---
status: draft
last_updated: 2026-10-05
---

# 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 *post-v1
contract extensions* (scheduler collapse surface, `QueueOpts` depth —
versioning discipline for such extensions is OQ-10's); 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)
    outbox_enqueue_tx(outbox, opts, payload) -> job_id
    commit(self: Box<Self>) -> Result<()>   // or rollback
}
```

`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):

```text
store.notify(channel, payload)          handle.notify_tx(channel, payload)
store.stream(name) -> Stream            handle.publish_tx / publish_with_key_tx
                                        handle.save_offset_tx
                                        (the Stream handle 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) -> Result<()>
```

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)).
Mechanism handles (`Stream`, `Queue`, `Outbox`, `Lock`) are core-owned
trait objects like the tx handle — engine types never appear in
consumer signatures; their trait methods pin at implementation,
mirroring the `TxHandle` pattern ([ADR-008](decisions/008-contract-v1-pinning.md)
§2's rationale applies identically).

## 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); 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) — see also
  OQ-08.
- `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 saved offset below the trim horizon
  ([ADR-015](decisions/015-streams-depth.md) §5) stays a valid
  position marker.
- `trim_to(horizon)` — delete events with `offset <= horizon`
  ([ADR-015](decisions/015-streams-depth.md) §5). 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).
  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 }`; queue-level
  `QueueOpts { visibility_timeout_s, max_attempts, backoff_base_s,
  dead_letter_retention_s }` ([ADR-010](decisions/010-queue-semantics-depth.md)
  §3/§4).
- `claim_one` / `claim_batch` — exactly-once handout under concurrency
  (POC-pinned on both engines); claim ordering: priority DESC, then
  ready-time, then enqueue order (FIFO under equal priority).
- Job handle: `ack / retry / fail / heartbeat`. `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`,
  `cancel` — unconditional delete, not an interrupt) pin in
  [ADR-010](decisions/010-queue-semantics-depth.md) §1.
- `sweep_expired(queue)` — 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);
  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).
- **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):
`run_once(worker_id, delivery)` 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
heartbeat inside delivery** (honker parity, inherited deliberately):
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)` runner
(leader-elected where the engine has peers — the leadership lock is
the reserved name `__alkstore_scheduler`). 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). 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). Post-v1 additions:
`InvalidSpec` (schedule spec grammar) and `LeadershipLost`
(`run_schedules` return on leadership loss) — both from
[ADR-009](decisions/009-scheduler-collapse.md) §6, the only
taxonomy deltas so far. 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 in contract v1. Whether the `Store` exposes engine capabilities
at all — and if so, which (payload limits, host semantics,
wake-cadence knobs) — is OQ-08's decision
([deployment.md](deployment.md)).

### Naming / reserved namespace

Consumer-visible names (channels, streams, queues, locks, and
schedule names) share engine-visible namespaces on Postgres (LISTEN
channel names are server-global per database). Pinned by
[ADR-008](decisions/008-contract-v1-pinning.md) §4 (schedule names
added 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). 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); 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 offset sequence → identical
  `read_since` output order on both engines, direct and subscriber
  reads alike; 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.
- **`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.

## 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 (post-v1 extension) | queues + `schedule()`/`run_schedules`, `@every`-only v1, boundary guarantee row |
| [010](decisions/010-queue-semantics-depth.md) | Queue depth (post-v1 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` |

## 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
  ([open](open-questions.md))
- **OQ-08**: capability-surface shape ([open](open-questions.md))

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)), and **OQ-12** (streams
depth — [ADR-015](decisions/015-streams-depth.md)), 2026-10-05.