alkstore/tasks/mem-engine-foundations.md

id, name, status, depends_on, scope, risk, impact, level, tags

id	name	status	depends_on	scope	risk	impact	level	tags
mem-engine-foundations	Mem engine — crate scaffold, `MemStore::new()`, guarded state core, wake routing	pending		moderate	medium	component	implementation	wave-6 mem-engine

Description

The mem engine's foundations (wave 6 — ADR-024): a new workspace member alkstore-mem and the state core every later task builds on.

• Crate + manifest: workspace member per ADR-024 §1 — dependencies are alkstore (versioned path pin per ADR-017 §4.1, matching the other engines' manifests) plus tokio restricted to the wasm-supported subset (sync + time only). No driver dependencies. thiserror via core; no other crates.io surface.
• Constructor: MemStore::new() — engine-native, no connection string, no file path (the one documented exception to the core-contract open prose, amended by ADR-024 §4). Each new() yields a fully isolated instance sharing nothing (the suite factory's isolation contract); teardown is drop. No PgOpts-equivalent opts struct exists — nothing to configure (if a knob surfaces during the wave, MemOpts with ..Default::default() follows the family opts pattern; not assumed now).
• Guarded per-mechanism state core: the store instance is the state — per-mechanism structures (queues+jobs, dead letters, streams+offsets, locks, schedules) behind the family's locking discipline (parking_lot short-held internal locks per AGENTS convention 3; tokio primitives for cross-await coordination). Layout is module-per-file under src/, mirroring the other engines. Subsequent mechanism tasks own their tables' shapes; this task owns the shared scaffolding: the guarded-state container idiom, name validation at entry (same InvalidName/ReservedName rules as every other engine — reserved prefix __alkstore_, the one reserved string __alkstore_listener_reconnected__ unused here but still rejected), and the wake-routing substrate (in-process channels; a mechanism registers its commit-ordered wake source, receivers attach per channel).
• Wake substrate shape: no batching boundary exists — wakes deliver at the commit boundary directly to each open receiver, per-subscriber FIFO by commit order (engine-mem.md §Architecture; coalescing not applicable). The routing core lives here; the listen()/WakeReceiver surface lives in mem-engine-notify-listen.
• Clock posture: every timestamp (unix-seconds stamps, visibility deadlines, TTLs, scheduler boundaries) reads the process clock — there is no server clock to defer to. Keep the clock behind one internal accessor so tests can observe timestamps deterministically the same way the SQL engines' row shapes do.

The Store trait impl itself is ADR-007-shaped and lands in mem-engine-seam-tx (the tx overlay is the load-bearing half); this task lands everything up to the trait boundary.

Acceptance Criteria

• alkstore-mem is a workspace member compiling clean; manifest dependency surface is core + tokio (sync + time) only
• MemStore::new() constructs; two instances share nothing (pinned by a unit test)
• The guarded state core, validation rules, wake-routing substrate, and clock accessor exist with unit pins
• cargo check --target wasm32-unknown-unknown -p alkstore-mem clean from this task onward (tokio subset discipline enforced here, not retro-fitted)
• Workspace gates green (cargo build, cargo test, clippy -D warnings, fmt)

References

• docs/architecture/decisions/024-mem-engine.md §1/§4
• docs/architecture/engine-mem.md §Identity and posture, §Architecture
• docs/architecture/core-contract.md §Naming / reserved namespace
• tasks/pg-engine-schema.md (the wave-4 rhythm this wave mirrors — its "foundations first" slot)

Notes

To be filled by implementation agent

Summary

To be filled on completion