# ADR-007: Transactional seam — caller-held tx handle with `*_tx` methods

## Status

Accepted (the seam shape both POCs verified); handle-type ergonomics
decided by [ADR-008](008-contract-v1-pinning.md) §2 — the `*_tx`
methods live on the handle trait, no downcast

## Context

The load-bearing property of the whole store (guiding principle 2) is
**transactional local-adjacency**: enqueue/publish/notify inside the
same transaction as the caller's business write; rollback drops both.
Serving it through an async trait over two transaction models —
rusqlite's sync, thread-bound connection vs tokio-postgres's
`Send + Sync` client — is the one genuinely driver-coupled design
point (OQ-ST-03's framing). Both POCs (2026-10-04) built the candidates
and measured them.

## Decision

The core contract's transactional seam is a **caller-held transaction
handle**:

```text
store.begin_tx() -> TxHandle
handle.enqueue_tx(name, opts, payload) -> job id
handle.publish_tx(name, payload) -> event id
handle.notify_tx(channel, payload)
handle.save_offset_tx(stream, consumer, offset)
handle.commit() / handle.rollback()
```

- The handle is owned by the caller across `await` points; every
  `*_tx` operation lands in *the caller's transaction*; commit/rollback
  are explicit and owned by the caller. Non-`_tx` operations are the
  auto-commit convenience counterparts, each atomic alone.
- **Postgres** ([ADR-004]): the handle holds the pooled connection
  object directly (tokio-postgres `Client` is `Send + Sync` — verified
  by a compile-time probe). Straight `.await`s, no bridge, no hop
  cost. Commit/rollback return the object to the pool.
- **SQLite** ([ADR-003]): the handle is a **writer-slot lease** —
  `begin_tx` acquires the `Writer` slot and opens `BEGIN IMMEDIATE`;
  every handle op round-trips `spawn_blocking` to that connection
  (rusqlite is not `Send`-across-await / not `Sync`). Holding the slot
  across `await` points *is* how the lease works — a long transaction
  parks the single WAL writer, which is SQLite's own shape.
- **Closure-scoped transactions** (`with_tx`) are *available as a
  wrapper over* the handle shape, not instead of it (the POC verified
  the reverse composition doesn't work: a closure cannot outlive
  itself, so caching-subscriber state can't escape it).
- Both engines deliver the property **natively** — no emulation:
  in-tx notify/Notify delivers only at commit; rollback drops job rows,
  business rows, and notifications together (the POC property tests on
  both engines green: no ghosts).

The POC's trait sketch carried two frictions, both now Phase 1
contract-pinning input rather than open risk:

1. The `as_any_mut` downcast per engine for `*_tx` methods on a
   `dyn TxHandle` — small, but a generic or enum-favored handle may be
   cleaner. Decided by [ADR-008](008-contract-v1-pinning.md) §2
   (with the full contract).
2. Thread-affinity of rusqlite tx ops (SQLite handle ops must
   round-trip the same blocking thread) — inherent to [ADR-003]'s
   bridge, documented as a contract note ("SQLite tx ops are
   serialized by the writer slot"), not a defect.

## Consequences

**Positive**

- The load-bearing property is one seam, both engines, POC-verified
  end-to-end — the dual-write problem is structurally prevented, not
  consumer-disciplined.
- Long transactions compose naturally (checkout-then-work shape);
  nothing about the seam forbids multi-statement business
  transactions with interleaved reads.
- Per-engine bridging differences are invisible to consumer code: the
  same `*_tx` calls, the same commit/rollback ownership.

**Negative**

- The SQLite handle parks the writer for its duration — a slow
  consumer transaction throttles all writers on the file. This is
  honest (WAL single-writer), but contract docs must say it so
  consumers budget transactions accordingly.
- Per-op `spawn_blocking` hop on SQLite tx ops (measured fine at
  ~0.35 ms p50; the dedicated-thread bridge is the recorded
  optimization).
- `with_tx` wrapping is a convenience surface we must ship and test
  so it doesn't accrete ad-hoc in consumer code.

## References

- `docs/research/poc-sqlite-posture-findings.md` §Arm A, §Probe 1/3;
  `docs/research/poc-pg-posture-findings.md` §Sub-module T.
- OQ-ST-04 (`docs/research/phase-0.md`), the `*_tx` seam bullet.
- [ADR-003](003-sqlite-driver.md), [ADR-004](004-postgres-driver.md) —
  the per-engine handle mechanics.
- [ADR-006](006-wake-and-delivery-contract.md) — commit-atomicity is
  the `*_tx` property this seam carries into the notify contract.
- [core-contract.md](../core-contract.md) — the spec whose transaction
  section this ADR defines.
[ADR-003]: 003-sqlite-driver.md
[ADR-004]: 004-postgres-driver.md