---
id: mem-engine-locks
name: Mem engine — named locks (in-memory TTL registry)
status: pending
depends_on: [mem-engine-seam-tx]
scope: narrow
risk: low
impact: component
level: implementation
tags: [wave-6, mem-engine, locks]
---

## Description

The named-locks mechanism over an in-memory TTL registry
(engine-mem.md §Mapping the contract, locks row):

- `store.try_lock(name, owner, ttl)` — acquire or fail (`Option<Lock>
  `; no-work is a value); validation at entry (empty → `InvalidName`,
  reserved → `ReservedName`); **duration guard at entry** —
  `ttl <= 0` rejects with opaque `Err`
  ([ADR-023](../architecture/decisions/023-fourth-review-round.md) §2,
  durations kind), never a silent immediately-expired lease.
- **`Lock` handle** ([ADR-019](../architecture/decisions/019-mechanism-handle-surfaces.md)):
  `renew(ttl)` (absolute reset; renews past expiry refused) and
  `release`. Registry entries carry owner + deadline; expiry is a
  clock check at try/renew boundaries (tokio time; no ambient
  revoker) — after TTL expiry exclusion lapses *silently* (the
  ADR-008 §7 row), re-acquirable; the holder may still release.
- **Single-instance contention semantics**: two `try_lock`s on the
  same name within the registry are serialized by the guarded-state
  discipline — first wins, second gets `None` cleanly (the SQLite
  busy-path question has no mem analog; loser is a value, never an
  error). Not part of the tx seam (no `lock_tx` — contract-verbatim:
  a lock held across a business tx is held by explicit acquisition;
  the overlay neither sees nor stages locks).

## Acceptance Criteria

- [ ] `try_lock`/`renew`/`release` over the TTL registry; entry
      validation (names + duration guard) — pinned
- [ ] TTL expiry: silent lapse, re-acquisition, renew-past-expiry
      refused — pinned (clock-controlled tests via the state core's
      clock accessor)
- [ ] Concurrent-contention shape: loser is `None`, winner's lease
      intact — pinned
- [ ] `cargo check --target wasm32-unknown-unknown -p alkstore-mem`
      clean; workspace gates green

## References

- docs/architecture/core-contract.md §named locks
- docs/architecture/decisions/008-contract-v1-pinning.md §7;
  019-mechanism-handle-surfaces.md; 023 §2
- docs/architecture/engine-mem.md §Mapping the contract (locks row)
- tasks/pg-engine-locks.md (the structural twin's task of record —
  the duration guards land identically)

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion