alkgit/docs/architecture/backend.md

status, last_updated

status	last_updated
draft	2026-09-21

Backend traits: the storage seam

What this is

The payload side of the protocol crate: the traits a service deployer implements (or consumes via the feature-gated gix implementation) so the git protocol can talk to storage. Per ADR-010 this mirrors alktty's TtyBackend pattern — traits in-crate, real implementation behind a feature. Unlike alktunnels (no backend trait), git needs the seam: pack generation/ingestion is too heavy to hard-wire.

The trait family (ADR-010 sub-decision 4, ADR-011/012)

Five traits, kept small and orthogonal — the protocol crate never sees gix types:

1. GitRegistry — repo id → RepoRecord (storage root, visibility, grants keyed on the stable logical identity id — ADR-011). Async resolve; the authoritative mapping (ADR-008); resolution failure is indistinguishable from authorization failure (ADR-007). Authorization is alkgit-core's policy function evaluated on the record (data in the backend, policy in core — an embedder with a foreign permission system maps its ACL into grants at resolve time).
2. GitRegistryStore: GitRegistry — the write supertrait (put_repo/update_repo/remove_repo; alknet ADR-035's read/write split shape). The git/repo/* ops wrap it (ADR-012 §3). Grants ride the record, so grant mutation is update_repo (convenience wrappers additive later, two-way).
3. GitRefs — listing for advertisement (refs + peeled tags + symref targets, the ls-refs response data) and ref transactions (CAS apply for receive-pack, name validation per git ref rules + reserved- namespace deny-list).
4. GitPackGen — (repo, wants, haves, limits) → streaming pack (io::Write consumer). Negotiation-agnostic. Missing objects abort with an error, never a broken pack (ADR-004).
5. GitPackIngest — client pack stream → indexed pack + fsck/ connectivity report. It prepares the validated ref updates; the transaction itself is applied by GitRefs (single CAS home — ingest validates, refs commits). Budgeted (ADR-009 max pack size); blocking-thread friendly.

Minimal-vs-full was the open sub-question; resolved as full family — the four traits are each one or two methods plus types, and collapsing them (e.g. refs into the registry) would force one impl block per downstream where independent seams are cheaper to satisfy. The gix feature implements all four; a downstream with its own object store implements 3–4 and reuses 1–2, or none of it.

Feature model (ADR-012 §4, amends ADR-010's single-gix story)

Two independent seams, two default-on features:

Feature	Contents	gitoxide
gix	GitRefs/GitPackGen/GitPackIngest impls — the local-disk object-storage engine (odb/pack/ref/object/fsck components)	yes
registry-file	GitRegistry/GitRegistryStore default impl (per-repo record files + in-memory index, config-seeded, op-mutable, atomic writes)	no

• default-features = false — wire/protocol layer only (an embedder brings its own backends). Both features default-on (batteries included); they are independent seams — DB-registry + gix-engine and S3-storage + simple-registry combinations both exist downstream.
• The gix facade crate is not a dependency (ADR-012 §4): the backends drive gix-odb/gix-pack/gix-ref at the component level (POC-2's shape); repo opening from a registry-resolved root is gix-discover + gix-odb::Store::at. Verified at implementation.
• The registry-file store needs no gitoxide; persistence adapters (SQLite et al.) are future, separate, additive — ADR-012 §2.
• Hash: sha1 pinned (the compile-time-rejected invariant from docs/research/gitoxide.md); sha256 passthrough feature (OQ-05 policy unchanged).
• Encodes the POC-2 prerequisites by construction: odb handle sharing (Arc<Store> shared, per-session handles, prevent_pack_unload() + ignore_replacements = true), generation on blocking threads, O(counts) memory, missing-objects abort.
• Received-pack ingestion via gix-pack::data::input (streaming-input)
  • gix-fsck + gix-ref transactions (ADR-004). Validation against real git push is OQ-04.

Concurrency model

• gix structures: parking_lot short-held locks; per-session handles moved into spawn_blocking tasks (POC-2's shape: store shared, handle per session, generation on blocking threads).
• Poisoned locks: unwrap_or_else(|e| e.into_inner()) (convention 2).
• The traits are Send + Sync object-safe; impls run under the adapter's tokio context. GitRegistry::resolve is async (one call per session/request, before the first protocol byte — ADR-011 §6); no sync-hot-path constraint exists here, unlike alkcall's accept loop.

Public API surface

Crate-root re-exports (the alktty pattern): backend traits + types (including RepoRecord, AccessAction, the authorize policy function, and the git/repo/* op spec+handler pairs), GitAdapter/ register_openable (producer), GitSession (consumer), substrate types, Limits, protocol error enums; feature types (GixBackend family under gix, the file registry under registry-file) exported under their features. The embedder-facing freeze point remains OQ-03 (narrowed: it is now this crate's own publish).

The two op kinds (ADR-012 §3, the classification)

alkgit ships both alkcall op kinds from one crate — the first family payload to do so (alktty/alktunnels: open ops only; alknet-docker, planned: call ops only):

• Open op (alk/git via register_openable; repo id in the open-op params) — the binary half: negotiation + ACL point (ADR-007), yields the duplex git session.
• Call ops (git/repo/{create,delete,update,get}) — the JSON half: thin OperationSpec+handler pairs over Arc<dyn GitRegistryStore>, Visibility::External, always-on (no gitoxide), gated by required_scopes (global tier) + ownership (self-owned tier, alkcall ADR-011 mint-and-own at create). Registered by the assembler wherever it wants them exposed. Managing records (op ACL) and git access (repo grants) are different capabilities — ownership never implies git access (ADR-011, ADR-012 §3).

Design Decisions

ADR	Decision	Summary
004	Pack pipeline	data::output gen / data::input ingestion
007	ACL first	registry returns rule inputs
008	Repo identity	wire names are registry ids
009	Budgets	limits flow into gen/ingest
010	Pure protocol crate	traits in-crate, impls behind features
011	Per-repo authorization	grants in records, policy in core
012	Registry + ops	write trait, file default, CRUD ops, feature split

Open Questions

• OQ-04: pack ingestion validation (deferred(unclear)).
• OQ-05: sha256 policy (deferred(scope)).
• OQ-03: publish-time API freeze inventory (the op set enters it).

References

• docs/research/gitoxide.md (API contract notes — normative for the gix impl)
• docs/research/poc2-findings.md (generation pipeline + prerequisites)
• alktty backend.rs/local module (the trait + feature template)
• alknet ADR-033/035 (the repo/adapter + read/write-split pattern)
• ADR-010 (the structural decision), ADR-011/012 (the seam this doc specifies)