md) §1):

- Reserved prefix **`__alkstore_`**, engine-independent, applies
  across all name kinds; the one v1-reserved string is
  `__alkstore_listener_reconnected__` (the Postgres reconnect-wake
  channel, [ADR-004](decisions/004-postgres-driver.md)).
- Reserved-prefix names are rejected at every entry point — the
  name-bearing methods *and their `*_tx` counterparts* — with the
  typed `ReservedName` error; empty names with `InvalidName`
  ([ADR-008](decisions/008-contract-v1-pinning.md) §4 — enforced as
  non-empty-after-whitespace-exclusion per that ADR's wave-1 review
  annotation: whitespace-only names are `InvalidName` on both name
  classes; names are not trimmed). Stream-consumer
  names are consumer-local identifiers, not a reserved-namespace kind.
- Engine-derived names in consumer namespaces carry the reserved
  prefix (the outbox's backing queue is derived under the prefix —
  honker's `_outbox:{name}` scheme is not inherited verbatim).
- SQLite: the substrate's `__alkstore_*` internal table family is
  storage-internal (not consumer namespace) — re-owned from upstream's
  `_honker_*` by the fork ([ADR-011](decisions/011-sqlite-substrate-fork.md),
  designed per [ADR-012](decisions/012-forked-substrate-design.md)).
  Postgres: engine tables are
  schema-scoped — one engine-owned schema
  ([ADR-010](decisions/010-queue-semantics-depth.md) §8) — the
  *channel* namespace is this section's.

## Verification backlog

Contract properties POC-pinned on one engine only (or sketched rather
than surface-verified) — the contract test suite must pin both engines
before the engine specs are called `stable`:

- **Named-lock TTL/expiry re-acquisition on SQLite** — pinned on
  Postgres (pg POC's lock probe); on SQLite it rests on the forked
  substrate's lock machinery (its `lock_renew` verified in the quality
  read; the re-acquire-does-not-refresh-TTL behavior is upstream's,
  inherited deliberately) — pin it in the contract suite.
- **Wake semantics under `WakeReceiver` shapes** — POCs verified
  wake delivery/coalescing through their own probe types; the pinned
  `Wake { channel }` / recv forms (§3 of
  [ADR-008](decisions/008-contract-v1-pinning.md)) need the contract
  suite's own property tests on both engines.
- **`save_offset_tx` exactly-once-within-a-business-tx shape** — both
  POCs verified it through their *sketch* implementations (SQLite's
  offset-save was a plain SQL upsert, not the forked substrate's full
  surface;
  the pg side likewise through its probe), so the property must be
  re-pinned against the real engines' `save_offset_tx` in the
  contract suite.
- **Concurrent `try_lock` loser/error behavior** on SQLite (the pg
  side returns cleanly; the substrate's busy-path under lock
  contention is the thing to pin).
- **Scheduler + depth properties on Postgres** — the SQLite side's
  tick/leader/catch-up machinery inherits the forked substrate's
  test-pinned implementation; the pg engine's re-derived tick (boundary
  advance,
  64-cap skip-forward, leadership-loss discipline) and the
  [ADR-010](decisions/010-queue-semantics-depth.md) depth properties
  (visibility reclaim consuming attempts, dead-letter moves, the
  no-stranded-rows sweep) pin in the contract suite at
  implementation.
- **Backoff-curve and stamp-resolution equivalence across engines** —
  the equal-jitter curve ([ADR-010](decisions/010-queue-semantics-depth.md)
  §3) and the opts-stamping resolution (§3a) are computed engine-side
  per [ADR-012](decisions/012-forked-substrate-design.md) §2's
  contract-blind boundary; the contract suite must pin both engines'
  arithmetic to identical outputs.
- **Job-handle validity predicate on both engines** — the uniform
  processing-state + unexpired-deadline rule
  ([ADR-010](decisions/010-queue-semantics-depth.md) §2): the
  late-heartbeat boundary (refused exactly when the deadline has
  lapsed), the post-lapse-ack-refusal (at-least-once), and the
  ack-vs-reclaim race (one wins atomically); `ack_batch` applies the
  predicate per id (lapsed or non-claimed ids silently not counted);
  `retry`'s refusal is `false` with the row untouched; the
  engine-default strings land identically (`fail(None)` →
  `"failed"`, exhaustion → `"max attempts exceeded"`); both engines'
  SQL pin identical outcomes in the contract suite.
- **`outbox_enqueue_tx` commit-atomicity on both engines**
  ([ADR-014](decisions/014-outbox-tx-enqueue.md)) — rollback drops
  the backing-queue job row together with the business write (no
  ghost job); commit makes it claimable by `run_once`; reserved/empty
  outbox names rejected identically on the tx path
  (`ReservedName`/`InvalidName`); stamped opts visible via `get_job`
  equal for both engines.
- **Cross-engine stream equivalence** ([ADR-015](decisions/015-streams-depth.md)
   §3/§4) — publish sequence → identical *(per-stream)* offset
   sequence → identical
   `read_since` output order on both engines, direct and subscriber
   reads alike (offsets are global counters engine-side — pg
   bigserial, SQLite AUTOINCREMENT — so equivalence is per-stream
   relative order, never absolute offset values); keyed/unkeyed
   interleavings preserve global FIFO;
   `key` round-trips exactly (`None` vs `Some`), `stream`/`created_at`
   fields equal.
- **`publish_with_key_tx` commit-atomicity on both engines**
  ([ADR-015](decisions/015-streams-depth.md) §2) — rollback drops the
  keyed event row with the business write (no ghost event); commit
  makes it visible to `read_since`/`subscribe`; empty-`Some`-key
  `InvalidName` on both engines' tx paths.
- **`PayloadTooLarge` occurrence asymmetry** ([ADR-016](decisions/016-deployment-honesty.md)
  §5) — with no capability surface, `PayloadTooLarge` is the one
  runtime carriage of an engine asymmetry, so its matchability pins
  in the suite: an oversized `notify`/`notify_tx` payload is rejected
  client-side by the Postgres engine before any round trip (the
  limit in the variant), and the SQLite engine never produces the
  variant at any size — engine-agnostic code writes the same match
  on both engines and the non-occurring arm simply never fires.
- **`trim_to` semantics on both engines** ([ADR-015](decisions/015-streams-depth.md)
  §5) — exact-boundary trim (`<=`), surviving rows keep their offsets
  (gaps legal, never renumbered), reads resume at the trim horizon's
  first remaining row, saved offsets below the horizon stay valid,
  trim wakes nothing, and a concurrent subscriber never loses its
  place.
- **Save-offset monotone composition** ([ADR-019](decisions/019-mechanism-handle-surfaces.md)
  §6) — direct-form and receiver-form saves interleave without
  regression on both engines; a lower-offset save is a silent no-op;
  receiver saves land the last-yielded event's offset exactly.
- **Payload encode/decode round-trip on both engines**
  ([ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md) §4) —
  same `Value` published/enqueued on both engines stores byte-identical
  rows and decodes back to `Value` equality (`Codec` only on
  non-serializable target types).
- **Enqueue-opts resolution equivalence** ([ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md)
  §1/§2/§3) — delay-over-run_at precedence, relative-expires
  resolution, and the scheduler-fired jobs' derived-default stamps
  (`get_job`-visible via the row) are identical on both engines;
  plain `enqueue_tx` stamps the same plain-queue derived defaults
  (300/3/5/none) on both engines — auto-commit-handle enqueues stamp
  the handle's opts, no-handle-open shapes stamp derived defaults
  (the outbox's 60/5/5 set being the one exception, ADR-014 §1).
- **In-tx read-your-own-writes on both engines**
  ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §1) —
  reads issued on the tx handle see the transaction's own writes
  (`get_job_tx` a row the same tx enqueued; `read_since_tx` an event
  the same tx published); another engine connection sees none of it
  pre-commit; post-rollback the reads' subjects are gone (no-ghosts
  for reads).
- **Drop = rollback on both engines**
  ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §4) —
  a handle dropped before commit leaves no job/event/notify/offset
  residue (no-ghosts via RAII); afterward the SQLite writer slot is
  reusable and the pg client re-poolable.
- **`schedule()` queue-argument validation on both engines**
  ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §3) —
  reserved-prefix and empty queue arguments rejected at registration
  with `ReservedName`/`InvalidName`; stored schedules fire only into
  non-reserved queue names.
- **Receiver error/close arms on both engines**
  ([ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) §5) —
  `recv()`'s `Err` carries `Database` only; `save_offset`'s `Err`
  carries `Database` only; regression saves are silent no-ops;
  SQLite's receiver closes on watcher death (terminal `None`); the
  pg receiver stays open across forwarder reconnects (reconnect-wakes
  arrive; no close) and closes only at engine shutdown.
- **Numeric-domain semantics on both engines**
  ([ADR-023](decisions/023-fourth-review-round.md) §2) — extents
  clamp to empty (`claim_batch(n <= 0)` claims nothing; stream reads
  with `limit <= 0` read nothing, never the whole queue/stream), so
  the SQLite `LIMIT -1` dialect artifact is dead on both engines;
  durations reject (`try_lock`/`renew` with `ttl <= 0` are opaque
  `Err`s on both engines); boundaries are total (a negative
  `trim_to` horizon deletes nothing, idempotently).
- **`encode_payload` typed failure**
  ([ADR-023](decisions/023-fourth-review-round.md) §1) — the helper
  returns `Result<Vec<u8>, Error::Codec>`; enqueue/publish store and
  decode the exact serialization of the input `Value` (byte-identical
  rows cross-engine, the §4 round-trip unchanged), and the failure
  arm is typed at the helper — no silent empty-byte fallback exists.
- **Plain-path SQLite open** ([ADR-023](decisions/023-fourth-review-round.md)
  §3) — the SQLite engine's `open` path argument is a plain
  filesystem path: a string carrying a `?name=value` suffix is
  treated as a filename, no URI interpretation, no connection-
  semantics mutation outside the constructor's opts.
- **Watcher cadence default + knob** ([ADR-023](decisions/023-fourth-review-round.md)
  §4) — the SQLite store's watcher runs at the inherited 1 ms
  default; `SqliteOpts::poll_interval` changes the configured
  cadence (config-surface accounting — wall-clock cadence
  measurements stay out of the contract suite's deterministic
  posture).

## Design Decisions

| ADR | Decision | Summary |
|---|---|---|
| [001](decisions/001-crate-split.md) | Crate split | core + per-engine crates; single-driver binaries |
| [002](decisions/002-feature-scope.md) | Feature scope | inventory-confirmed features only; cut-flags explicit |
| [006](decisions/006-wake-and-delivery-contract.md) | Wake contract | opaque wake + re-read; notify-vs-streams guarantee split |
| [007](decisions/007-transactional-seam.md) | Tx seam | caller-held handle, `*_tx` methods, native commit-atomicity |
| [008](decisions/008-contract-v1-pinning.md) | Contract v1 pinning | surface partition, `TxHandle` trait shape, `Wake` type, reserved strings, error taxonomy, config split, locks guarantee row |
| [009](decisions/009-scheduler-collapse.md) | Scheduler collapse (pre-release extension) | queues + `schedule()`/`run_schedules`, `@every`-only v1, boundary guarantee row |
| [010](decisions/010-queue-semantics-depth.md) | Queue depth (pre-release extension) | visibility/renewal, opts stamping, backoff curve, dead-letter, sweep, layout |
| [011](decisions/011-sqlite-substrate-fork.md) | Substrate fork | SQLite substrate owned (`__alkstore_*` naming); queue ops re-derived on contract v1 |
| [012](decisions/012-forked-substrate-design.md) | Fork design | contract-blind substrate boundary; engine-side formula arithmetic pinned equivalent by the contract suite |
| [014](decisions/014-outbox-tx-enqueue.md) | Outbox tx enqueue (amends 008) | `outbox_enqueue_tx` on `TxHandle`; outbox-name validation; derived backing queue reached only through the outbox surface |
| [015](decisions/015-streams-depth.md) | Streams depth (amends 006/008) | key = carried metadata, global-FIFO ordering row, `StreamEvent` shape, `publish_with_key_tx`, `trim_to` |
| [016](decisions/016-deployment-honesty.md) | Deployment honesty (decides 008's parked question) | no runtime capability surface — compile-time engine identity + documented matrix; `PayloadTooLarge` occurrence asymmetry pinned |
| [017](decisions/017-contract-versioning.md) | Contract versioning (governs this surface's changes) | core crate's semver *is* the contract version; four change classes; amend-in-place ends at first release; pairing = manifest pin + version-stamped contract suite + docs |
| [019](decisions/019-mechanism-handle-surfaces.md) | Mechanism handles (amends 008 §1/§2/§8) | handle traits pinned (`Queue`/`StreamHandle`/`Outbox`/`Lock`/`JobHandle`); `worker_id` claimant identity; `Job`/`Schedule` struct shapes; core-owned `StopToken`; save-offset composition |
| [020](decisions/020-enqueue-opt-semantics-and-bridges.md) | Enqueue options + bridges (amends 008 §1/§2, 009 §3) | `delay` wins over `run_at`; `expires` = relative seconds; scheduler stamps from derived queue defaults; serde_json byte-level payload encoding |
| [021](decisions/021-tx-reads-and-value-shape-fixes.md) | Third review round (amends 008 §2/§8, 019 §1/§3, 009 §1) | tx-side read methods on `TxHandle` (read-your-own-writes); `Job.claimed_at` restored; `schedule()` queue argument validated; drop = rollback; receiver close/error arms pinned |
| [023](decisions/023-fourth-review-round.md) | Fourth review round (amends 020 §4, 008 §5, 012 §4) | numeric-argument domains by kind (extents clamp empty, durations reject, boundaries total); `encode_payload` typed (`Codec`); plain-path SQLite open (URI flag dropped); watcher cadence 1 ms default + `SqliteOpts` knob |

## Open Questions

Open questions are tracked in
[open-questions.md](open-questions.md). Key
questions affecting this document:

- **OQ-10**: contract versioning discipline across engine crates —
  **resolved** ([ADR-017](decisions/017-contract-versioning.md)):
  the core crate's semver is the contract version; change classes
  pinned; the verification backlog below matures into the
  version-stamped contract suite (the pairing instrument).

Resolved on this document's surface: **OQ-09** (scheduler collapse —
[ADR-009](decisions/009-scheduler-collapse.md)), **OQ-05** (queue
semantics depth — [ADR-010](decisions/010-queue-semantics-depth.md)),
**OQ-13** (transactional outbox enqueue shape —
[ADR-014](decisions/014-outbox-tx-enqueue.md)), **OQ-12** (streams
depth — [ADR-015](decisions/015-streams-depth.md)), **OQ-08**
(capability-surface shape — none, by default ever;
[ADR-016](decisions/016-deployment-honesty.md)), and **OQ-10**
(the versioning discipline this document's surface is governed by —
[ADR-017](decisions/017-contract-versioning.md)), 2026-10-05/06.
The second review round (2026-10-06) found no new open questions —
its findings resolved directly as
[ADR-019](decisions/019-mechanism-handle-surfaces.md) (handle
surfaces, value shapes, stop token) and
[ADR-020](decisions/020-enqueue-opt-semantics-and-bridges.md)
(enqueue-option semantics, scheduler stamp source, payload encoding).
The third review round (2026-10-07, pre-decomposition) likewise found
no new open questions — its five findings resolved directly as
[ADR-021](decisions/021-tx-reads-and-value-shape-fixes.md) (tx-side
reads, `Job.claimed_at`, `schedule()` queue-validation, drop =
rollback, receiver close/error arms).