omnigraph/docs/dev/invariants.md

Architectural Invariants

Type: standing review checklist Status: living document Audience: anyone proposing, reviewing, or implementing an OmniGraph change

This file is intentionally short. It records the rules that should be in working memory for every non-trivial change. Detailed mechanics live in the area docs linked below.

Use it this way:

• Review the change against Hard Invariants and the Deny-list.
• If code and docs disagree, either fix the code or add/update a Known Gap.
• Keep implementation ledgers, roadmap detail, and historical MR notes in the per-area docs. This file is the filter, not the encyclopedia.

Governing principle: logical contract over physical state

The hard invariants below are instances of one rule. Keep it in view whenever a change touches the boundary between what the graph means and how it is physically stored.

Logical state is the contract. Physical state — index coverage, fragment layout, compaction versions, staged writes — is derived, rebuildable, and may be produced asynchronously. A physical operation must never fail a logical one. Preconditions are checked against logical state; physical reconciliation is idempotent and may lag or retry. Genuine logical conflicts still fail loudly: the licence to lag covers physical convergence, not correctness.

Invariants that instantiate it: 2 (manifest-atomic visibility) and 5 (recovery is part of the commit protocol) — a partially-written physical layer never changes what a graph commit means; 7 (indexes are derived state) — a query is correct under partial index coverage, and expensive index work converges from manifest state instead of gating the write path; 13 (failures bounded and observable) — the licence to lag is not a licence to drop, so a physical step that cannot make progress is surfaced, not swallowed. Deny-list items that enforce it: synchronous inline vector/FTS index rebuilds on the commit path; state that drifts from Lance or the manifest when it can be derived; job queues for manifest-derivable state where a reconciler fits.

The failure shape it rules out: a legitimate background operation on the physical layer (compaction, an index build, an interrupted staged write) is allowed to break a logical operation (a query's correctness, a migration's success, a branch's writability). The smell to watch for is a logical operation whose precondition is a physical fact — a cached file version, an index's existence, a fragment count. Make the precondition logical and let a reconciler converge the physical state.

Hard Invariants

1. Respect the substrate. Lance owns columnar storage, per-dataset versioning, fragments, branches, compaction, cleanup, and index primitives. DataFusion should own relational execution where it fits. Do not add custom WALs, transaction managers, buffer pools, page formats, or local clones of substrate behavior. Read lance.md before guessing. Respecting the substrate also means using it idiomatically, not only refraining from rebuilding it: reuse long-lived handles instead of re-opening per call, resolve latest state through the substrate's cheap primitive instead of re-scanning, and share its caches/session. Re-deriving per call what the substrate keeps warm is a substrate violation even when no code is reimplemented.

2. Graph visibility is manifest-atomic. Lance commits are per dataset. OmniGraph's graph-level atomicity comes from publishing one manifest update for the whole graph, guarded by expected table versions and sidecar recovery. No write path may make a subset of touched node/edge tables visible as a graph commit.

3. A query reads one snapshot. Query execution captures a manifest snapshot for its lifetime. Do not re-read branch head mid-query to discover newer table versions.

4. Mutations publish at one boundary. A mutate_as or load operation accumulates constructive writes, commits each touched table at the end, then publishes one manifest update. Do not commit per statement. Delete-only queries are the documented inline residual; the parse-time D2 rule prevents mixing deletes with insert/update until Lance exposes two-phase delete. Read writes.md and execution.md.

5. Recovery is part of the commit protocol. Writers that can advance Lance HEAD before manifest publish must write __recovery/{ulid}.json sidecars. Omnigraph::open in read-write mode runs the all-or-nothing sweep; the write entry points (load_as, mutate_as, apply_schema_as, branch_merge_as) and refresh run roll-forward-only recovery in-process, so a long-lived process converges on its next write rather than at restart. Do not add a new writer kind without sidecar coverage or an explicit proof that no Lance HEAD can move before manifest publish.

6. Strong consistency is the default. Reads are snapshot-isolated, writes are durable before acknowledgement, and branch reads observe the current committed graph state. Any eventual-consistency mode must be explicit, read-only, auditable, and non-default.

7. Indexes are derived state. Reads must see the correct result for the branch they read even when index coverage is partial. Expensive index work should converge from manifest state instead of extending the critical write path. Scalar staged index builds and vector inline residuals are documented in writes.md and indexes.md.

8. Schema identity survives renames. Accepted schema identity must remain stable across type and property renames. Rename support belongs in migration planning, not in "drop and recreate" behavior. See the known gap below.

9. Schema/data integrity failures are loud. Type errors, required-field misses, invalid edge endpoints, cardinality violations, and unsupported mixed mutation modes fail before a graph commit is published. The system must not invent placeholder nodes or silently weaken integrity.

10. Query semantics are first-class IR concepts. Search modes, mutations, polymorphism, traversal, retrieval scores, imports, and policy predicates belong in typed AST/IR/planner structures. Do not smuggle semantics through strings, side tables, global state, or transport-specific flags.

11. Transport/auth stay at the boundary. Kernel crates should not depend on HTTP, OpenAPI, bearer-token parsing, or future transport protocols. The server resolves bearer tokens to actors; clients cannot set actor identity directly.

12. Bearer-token plaintext is not retained. Server startup hashes bearer tokens, authentication uses constant-time comparison, and request handling carries only the resolved actor identity and hash-derived match state.

13. Operational failures are bounded and observable. Timeout, memory, OOM, partial result, recovery, and conflict paths must fail loudly or degrade in a documented way. If a metric affects plan choice or operator behavior, it must be exposed through the relevant trait or observability surface.

14. Tests match the boundary being changed. Prefer extending the existing test that owns the area. Planner changes need planner-level coverage, storage changes need storage/recovery coverage, and end-to-end tests are not a substitute for missing lower-level assertions. Read testing.md before adding tests.

15. One source of truth, cheaply derived. Lance and the manifest are the source of truth. Everything the engine needs at runtime is a derived view of them: read or projected on demand, held warm, refreshed by a cheap probe. Two failure modes are forbidden. A parallel copy the engine maintains can drift from the source, and that divergence compounds over time. Cold re-derivation rebuilds the view from the full source on every call, so its cost grows with history. Invariants 1 and 7, and the deny-list "state that drifts" and "manifest-derivable reconciler" items, are instances; so is bounding a read's cost to its working set rather than the commit count. This is the structural face of "engineering is programming integrated over time": both failure modes are liabilities that compound as the system grows.

Current Truth Matrix

Area	Current state	Source
Multi-table commit	Manifest CAS plus recovery sidecars; not a single Lance primitive	writes.md, architecture.md
Constructive mutations	In-memory MutationStaging, one end-of-query table commit per touched table, then one manifest publish	writes.md, execution.md
Deletes	Inline-commit residual; delete-only queries allowed, mixed insert/update/delete rejected by D2	query-language.md, writes.md
Branch delete	Manifest is the single authority, flipped atomically first; per-table forks + commit-graph branch are derived state, reclaimed best-effort (force_delete_branch) with the cleanup reconciler as the guaranteed backstop. Reusing a name whose reclaim failed before cleanup surfaces an actionable error	branches-commits.md, maintenance.md
Schema validation	Type checks, required fields, defaults, edge endpoint checks, and edge cardinality are enforced on write paths	schema-language.md, execution.md
Unique constraints	Intra-batch and write-path checks exist; intake and branch-merge derive the composite key through one shared function (loader::composite_unique_key, a separator-free Vec<String> tuple) and fail loudly on an un-keyable column type rather than silently exempting it; full cross-version uniqueness against already-committed rows is still a gap	schema-language.md
Storage trait	TableStorage (via db.storage()) is staged-only; the inline-commit residuals (delete_where, create_vector_index) are split onto a separate sealed InlineCommitResidual trait reached via db.storage_inline_residual() (MR-854), so §1 holds by construction; capability/stat surfaces are roadmap	writes.md, architecture.md
Index lifecycle	@index/@key declares intent; the physical index is derived state and never fails a logical op. schema apply builds no indexes (records intent only; index-only changes touch no table data). load/mutate build inline through one chokepoint (build_indices_on_dataset_for_catalog, type-dispatched by node_prop_index_kind: enum + orderable scalar → BTREE, free-text String → FTS, Vector → vector) that fault-isolates an untrainable Vector column into a pending index instead of aborting. optimize/ensure_indices is the reconciler: it creates declared-but-missing indexes and folds appended/rewritten fragments into existing ones (optimize_indices), reporting still-pending columns. Explicit maintenance call, not yet a background loop	indexes.md, maintenance.md
Traversal IDs	Runtime still builds TypeIndex; Lance stable row-id based graph IDs are roadmap	architecture.md, query-language.md
Auth	Bearer token hashing and server-side actor resolution are implemented at the HTTP boundary	server.md, policy.md
Tests	Tempdir-backed Lance tests are the current substrate; the storage adapter has an in-memory backend for adapter-level contract tests, but Lance datasets bypass it	testing.md

The branch-delete reconciler is authority-derived: it reclaims orphaned forks today and degrades to a no-op if Lance ships an atomic multi-dataset branch operation, so the design composes with that future rather than blocking it. This is the same shape as invariant 7 (indexes are derived state); prefer it over a recovery-sidecar-style approach for any new multi-dataset metadata operation, since the sidecar would be scaffolding to remove once the substrate closes the gap.

Known Gaps

Do not hide these behind invariant wording. Either move them forward or keep them explicit.

• Rename-stable schema identity: the invariant is that accepted IDs survive renames. The current compiler still derives type IDs from kind:name; this must be fixed before relying on renamed IDs across accepted schemas.
• Storage abstraction: TableStorage is present, sealed, and canonical for staged writes. MR-854 sealed it: db.storage() exposes only staged primitives
  • reads, and the inline-commit residuals are split onto a separate sealed InlineCommitResidual trait reached via db.storage_inline_residual(), so a new writer cannot couple a write with a HEAD advance through the default surface. The dead legacy methods (append_batch on the trait, merge_insert_batch{,es}, create_{btree,inverted}_index) were removed. The remaining residuals are delete_where and create_vector_index. The Lance 6.0.1 → 7.0.0 bump landed, so the staged two-phase delete API (DeleteBuilder::execute_uncommitted, Lance #6658) is now available and MR-A is unblocked — but the migration itself is still pending, so delete_where stays inline for now. create_vector_index remains gated on Lance #6666 (still open). See lance.md and writes.md. New write paths should use the staged shape unless a documented Lance blocker applies.
• Deletes and vector indexes: delete_where and vector index creation still advance Lance HEAD inline. The public delete two-phase API now exists (Lance #6658 shipped in 7.0.0), so the delete residual is unblocked pending the MR-A migration; vector index creation is still blocked (Lance #6666 open). Keep D2 and recovery coverage in place until those residuals are removed.
• Blob-column compaction: Lance compact_files mis-decodes blob-v2 columns under its forced BlobHandling::AllBinary read ("more fields in the schema than provided column indices"), so optimize skips any table with a Blob property — reporting SkipReason::BlobColumnsUnsupportedByLance (loud, not a silent drop) behind the LANCE_SUPPORTS_BLOB_COMPACTION gate. Reads and writes are unaffected; only space/fragment reclamation on blob tables is deferred. Remove the skip when the upstream Lance fix lands — the lance_surface_guards.rs::compact_files_still_fails_on_blob_columns guard turns red on that bump to force it.
• Recovery is serialized against live writers in-process only: the write-entry heal (and refresh) serialize against a live writer's sidecar lifetime via the per-(table, branch) write queues plus the schema-apply serialization key — all in-process primitives. A recovery pass in one process cannot serialize against a live writer in another (the open-time sweep has the same exposure, and always has): it may roll a live foreign writer's sidecar forward, which degrades to publisher-CAS contention for data writes but can race the schema-staging promotion for a foreign live schema apply. Multi-process writers on one graph are already documented one-winner-CAS territory; closing this fully needs a cross-process serialization primitive (e.g. lease-based use of the schema-apply lock branch) — design it before promoting multi-process write topologies.
• Fork reclaim is in-process-safe only: the first write to a table on a branch forks it (a Lance create_branch that advances state before the manifest publish). An interrupted fork (crash, or a cancelled request future) leaves a manifest-unreferenced branch ref. The next write self-heals it — reclaim_orphaned_fork_and_refork (force_delete_branch + re-fork) — but reclaim is only safe because the writer holds the per-(table, branch) write queue from before the fork through the publish AND re-checks the live manifest under it, so no in-process writer can be mid-fork. A reclaim cannot serialize against a foreign-process in-flight fork: it may force-delete a peer's just-created ref, which makes that peer's commit fail and retry — the same one-winner-CAS exposure as above, not corruption. The reclaim never fires unless in-process-queue + manifest authority both prove the ref is manifest-unreferenced. cleanup's per-table reconciler (reconcile_orphaned_branches) is the guaranteed backstop for any fork the write path never revisits. Both degrade to a no-op if Lance ships an atomic multi-dataset branch op.
• Local write_text_if_match is not a cross-process CAS: object-store backends use a true conditional put (ETag If-Match; the in-memory test backend too), but upstream object_store leaves PutMode::Update unimplemented for LocalFileSystem, so the local path emulates CAS with a content-token compare followed by an atomic replace — a check-then-act gap plus content-token ABA. Every current caller goes through the cluster lock protocol first, which makes this safe. A lock-free caller would get S3-correct but local-racy behavior — the same divergence shape as the acknowledged-before-visible bug this branch fixed. Close it (local CAS primitive, or a trait-level lock requirement) before admitting any lock-free if_match caller.
• Manifest→commit-graph publish atomicity: a graph commit advances __manifest (the visibility authority) and then appends _graph_commits as two separate writes (commit_updates_with_actor_with_expected, failpoint graph_publish.before_commit_append). A crash between them leaves the manifest at version N with no commit-graph row for N. Live reads and durability are unaffected — the live version resolves via the manifest (GraphCoordinator::version()), not the commit-graph head — and the open-time recovery sweep does NOT repair it (lance_head == manifest_pinned classifies NoMovement; a recovery sidecar would not change this). Impact is bounded to commit history: commit list misses N, time-travel by commit id to N fails, and merge-base loses a node (a likely-benign off-by-one re-merge). This affects every publish, not a specific maintenance command. Eventual fix: make the commit graph reconcilable from the manifest (or the two writes atomic) — not a recovery-sidecar concern.
• Planner capability/stat surfaces: cost-aware planning, complete capability advertisement, and explain-with-cost are roadmap. Do not describe them as implemented.
• Traversal execution: current multi-hop execution still uses TypeIndex, ad-hoc ID filtering, and eager materialization in places. Stable row IDs, SIP, and factorization are target patterns, not current fact.
• Retrieval ranks: hybrid search works, but rank/score are not yet carried everywhere as ordinary columns through the plan.
• Policy pushdown and Source: Cedar enforcement is at the HTTP boundary today, and imports are still loader-shaped. Planner predicates and a unified Source operator are roadmap.
• Resource bounds: some operations still lack enforced per-query memory or time budgets. New long-running work should add explicit bounds rather than widening the gap.
• Read-path re-derivation (largely closed by the query-latency work): snapshot resolution used to re-open a fresh coordinator per read (a full __manifest re-scan plus two commit-graph scans), open each table through the namespace (two more __manifest scans per table), validate the schema twice, and share no Lance Session. That was an O(commits) cost that never warmed up. Fix 1 (warm coordinator reuse behind a latest_version_id probe), Fix 2 (open tables by location+version), finding A (validate once), and Fix 3 (a held Dataset-handle cache keyed by (table, branch, version, e_tag when Lance exposes it) plus one shared Session per graph) remove that tax: a warm same-branch read does one probe, one schema read, and zero opens on a repeat. Non-main branch freshness compares the manifest incarnation (version plus manifest-location e_tag when available, otherwise Lance manifest timestamp), because Lance branch names can be deleted/recreated at the same version number; the manifest e_tag is carried into synthetic snapshot ids when available, and a detected same-branch manifest refresh clears read caches as the fallback for e_tag-less table locations/topology. Remaining: optimize now compacts the internal metadata tables (__manifest, _graph_commits) too (RFC-013 step 2), so a periodically-optimized graph keeps the probe/refresh/per-write scan flat in history; but they are not yet brought into cleanup (version GC), so the _versions/ chain still grows until an explicit cleanup (the cleanup half is deferred — it needs the Q8 cleanup-resurrection watermark first). The commit graph is not yet reconcilable from the manifest; and the traversal id-map is still rebuilt.
• Commit-graph parent under concurrency: record_graph_commit now refreshes the commit-graph head from storage before appending, so a same-branch write after an external commit no longer forks the commit DAG by parenting off a stale cached head (the single-process fork, pre-existing for non-strict inserts and widened to strict ops by Fix 1's refresh_manifest_only, is now closed). Residual: two processes writing disjoint tables can still pass their per-table manifest CAS and append off the same parent (a refresh-then-append TOCTOU). The convergent fix is reconcile-from-manifest (parent = the commit at the manifest version the publisher CAS'd against; manifest_version is on every commit row), composing with the manifest-to-commit-graph atomicity gap; it needs commit-graph append ordering or a Lance append-CAS to fully close.

Deny-list

If a proposal fits one of these, the burden is on the proposer to prove why the case is exceptional.

• Custom WAL, transaction manager, buffer pool, page format, or storage engine.
• Per-table graph publishing outside the manifest publisher.
• Re-reading current branch head during a query instead of using the captured snapshot.
• New write paths that can advance Lance HEAD before manifest publish without a recovery sidecar.
• Cross-query BEGIN/COMMIT transactions in the OSS engine. Use branches and merges for multi-query workflows.
• Acknowledging writes before durable Lance and manifest persistence.
• Silent fallback to eventual consistency, partial results, or dropped rows.
• State that drifts from Lance or the manifest when it can be derived.
• Job queues for manifest-derivable state where a reconciler is the right shape.
• Synchronous inline vector/FTS index rebuilds on the query commit path, except for documented Lance API residuals.
• Side-channels for query semantics: hidden globals, magic strings, transport flags, or out-of-band metadata.
• Cost-blind plan choice when statistics are available or required.
• Hidden statistics for behavior that affects planning or operator choice.
• Hash-map iteration order in result ordering, plan choice, or migration output.
• Cold re-derivation on the hot path: rebuilding from the full source what could be held warm and refreshed cheaply, so cost scales with history rather than the working set (the cost face of invariant 15; "state that drifts" above is its shadow-copy face).
• String-flattened SQL/filter generation when a structured pushdown API is available.
• Eager multi-hop cross-product materialization when factorization fits.
• Ad-hoc IN-list filtering where SIP or another structured selectivity path fits.
• Discarding retrieval score/rank before fusion or projection decisions.
• Auto-creating placeholder nodes for orphan edges.
• Raw filesystem I/O for cluster-stored state (ledger, lock, sidecars, approvals, catalog) outside the cluster crate's storage module — every stored byte goes through the engine StorageAdapter so file:// and s3:// stay one code path.
• Wire-protocol-specific code in compiler or engine crates.
• Cloud-only correctness fixes or forks of the OSS engine for correctness.
• Mutating immutable substrate state in place, including Lance fragments or index segments.
• Shipping observable behavior as if it were not part of the contract. Output ordering, error text, timestamp precision, defaults, and latency profiles all become dependencies once exposed.

Review Checklist

Use this as yes/no/NA for any non-trivial design or PR:

• Does it respect Lance/DataFusion instead of rebuilding them?
• Does it preserve manifest-atomic graph visibility?
• Does every query keep one snapshot for its lifetime?
• Do mutations publish once at the commit boundary?
• Can every Lance-HEAD-before-manifest gap recover all-or-nothing?
• Are schema and edge integrity checks strict by default?
• Are query semantics represented in AST/IR/planner structures?
• Are transport, auth, and policy boundaries preserved?
• Are failures bounded, typed, and observable?
• Are result ordering and plan choices deterministic within a snapshot?
• Are stats/capabilities exposed when behavior depends on them?
• Are existing known gaps left no worse and documented if touched?
• Does the test live at the same boundary as the change?
• Is this operation's cost bounded with respect to history and scale, or does it re-derive warm state from cold storage per call?
• Does the change avoid every deny-list pattern, or justify the exception?

Maintenance Policy

Update this file when an invariant changes, a known gap opens or closes, or a new review anti-pattern deserves deny-list treatment. Prefer stable headings over numbered sections so other docs can link here without churn.

Removing or relaxing a hard invariant requires the same review process as code. Adding a known gap is acceptable when it makes reality explicit; leaving stale claims is not.