alkblobs/docs/architecture/README.md

status, last_updated

status	last_updated
draft	2026-10-03 (ADR-010/011 — I/O seams + GC-state home; vocabulary pinned, fs → large, mem joins kv)

alkblobs — Architecture

Current State

Phase 1 (Architecture). Phase 0 (Exploration) converged — see docs/research/phase-0.md for the vision, the evidence trail, and the POC register (all POCs passed or were absorbed; POC code lives in standalone crates under /workspace/ — alkblobs-trait-poc, alkblobs-largeblob-poc, alkblobs-postgres-poc, alkblobs-redb-poc, alkblobs-pglo-poc; findings live in docs/research/). No implementation exists yet (src/lib.rs is a placeholder).

The converged Phase 0 posture has been promoted into the specs and ADRs below. Two framing corrections from the convergence review are captured there — not inherited as hedges:

• ADR-001 makes the substrate posture explicit: the store layer carries no alkcall dependency by design — which is conformance to the family pattern, not deviation from it — and the ops surface rides alkcall, feature-gated.
• ADR-004 fixes the "two backends" framing: the crate ships exactly two tiers (kv small / large — both required by measured scale economics); the path→hash mapping layer (the alkfs/alknet "vfs" shape) is a consumer-layer concern and must never become a third backend.
• ADR-007 widens the kv tier's engine set rather than hedging it: sqlite (default) + postgres (feature-gated) behind one trait. The deployment economics this records: a downstream node (git server, vfs) classically needed three storage systems — kv + large content + relational — and this design collapses the three to two: one relational engine + large-tier storage, with the engine a per-node constructor choice. Resolved from evidence in hand (POCs #3/#5/#6); the would-be "deployment trigger" framing was caught at review as circular hedging (a deployment running alkblobs cannot pre-exist the engine) and closed on the OQ-09 precedent.
• ADR-008 (2026-10-03) repairs the seams ADR-007 stressed, against the pinned consumer requirement REQ-2 (requirements.md — a replicator node may run a fleet of store instances sharing one pg pool, a planning fact predating all POCs): the Backend trait gains a size probe (before any backend ships data); fleet GC gets DB-backed pins, an advisory-locked single sweeper, and SQL-level delete arbitration (single-node postures unchanged); and the large tier becomes engine-selectable (local default; pg-lo named candidate, admission-gated on POC #7). It also adds requirements.md — the REQ anchor that ends per-session re-derivation of consumer facts.
• ADR-009 (2026-10-03) admits pg-lo — postgres Large Objects as the large tier's second engine (feature pg-lo, default-off) — on POC #7's passed gate (docs/research/poc-pglo-findings.md): contract 10/10 exact-count tests, durable put ≈ fs durable put at ≥1 MiB, descriptorless lo_get window gets, transactional LO lifecycle (zero crash orphans), companion-table contract authority. REQ-2's fleet consolidation option is now shipped: a fleet node can run postgres-only. Named deltas: cached-get 20–50× behind page-cache fs single-stream (aggregate serving ~700 MB/s at 16 readers), catalog space reused-but-never-returned, autovacuum inherited. The admission door (ADR-003, exercised by ADR-007 and ADR-009) has now shipped both its tier-extending engines.
• ADR-010 (2026-10-03) pins the Backend trait's I/O seams — the last unpinned signatures before implementation: get returns a crate-internal read cursor (kv engines materialize; large-tier engines stream — local's pread loop, pg-lo's lo_get windows), put has two named forms (whole-value for kv, staged put for large — stage-then-commit-rename promoted from behavior to contract); rules that GC state is store-core-owned (backends never learn liveness — ADR-005 verbatim) hosted by SQL-backed engines via a crate-internal contract-tested engine-state companion seam (schema/pin/window/lock clauses), rules the store core holds the joint entry+pin transaction (correcting ADR-009 §2's ownership statement), adds the kv tier's fleet-validity rule (mirror of ADR-008 §3 — postgres is the fleet engine; a sqlite-fleet posture is not on offer), resolves the ops pin-token TTL race (the token is the pin; renewal rides have/re-put; pause-past-TTL falls to delete-then-recover), and pins the feature graph (pg-lo does not imply postgres).
• ADR-011 (2026-10-03) pins the vocabulary: tier (contract + routing class — exactly two) vs engine (concrete impl — exactly five: kv's sqlite/postgres/mem, large's local/pg-lo); mem demoted from "backend"/"testing tier" to the kv tier's third engine — the contract-reference engine; store instance / node / fleet split so REQ-2's "one node may run multiple instances" phrasing is no longer self-contradictory (fleet = pool-sharing, not node count); and the large tier is renamed large (feature fs → large) — its routing criterion is length, not medium — with constructor modes (dual-tier default, kv-only, mem-only) pinned in the same ADR's table.

Architecture Documents

Document	Scope	Status
overview.md	Purpose, consumers, layer map, dependency posture	draft
requirements.md	Consumer requirements REQ-1..4, pinned vocabulary (tier/engine/instance/node/fleet)	draft
hashing-and-keys.md	Canonical hash, key encoding (the one-way door)	draft
store-api.md	Store facade: put/get/stat/range/pin/batch/sweep surface, errors, invariants	draft
backends-and-dispatch.md	Backend trait contract, tiers + engines table, size-threshold dispatch, constructor modes	draft
gc-and-namespaces.md	Pooled CAS, liveness seams, mark-and-sweep, delete windows	draft
ops-surface.md	alkcall-backed have/need + fetch/put ops, ACL mapping	draft
open-questions.md	Centralized OQ tracker (incl. promoted Phase 0 register)	draft

Decision Records

ADR	Decision	Status
001	Substrate posture & ops-surface placement (conformance, not deviation)	Accepted
002	Canonical hash + key encoding (the wire-format ADR; precedes any consumer)	Accepted
003	Backend contract, shipped backends, dispatch policy	Accepted
004	Exactly two backends; manifest layers stay above	Accepted
005	Pooled CAS, liveness seams, mark-and-sweep, delete windows	Accepted
006	Whole-blob verification; transfer encoding excluded by scoping	Accepted
007	Two kv engines (sqlite + postgres) behind one trait; constructor-selected	Accepted
008	Trait size probe; fleet GC (DB-backed pins, advisory-locked sweeper); large-tier engines	Accepted
009	pg-lo — postgres Large Objects as the large tier's second engine (admitted on POC #7)	Accepted
010	Backend I/O seams (read cursor, staged put); GC state store-core-owned via the engine-state seam; kv fleet-validity; pin-token renewal	Accepted
011	Vocabulary: tier/engine/instance/node/fleet; mem joins kv; large tier renamed large; constructor modes	Accepted

Open Questions

Tracked in open-questions.md. The Phase 0 register (OQ-BL-01..06) is promoted there with its resolutions; two questions remain parked — OQ-07/OQ-08 externally-owned (alkgit seam mapping, alkfs intake; carried for visibility, gating nothing here). OQ-09 (namespace-visibility default) resolved closed-by-default; OQ-10 (second kv engine) resolved by ADR-007. The pg-lo large-engine question resolved to sequenced work, not a parked question: REQ-2/ADR-008 named it the candidate large-tier engine and POC #7 (docs/research/poc-pglo-spec.md) was its admission evidence — a task, not a wait; that task completed 2026-10-03 (docs/research/poc-pglo-findings.md, passed) and ADR-009 shipped the engine. Deferral policy (the "Schrödinger's code" rule): a decision this crate needs before shipping may not be deferred on a dependency that is itself waiting for this crate to exist. Legitimate parked kinds: deferred(scope) on a deciding fact that exists independently of this crate, and externally-owned questions (how a consumer maps onto this crate) that gate no decision here — full definitions in the header of open-questions.md. A third category is not a parked kind: decided-but-sequenced work (a resolved decision whose admission gate requires evidence — the pg-lo round's POC #7 is the worked example; a task with an owner, not a parked question). A corollary worth restating, because it recurs: a fact this crate must create (its own first deployment, its own first consumer) is never a deciding input — decisions stand on evidence in hand, and future needs reopen via named requirements, not via waiting. (Corollary applications so far: OQ-09 and OQ-10 both resolved on it.)

Lifecycle

Spec docs: draft → reviewed → stable → deprecated. A doc moves reviewed when every crate-owned open question it references resolves and an architecture review pass clears it — externally-owned OQs (OQ-07/OQ-08) gate nothing here by definition and do not block the lifecycle; their outcomes arrive through their owners' processes. stable when implementation verifies against it; deprecated when superseded (kept for reference). ADRs use a separate status set (Accepted | Proposed | Deprecated | Superseded), defined per ADR file. This tree is in draft pending the first architecture review cycle.