alkblobs/docs/architecture/backends-and-dispatch.md

---
status: draft
last_updated: 2026-10-03 (ADR-010 I/O seams + ADR-011 rename — fs tier → large; mem is a kv engine)
---

# Backends, tiers, and dispatch

## What this is

The physical storage layer: the `Backend` trait contract every engine
implements, the two shipped tiers (kv, large — the tier formerly named
`fs`; renamed in ADR-011), and the size-threshold dispatch that routes
between them. The trait is the crate's second stable seam (with the
key encoding, ADR-002): engines must survive digest-layer evolution,
so the boundary stays dumb.

Vocabulary is pinned in requirements.md (ADR-011): **tier** = contract
+ routing class (exactly two; ADR-004), **engine** = concrete impl of
one tier's contract (exactly five), and this document's table is the
spec home of the constructor table (ADR-011 §4).

## What the Backend trait is (and is not)

The trait is a **CAS-tier contract**: it accepts, serves, and enumerates
immutable content-addressed entries (`hash → bytes`), and its `list()`/
`delete()` shapes exist to feed GC (ADR-005). That is its whole job, and
the contract assumes it: complete-list-equals-sweep-safety only makes
sense for a store where every entry is a standalone immutable blob.

It follows that the trait is *not* a general-purpose storage abstraction,
and a downstream must never implement it to host data that is not
content-addressed blobs — a manifest table, a queryable index, or any
mutable/structured state. Such storage is **consumer-owned, beside the
pool** (a downstream's own sqlite/whatever, ADR-004): it needs no
digest-byte addressing, no `list()`-completeness, and no sweep
participation, so the CAS-tier contract fits it worse than it fits any
purpose-built option. The wrong turn to rule out is "my data isn't
blobs, therefore I implement a Backend for it"; nothing a consumer needs
is ever reachable through that door.

## The Backend trait contract (ADR-003; I/O seams per ADR-010)

- **Opaque byte keys, opaque byte-valued entries.** Engines never learn
  what a digest is; typed `Key` converts at the store boundary
  (ADR-002). This is also the persistence boundary — a kv row, a
  `local` filename, or an LO companion row has no schema-migration story, so
  the byte layout must be self-describing (algorithm-tagged keys).
- **Methods:** `has` / `get` / `put` / `delete` / `list` / `name` /
  `size` (the length probe added by ADR-008). `size(key)` returns the
  entry's length or `None` if absent — metadata only, never a content
  fetch: kv engines probe the indexed `size` column both row shapes
  carry (POC #5 B5); `local` stats the file; pg-lo reads the companion
  row. Store-level `stat` and pool accounting ride this method.
- **The I/O seams are crate-internal abstractions, not engine shapes**
  (ADR-010 §1 — the gap that would otherwise force guessing):
  - `get` returns a **read cursor** the store core drives. kv-engine
    cursors materialize the whole value (bounded by tier policy);
    large-engine cursors stream — `local`'s is a pread loop over the
    committed file, pg-lo's drives descriptorless `lo_get(oid, off,
    len)` windows (ADR-009 §3, the C4 posture). Both serve
    `read_range` (slice + slice digest, ADR-006) at one seam. The
    cursor is not public API: store consumers see the facade's
    `(len, stream)` shapes with the cursor behind them.
  - `put` has **two forms**: the **whole-value put** (kv tier — the
    settled CAS row write) and the **staged put** (large tier —
    ADR-003's stage-then-commit-rename as a named per-engine
    contract: the engine hands out a *staging cursor*; commit
    publishes; every failure path converges on discard with zero
    residue). Both land in the CAS entry shape; both participate in
    pin-before-publish (ADR-005/010 §2 — the store core holds the
    joint entry+pin transaction via the engine-state seam).
  - Per-engine conformance: an engine is admitted only if its
    cursor/stage/list bodies pass the uniform contract suite — the
    POC #7 gate (10/10 exact-count sweep-outcome tests) is the norm;
    suite details in store-api.md's invariants.
- **`list()` complete by contract.** A *malformed* list is as deadly as
  an incomplete one: list correctness is only observable through GC
  (POC #1 finding 2 — the redb key-vs-value trap produced a sweep that
  deleted the wrong blobs, silently). Therefore:
  - every engine implementation must prove list correctness through
    sweep-outcome tests (the invariant's test gate lives in
    store-api.md);
  - an implementation that cannot enumerate (the rudolfs S3
    anti-lesson) must not ship — GC is a structural requirement, not an
    optional extra (ADR-005).
- **Virgin-store reads are no-ops:** read paths treat a missing table
  as absent/empty (POC #1 finding 2).
- **Namespace-blind:** engines never see namespaces, tenants, or
  reference structure (ADR-005 — the rudolfs inversion; physical
  storage is `hash → bytes` flat).
- **GC state never rides this trait** (ADR-010 §2): pins, window rows,
  and sweep locks are store-core-owned via a crate-internal
  engine-state companion seam on the SQL-backed engines (sqlite,
  postgres, pg-lo) — invisible at the public trait; `local` and `mem`
  cannot express it and are never fleet engines.

## Shipped tiers and engines

Two tiers, exactly — this counts *trait implementations this crate
ships as CAS tiers* (the contracts dispatch routes between); it is the
complete set the problem requires, and the "third backend" fear is a
category error fixed in ADR-004 (tier count is not engine count). It
says nothing about other storage existing in a deployment: a downstream
runs whatever else it needs on its own media, beside the pool, above
the store's seams (see "What the Backend trait is").

| Tier | Engine | Feature | Default | Fleet-valid |
|---|---|---|---|---|
| kv | sqlite | — (tier default) | yes (tier default-on) | no |
| kv | postgres | `postgres` | no | **yes** |
| kv | mem | `mem` | no | no (test/reference engine) |
| large | local | — (tier default) | yes (tier default-on) | shared media only, explicit assertion |
| large | pg-lo | `pg-lo` | no | **yes** |

### kv tier (tier default-on, feature `kv`); engines sqlite (default) + postgres (feature `postgres`, default-off) + mem (feature `mem`, default-off)

Small blobs — **`kv` is the tier name; the engine is a per-node
constructor choice** between the shipped engines (ADR-007; ADR-003
carries sqlite's on-disk format trade: file compat is upstream
sqlite's guarantee). The shipped trait impls write plain
content-addressed rows into the chosen engine, nothing else (ADR-010:
the fleet state rides the companion seam, never the public trait). A
*third* non-mem engine remains the substitution door as amended: a new
ADR with its own sweep-safety proof (the door is exercised — ADR-007
in-repo); and a downstream's *other* storage — schemas, manifests,
whatever it likes including some other sqlite file — never comes near
this tier at all, per "What the Backend trait is".

The collapse ADR-007 buys: the classical downstream stack (git
server, vfs node) needed three storage systems — kv + large content + relational.
Here the kv tier rides the relational engine itself, so a node
provisions **one relational engine + large-tier storage**; consumer
tables (refs, manifests, queues) sit beside the pool on the same
engine or file by the deployment's own choice (co-tenancy note: same-
file consumer writes share sqlite's single-writer ceiling — that
sharing is the node's trade; separate files remain available).

Evidence (POC #3 finding A4, first-party measured): sqlite is ~9-10×
faster than fs at 1-16 KiB (the git small-blob regime — most git
objects, workspace files, manifests), with the crossover at ~128-256
KiB where fs stops paying the B-tree row rewrite and wins.

**sqlite engine** (the default): WAL + `synchronous=NORMAL` as the
shipped durability tier (matching what the benchmark measured and what
iroh's store ships); bounded reads; `has` as an EXISTS probe; prepared
statements; read paths tolerate the no-tables-yet database (see
contract). Solo economics win the single-machine case by measurement;
the WAL single-writer ceiling (~1.2k objects/s under contention,
POC #5 B3) is an order above single-node push rates.

**postgres engine** (feature `postgres`, default-off; ADR-007): the
cross-machine-writers / many-client-node / already-running-pg engine.
Contract-identical row shape (`key bytea PK / value bytea / size`),
`ON CONFLICT DO NOTHING` CAS, complete `list()` as a stable cursor
over a vacuuming table, batch-delete via `key = ANY($1)`. Impl
requirements per POC #5 finding B5: configurable `synchronous_commit`
posture (the parity knob with the sqlite shipped tier), pooled
connections with per-connection prepared-statement discipline, stated
autovacuum + fillfactor tuning, ~40× small-tier storage overhead
accepted (B6). Sweep-safety proof via the same exact-count
sweep-outcome CI gate, run against dockerized postgres under
`--all-features`. Concurrency evidence (POC #5 B3/B4): gets/puts scale
near-linearly to ~37k puts/s at 24 conns — the only engine whose
throughput *increases* under concurrency.

**mem engine** (feature `mem`, default-off; ADR-011 §2 — formerly
described as the "mem backend"/"testing tier" before the vocabulary
was pinned): a `BTreeMap`-shaped ephemeral implementation of the kv
tier's contract, including `size` (trivially). For tests and
in-process ephemerality — never a production story, never
fleet-valid, and the **contract-reference engine**: every other
engine's conformance tests mirror its suite (the role it already
played in POC #1's miniature).

### large tier (tier default-on, feature `large`; the tier formerly named `fs` — renamed in ADR-011); engines `local` (default) + `pg-lo` (feature `pg-lo`, default-off)

Large blobs. Like the kv tier (ADR-007), the large tier is **one
contract, multiple engines** — the medium is a per-node constructor
choice (ADR-008; REQ-2 in requirements.md makes the engine concept
load-bearing for fleets):

- **`local` engine** (the default; the ADR-003 backend as originally
  specced): flat sharded layout:
  `{hex-prefix}/{hex-prefix}/{hash}` sharding survives from iroh's
  conclusion (limits directory size on huge pools); stage-then-commit-
  rename for the two-pass unknown-length path (the staged-put form,
  ADR-010); pread-based range reads (POC #3 finding A2 — local range
  serving is sound, e.g. for packfiles).
- **`pg-lo` engine** (shipped, feature `pg-lo`, default-off; ADR-009):
  postgres Large Objects as the large tier's storage — the same
  engine-behind-one-trait move as ADR-007, one tier over, admitted on
  POC #7's measured evidence (contract 10/10 exact-count tests;
  durable put ≈ fs durable put at ≥1 MiB, beats it <1 MiB on the POC
  disk; gets ride descriptorless `lo_get(oid, off, len)` windows over
  the companion table as contract authority; LO creation
  transactional — zero crash orphans). Named deltas: catalog space
  reused-but-never-returned (monitor rel size; `VACUUM FULL` is the
  operator's shrink path), autovacuum inherited, cached-get 20–50×
  behind page-cache fs single-stream (~700 MB/s aggregate at 16
  readers — the fleet serving picture).
- **Fleet locality contract (ADR-008, extended by ADR-009/010):** over
  one shared pool, the large tier is one of: shared media (every
  node's `local` root on the same fleet-shared media — soundness
  caveats and the required media guarantees in ADR-008), re-routed
  (one storage node serves pool large-blob content via the ops
  surface; client nodes are kv-only over pool content), **or
  `pg-lo` for the whole fleet** (ADR-009 — one engine, no shared
  media, no re-routing; pool content lives in the same pg instance
  the kv tier rides; the joint entry+pin tx is store-core-held,
  ADR-010 §2). Mixed per-node-local large tiers over one pool is the
  partitioning failure this contract exists to prevent — a documented
  deployment invariant the constructor cannot fully prove but must be
  declared against (ADR-008 names the enforceable seam and the
  detection symptom). The kv tier has the mirrored rule (ADR-010 §3):
  a fleet constructor requires a pool-shared kv engine (postgres);
  two sqlite engines are two pools, never one.
- **The composite fleet-validity rule (one sentence, both tiers):** a
  constructor configuration is fleet-valid iff **kv = postgres AND
  (large = pg-lo OR large = `local`-on-declared-shared-media OR
  large = none with the re-routing posture carrying pool large
  content)**. Any other combination over one shared pool is invalid —
  the constructor requires the declarations that make this predicate
  checkable per node; cross-node truth remains the deployment's
  verified invariant (ADR-008's seam). This predicate is
  backends-and-dispatch.md's and ADR-011 §4's table combined; it is
  stated here once so no reader composes it by inference.

## Size-threshold dispatch (ADR-003)

- **Routing is a pure function of content length.** Same content ⇒ same
  length ⇒ same tier; re-puts are deterministic. No content ever
  migrates between tiers — the migration question existed only to
  patch the unknown-length asymmetry, which ADR-003's pre-threshold
  buffering eliminates by construction.
- **Default threshold: 128 KiB** (constructor-tunable) — the bottom of
  the measured crossover zone (~128-256 KiB, POC #3 A4; chosen at the
  zone's conservative edge, not its midpoint). Re-tuning per
  deployment media is a constructor parameter, not an API change.
- **Get fall-through:** small-tier miss queries the large tier
  (deterministic, cheap — a stat probe).
- Per-namespace or per-tenant engine configuration: **rejected**
  (ADR-003 §Consequences — it would re-weld namespacing into the
  physical layer, the rudolfs anti-pattern ADR-005 inverts).

### Constructor modes (ADR-011 §4)

- **dual-tier** (the default): kv + large, size-threshold dispatch
  (ADR-003).
- **kv-only** (ADR-008's re-routing client posture, now named): no
  large tier; over-threshold puts are a constructor-time error (the
  re-routing posture handles them via the ops surface — the
  consumer's composition, not a store mode).
- **mem-only:** the mem engine alone, no dispatch threshold in
  effect; a testing/embedder-ephemeral posture, never production.
- Single-tier SQL modes (postgres-only, pg-lo-only *tiers*) do not
  exist: both tiers are load-bearing (ADR-004). "Postgres-only" as in
  one SQL *instance* serving both tiers exists and is the ADR-009
  consolidation: dual-tier mode with kv=postgres + large=pg-lo over
  one pool.

## Where a *new* engine could come from

The trait is open to future implementations (network stores, S3-like
tiers), but nothing in the current consumer set requires one, and the
contract is deliberately hostile to half-implementations (complete
`list()`, GC-participating `delete`). Any future engine — per tier —
is a new ADR carrying its own sweep-safety story (ADR-007 is the
precedent for a kv engine addition; ADR-008 named pg-lo and ADR-009
admitted it with POC #7's evidence — the door is exercised twice).
This crate's roadmap is not blocked on one (see open-questions.md —
alkfs intake may name needs externally; OQ-08).

## Design Decisions

| ADR | Decision | Summary |
|---|---|---|
| [003](decisions/003-backend-contract-and-dispatch.md) | Backend contract & dispatch | opaque keys, complete list, pure-function routing, no migration |
| [004](decisions/004-two-backends-no-third.md) | Two tiers, no third | scope boundary against manifest-layer absorption |
| [005](decisions/005-pooled-cas-and-gc-mechanism.md) | Namespace-blindness | engines see hashes only |
| [007](decisions/007-two-kv-engines-sqlite-and-postgres.md) | Two kv engines | sqlite (default) + postgres behind one trait; one relational engine + large tier per node |
| [008](decisions/008-trait-size-probe-fleet-gc-and-large-engines.md) | `size` probe + fleet GC + large-tier engines | trait length probe; DB-backed pins/advisory-locked sweeper for fleets; large tier engine-selectable (`local` default) |
| [009](decisions/009-pg-lo-large-tier-engine.md) | pg-lo admitted | postgres Large Objects as the large tier's second engine, on POC #7 |
| [010](decisions/010-backend-io-seams-and-fleet-state-home.md) | I/O seams + GC-state home | read cursor / staged put; store-core-owned GC state via the engine-state seam; the kv fleet-validity rule |
| [011](decisions/011-vocabulary-tiers-engines-and-fleet-state.md) | Vocabulary | tier/engine/instance/node/fleet; mem is a kv engine; fs → large; constructor modes |

## Open Questions

- **OQ-08**: alkfs requirement intake may name storage requirements
  (e.g., durability tiers, sync-friendly layouts) that touch this
  layer — open, external owner (alkfs Phase 0).

## References

- `docs/research/poc-trait-dispatch-findings.md` findings 1/2/6
- `docs/research/poc-largeblob-findings.md` findings A2/A4 (+ the
  re-runnable benchmark harness)
- `docs/research/poc-postgres-kv-findings.md` — the pg arm's measured
  curves + engine-posture deltas (ADR-007's evidence base)
- `docs/research/poc-redb-kv-findings.md` — redb ruled out at a
  durability-tier mismatch (POC #6; recorded in ADR-007)
- `docs/research/poc-pglo-findings.md` — pg-lo's admission evidence
  (POC #7; ADR-009)
- `docs/research/iroh-blobs-eval.md` — the fs-layout conclusions borrowed
  (sharding, crash ordering, inline thresholds rejected as weld)
- rudolfs notes — `list()` anti-lesson, decorator alternative noted and
  not adopted (threshold dispatch chose the simpler policy;
  ADR-003 §Context)
- ADR-003, ADR-004, ADR-005; [store-api.md](store-api.md) (the invariants
  engines must satisfy); [requirements.md](requirements.md) (the pinned
  vocabulary)