alkblobs/docs/architecture/store-api.md

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Store API

What this is

The store core's public surface: the typed facade above the backends that consumers (alkgit, alkfs, the ops module) program against. It owns typed keys and hashing (ADR-002), size-threshold dispatch (ADR-003), the put-pinning seam, and the liveness/sweep seam (ADR-005) — and nothing else: no paths, no manifests, no wire (ADR-004, ADR-001). The facade is engine-blind (ADR-007/008): which kv/fs engine a node runs changes nothing here except fleet-GC mechanism wiring (ADR-008).

Public surface

Write path

• put(len: Option<u64>, stream) → (Key, Pin) — the streaming put seam (POC #3 finding A1). Two internal paths behind one signature:
  • Known length (encouraged, documented as such on the API): the preamble hashes before content flows; one pass, no staging needed for hashing. Covers git objects, pre-staged files (stat-derived length), network offers, Content-Length-bearing uploads.
  • Unknown length: content buffers in memory up to the dispatch threshold; on commit the buffer is the whole stored artifact (kv tier) — or, on mid-stream threshold overflow, the buffer flushes into a stage file and the stream continues into it (fs tier — the streaming pass cannot include the preamble, so the derivation pass restarts over preamble + staged bytes; forced by the preamble being inside the hashed input, not a design choice). Verification runs over the eventual stored artifact as a whole; a stream error on either path leaves no entry and no stage file. Eliminates the A6 dispatch asymmetry by construction: routing is a pure function of content length (ADR-003).
• Stage-file hygiene invariant (POC #3 finding A5, codified): every failure path converges on stage-discard; every success path converges on commit-rename. No early returns that bypass cleanup; this is an asserted invariant with dedicated exact-count tests, not review hygiene.
• Pin / Batch — RAII guard on the put path (ADR-005): the entry is liveness-protected until the guard drops or the caller converts it into a consumer-side reference (registered liveness, a root tag). Batch groups multi-put writes under one batch-scoped pin (manifest writes are exactly this); a batch's pins drop together on batch commit or drop (ADR-005 §Decision).

Read path

• get(key) → Option<(len, stream)> — small tier returns a bounded read; large tier streams a file handle. Dispatch fall-through: a small-tier miss queries the large tier (POC #1 finding 6).
• stat(key) → Option<EntryMeta> — cheap length/type probe (phase-0 OQ-BL-04 addendum; gix's header-only read is git's cheapest primitive and the packfile-serving surface rides this). EntryMeta carries len: u64 and the key's algorithm (the "type" from the store's perspective is the hash algorithm — there is no other type at this layer). stat rides the Backend trait's size method (ADR-008): an indexed-column probe on kv engines, a file stat on the fs local engine — never a value/content load.
• read_range(key, range) → (slice, slice_digest) — slice plus an out-of-band slice digest: SHA-256 over the slice bytes, fixed by convention so callers and ops handlers agree without negotiation (per-range verification against the canonical digest is impossible — ADR-006). Range reads fall through tiers exactly as get does (small-tier miss slices the large tier's value). Local range serving (packfiles) is the consumer of this.
• get_stream / fanout — the ops module's fetch handler is a broadcast above the store: one reader, store arm + subscriber arms (POC #3 finding A3); late joiners degrade to normal verified get after commit. The store exposes the seam; the fanout policy lives in the ops layer (ADR-001).

Lifecycle

• has(key), delete(key) — deletion participates in ADR-005's delete windows; direct deletes are permitted but refuse protected keys — pinned or re-observed live via registered sources (the same per-key arbitration sweeps apply, typed error) — and are unusual by posture; most deletion flows through sweeps.
• list() → stream of keys — whole-pool enumeration; complete by contract (see backends doc for why list correctness is load-bearing).
• register_liveness_source(...) — the live-shared GC seam (ADR-005, §Decision; the POC's "install implies copy" failure (finding 7) is why the verb/name is pinned here).
• sweep() → SweepReport — explicit mark-and-sweep; embedders own the cadence (ADR-005, no ambient timers). A sweep with no registered liveness sources aborts without deleting — the safe default. Under a fleet (multiple nodes over one pool — requirements.md REQ-2), sweep/put/delete semantics are mechanism-extended by ADR-008: pins are DB-backed rows committed atomically with entries, one sweeper holds the advisory lock, and delete arbitration is SQL-level — the facade's invariant text is unchanged.

Error model

thiserror; no panics in library code; no unwrap/expect outside tests (AGENTS convention 2). Distinguished failure families:

• Missing — key absent (get/stat miss)
• Verification — put/get hash-check failure (content ≠ key)
• Io(String) — backend media failure (stringly because std::io::Error is not stable across versions)
• GcAborted — nothing deleted; sweep not run. Two cause classes (ADR-005: a protection source failed; ADR-008: another node holds the fleet sweeper lock) — the typed error distinguishes them.
• KeyInvalid — malformed key bytes at the boundary (hashing doc)

Virgin-store semantics: read paths on a fresh store see absent/empty, never "table does not exist" errors (POC #1 finding 2 — the redb lesson generalizes to any kv engine).

Concurrency posture

• Async I/O throughout; tokio::sync for lifecycle correlation; parking_lot for short-held internal locks (AGENTS convention 3); poisoned locks degrade via unwrap_or_else(|e| e.into_inner()).
• Blocking file work lives in spawn_blocking inside backend impls (the alkgit trait-execution pattern); the store never blocks the executor.
• The sweep-vs-put window is an architectural mechanism — delete windows — specified in ADR-005, not an implementation note.

Invariants (the test gate)

1. Whole-blob put/get round-trips byte-identical under the canonical derivation, both tiers, both put paths, and every shipped engine of both tiers (kv: sqlite + postgres; fs: local + pg-lo; ADR-007/008/009) (interop with real git remains the source of truth, per POC #1's lesson about hardcoded vectors).
2. Dedup: putting identical content twice (same or different path) is one pool entry.
3. Stage hygiene: any failure mid-put leaves zero stage files; any success leaves exactly one committed entry.
4. Sweep safety: with correct liveness registered, sweep counts are exact; with aborting sources, sweep deletes nothing. Engine-gated form (ADR-008): single-node postures assert this under the in-process protocol; fleet postures (shared-pool engines) assert it under DB-backed pins + the advisory-locked sweep, with the exact-count sweep-outcome shape run against dockerized postgres under --all-features.
5. list() correctness is observable only through GC — list-related tests assert through sweep outcomes (POC #1 finding 2's lesson codified).
6. size/stat on any engine returns the entry length without fetching content (asserted per-backend: kv probe, fs stat, pg-lo companion-row probe — ADR-008/009).

Design Decisions

ADR	Decision	Summary
002	Hashing & keys	derivation + encoding this surface is built on
003	Dispatch	routing is a pure function of length; pre-threshold buffering
005	Pools & GC	pins, liveness seams, delete windows, sweep semantics
006	Verification	whole-blob checks; slice digests out-of-band
008	Trait size + fleet GC	stat/accounting ride the trait's size probe; fleet pins/sweeps are engine-backed

Open Questions

None owned by this document beyond the cross-references above.

References

• docs/research/poc-largeblob-findings.md findings A1/A5/A6, benchmark table
• docs/research/poc-trait-dispatch-findings.md findings 1–3, 7
• ADR-003 (put-path buffering), ADR-005 (pin/Batch/Pin lifecycle), ADR-006 (range-read semantics), ADR-002 (keys)
• ops-surface.md — the fanout/fetch consumer of the read path
• gc-and-namespaces.md — the lifecycle seam details