omnigraph/docs/branches-commits.md

Branches, Commits, Snapshots

L1 — Lance per-dataset branches

Lance supports branching at the dataset level: a branch is a named lineage of versions, and fork_branch_from_state(source_branch, target_branch, source_version) creates a copy-on-write fork.

L2 — Graph-level branches

OmniGraph builds graph branches on top by branching every sub-table coherently:

• branch_create(name) / branch_create_from(target, name) — disallowed name main; fails if branch exists; ensures the schema-apply lock is idle.
• branch_list() — returns public branches, filters internal __run__… and __schema_apply_lock__ prefixes.
• branch_delete(name) — refuses if there are descendants or active runs on the branch; cleans up owned per-branch fragments.
• Lazy forking: a branch only forks a sub-table when that sub-table is first mutated on it. Pure-read branches share fragments with their source.
• sync_branch(branch) — re-binds the in-memory handle to the latest head of the branch.

L2 — Commit graph (db/commit_graph.rs)

In-memory shape of a graph commit:

GraphCommit {
  graph_commit_id: ULID,
  manifest_branch: Option<String>,
  manifest_version: u64,
  parent_commit_id: Option<String>,
  merged_parent_commit_id: Option<String>,   // populated for merge commits
  actor_id: Option<String>,                  // joined in-memory from _graph_commit_actors.lance, NOT a column on _graph_commits.lance
  created_at: i64 (microseconds since epoch),
}

Storage is split across two Lance datasets (both with stable row IDs):

• _graph_commits.lance — every column above except actor_id.
• _graph_commit_actors.lance — optional separate (graph_commit_id, actor_id) map, created on demand. The actor_id field above is populated by joining this dataset in-memory at load time.

Notes:

• Every successful publish (load / change / merge / schema_apply) appends one commit.
• Merge commits have two parents; linear commits have one.
• API: list_commits(branch), get_commit(id), head_commit_id_for_branch(branch).

L2 — Snapshots & time travel

• snapshot() — current snapshot for the bound branch; cached.
• snapshot_of(target) — snapshot at a ReadTarget (branch | snapshot id).
• snapshot_at_version(v: u64) — historical snapshot from any manifest version.
• entity_at(table_key, id, version) — single-entity time travel without building a full snapshot.
• A Snapshot is a (version, HashMap<table_key, SubTableEntry>) — cheap to build, snapshot-isolated cross-table reads.

L2 — Internal system branches

Filtered from branch_list() but visible to internals:

• __schema_apply_lock__ — serializes schema migrations.
• __run__<run-id> — legacy from the pre-v0.4.0 Run state machine (removed in MR-771). The branch-name guard predicate is_internal_run_branch is kept as defense-in-depth so users cannot create a branch matching the legacy prefix; the filter will be removed once production legacy branches are swept (MR-770).

L2 — Recovery audit trail (MR-847)

The four migrated writers (MutationStaging::finalize, schema_apply, branch_merge, ensure_indices) protect their multi-table commits with a sidecar at __recovery/{ulid}.json written before Phase B and deleted after Phase C. The next Omnigraph::open (gated on OpenMode::ReadWrite) runs the recovery sweep in crates/omnigraph/src/db/manifest/recovery.rs: classify per-table state, decide all-or-nothing per sidecar, roll forward / back, record an audit row.

Audit rows live in _graph_commit_recoveries.lance (sibling to _graph_commits.lance) and reference the commit graph by graph_commit_id. The linked _graph_commits.lance row carries actor_id="omnigraph:recovery" (the system actor). To find recoveries for a specific original actor: omnigraph commit list --filter actor=omnigraph:recovery, then join to _graph_commit_recoveries.lance by graph_commit_id to read recovery_for_actor. Schema: see crates/omnigraph/src/db/recovery_audit.rs.