alkblobs/docs/architecture/overview.md

status, last_updated

status	last_updated
draft	2026-10-02

Overview

What this crate is

alkblobs is content-addressed blob storage in the alk* family: a put/get/verify store keyed under one canonical content hash (the git-family derivation; legacy SHA-1-tolerant), with pluggable backends (small blobs in a kv store, large blobs on a filesystem fallback), split from any wire/ops-protocol layer. Network ops ride the alkcall substrate behind a feature-gated ops module (ADR-001).

Why it exists — the extracted shared core

Two downstream projects were converging on the same storage stack, so it is being built once, here:

• alkgit was paused mid-Phase-1 (its architecture is reviewed, docs/architecture/ in the alkgit repo) to stop duplicated storage-layer work. Its object backend needs git's own oid derivation, a better small-object backend than its proposed default file layout, and a git-lfs-shaped large-blob story. Under the canonical-hash resolution (ADR-002) git objects are first-class pool entries with zero indirection.
• alkfs (Phase 0 pending) needs the appfile/workspace shape: small content in a fast kv store, large content on the filesystem, path→hash mapping above. The path-tree/manifest layer is alkgit's and alkfs's, not this crate's (ADR-004).

Both demanding consumers (p2p git replicators, agent workspaces) reduce to the same shape: a pooled CAS per node, where repos and workspaces are sets of hash references, not walled-off object stores — cross-repo dedup by construction (ADR-005). That pooling property is why the storage layer was pulled out of both consumers rather than built twice or three times.

Layer map

┌────────────────────────────────────────────────────────────┐
│ consumers (above this crate)                               │
│   alkgit object storage · alkfs path trees/workspaces      │
│   refs, manifests, path→hash maps, git semantics           │
└────────────┬───────────────────────────────────────────────┘
             ▼
┌────────────────────────────────────────────────────────────┐
│ ops surface   (feature "ops"; ADR-001)                     │
│   have/need + verified fetch + put; JSON control ops,      │
│   binary data channels; ACL via alkcall AccessControl      │
└────────────┬───────────────────────────────────────────────┘
             ▼
┌────────────────────────────────────────────────────────────┐
│ store core    (always on)                                  │
│   hashing & keys (ADR-002) · put/get/stat/range/pin seams  │
│   mark-and-sweep GC + liveness registration (ADR-005)      │
│   size-threshold dispatch (ADR-003)                        │
└───────┬───────────────────┬───────────────────────────────┘
        ▼                   ▼
   kv backend           fs backend        (ADR-003: exactly two)

   transport: none, anywhere. alkgit/alkfs or the embedder dials;
   an alkcall Connection is handed in at the ops boundary.

Dependency posture

Layer	Dependencies	Notes
base crate (store core + backends)	tokio, thiserror, parking_lot, plus rusqlite behind the kv feature and nothing behind fs/mem	no alkcall, no serialization frameworks
ops feature	adds alkcall (feature-gated, default-off)	JSON op specs + binary channels + AccessControl; ADR-001
consumer crates	depend on alkblobs (and on alkcall directly when they speak ops)	alkgit, alkfs

Backends are feature-gated; both kv and fs ship default-on (batteries-included, the alkgit feature-model pattern); the ops feature is default-off. Wasm: the backends are not a wasm story; a wasm client is a consumer of the ops surface (which rides alkcall, itself wasm-clean), never an in-process embedder (ADR-001 §Consequences).

Non-goals (each enforced by an ADR)

• No wire framing, op dispatch, ACL, or transport inside the store layer (ADR-001).
• No adopted iroh-blobs wire surface — no tickets, no postcard, no provider protocol (AGENTS convention 7; ADR-001 §Context).
• No namespaces-as-physical-storage; the pool is flat, namespaces are consumer-level reference sets (ADR-005).
• No manifest/path-tree/vfs tables; consumers never implement a Backend to model paths (ADR-004).
• No chunk-tree/CDC storage encoding; whole-file CAS is the store's only granularity (ADR-006).
• No per-repo or per-namespace walled stores — one pooled CAS (ADR-005).

Design Decisions

ADR	Decision	Summary
001	Substrate posture & ops placement	store stays substrate-free; ops ride alkcall feature-gated
002	Canonical hash + key encoding	git-blob-sha-256 canonical; algorithm in-key; one-way door
003	Backend contract & dispatch	two backends, size-threshold routing, no migration
004	Two backends, no third	manifest layers are consumer-side
005	Pooled CAS & GC	flat pool, registered liveness, delete windows
006	Verification posture	whole-blob verification; chunk trees excluded by scoping

Open Questions

Tracked in open-questions.md. Ones affecting the crate level:

• OQ-07: how alkgit's object-storage seam consumes alkblobs (externally-owned: alkgit's architecture process)
• OQ-08: alkfs requirement intake (externally-owned: alkfs Phase 0)
• OQ-09: default namespace visibility in network ops — resolved closed-by-default (deny-unlisted; reopen only on a named open-read requirement)

References

• docs/research/phase-0.md — the converged Phase 0 record; inputs to every ADR above
• docs/research/poc-trait-dispatch-findings.md, docs/research/poc-largeblob-findings.md, docs/research/iroh-blobs-eval.md — the evidence trail
• docs/sdd_process.md — the process governing this tree
• /workspace/@alkdev/alkgit/docs/architecture/backend.md — the reviewed consumer-side seam that OQ-07 maps onto alkblobs
• AGENTS.md — family conventions (conventions 4–8 gate the decisions)