omnigraph/docs/dev/lance.md

Lance Docs Index (for OmniGraph agents)

OmniGraph sits on top of Lance. Many problems — index lifecycle, branching, transactions, fragments, compaction, vector/FTS internals — are answered upstream in Lance's docs, not in this codebase.

This file is the curated entry point. When you hit a Lance-shaped problem, find the matching topic below and fetch the listed URL(s) before guessing. Don't grep our codebase for behavior that is documented authoritatively in Lance.

Base URL: https://lance.org. Fetch the FULL page content, not summaries — use curl -sL <url> | pandoc -f html -t markdown or paste the rendered page text manually. Tools that summarize pages (like Claude's WebFetch) routinely drop load-bearing details — defaults, pub(crate) blockers, sub-specs hidden behind navigation hubs. Never act on a summarized fetch alone. Keep this index curated to relevant material — the upstream sitemap has hundreds of URLs (notably the Namespace REST API model surface, Spark/Trino/Databricks integrations) that we don't use.

Substrate boundary check. Before fetching, recall docs/dev/invariants.md: if Lance already does the thing, we don't reimplement it. The most common reason to read these docs is to confirm a substrate behavior, not to learn what to clone.

Quick-start (read these once per project)

Read when	URL
Onboarding to Lance — concepts in 10 min	https://lance.org/quickstart/
Onboarding to vector search	https://lance.org/quickstart/vector-search/
Onboarding to full-text search	https://lance.org/quickstart/full-text-search/
Onboarding to versioning / time travel	https://lance.org/quickstart/versioning/
Lance's own AGENTS.md (its agent guide)	https://lance.org/format/AGENTS/

By problem domain

Storage format & file layout

Touching db/manifest, fragment lifecycle, dataset reconstruction, or anything that reads/writes raw Lance state.

Topic	URL
Lance file format overview	https://lance.org/format/
File-level format spec	https://lance.org/format/file/
File encoding	https://lance.org/format/file/encoding/
File-level versioning	https://lance.org/format/file/versioning/
Table layout (fragments, manifest)	https://lance.org/format/table/layout/
Table schema metadata	https://lance.org/format/table/schema/
Table-level versioning	https://lance.org/format/table/versioning/
Transactions (commit semantics, conflict types)	https://lance.org/format/table/transaction/
MemWAL (durability story)	https://lance.org/format/table/mem_wal/
Row-ID lineage (stable row IDs)	https://lance.org/format/table/row_id_lineage/
Branches & tags (Lance native)	https://lance.org/format/table/branch_tag/

Branching / tags / time travel

Touching graph-level branches, snapshots, run isolation, the commit graph.

Topic	URL
Branch & tag format	https://lance.org/format/table/branch_tag/
Tags & branches operational guide	https://lance.org/guide/tags_and_branches/
Versioning quick-start	https://lance.org/quickstart/versioning/
Table-level versioning spec	https://lance.org/format/table/versioning/

Indexes

Adding/changing index types, fixing coverage, debugging FTS or vector recall, designing the reconciler.

Topic	URL
Index spec overview	https://lance.org/format/index/
BTREE scalar index	https://lance.org/format/index/scalar/btree/
Bitmap scalar index	https://lance.org/format/index/scalar/bitmap/
Bloom-filter scalar index	https://lance.org/format/index/scalar/bloom_filter/
Label-list scalar index	https://lance.org/format/index/scalar/label_list/
Zone-map scalar index	https://lance.org/format/index/scalar/zonemap/
R-Tree scalar index (spatial)	https://lance.org/format/index/scalar/rtree/
Full-text search (FTS) index	https://lance.org/format/index/scalar/fts/
N-gram scalar index	https://lance.org/format/index/scalar/ngram/
Vector index	https://lance.org/format/index/vector/
Fragment-reuse system index	https://lance.org/format/index/system/frag_reuse/
MemWAL system index	https://lance.org/format/index/system/mem_wal/
HNSW Rust example	https://lance.org/examples/rust/hnsw/
Distributed indexing	https://lance.org/guide/distributed_indexing/
Tokenizer (FTS, n-gram)	https://lance.org/guide/tokenizer/

Reads & writes

Touching the bulk loader, mutation execution, merge_insert, WriteMode selection.

Topic	URL
Read-and-write guide	https://lance.org/guide/read_and_write/
Distributed write	https://lance.org/guide/distributed_write/
Rust example: write & read a dataset	https://lance.org/examples/rust/write_read_dataset/

Schema evolution

Touching apply_schema, the migration planner, additive evolution.

Topic	URL
Data-evolution guide	https://lance.org/guide/data_evolution/
Migration guide	https://lance.org/guide/migration/

Object store / S3

Touching storage.rs, S3-compatible backends (RustFS, MinIO), env vars.

Topic	URL
Object-store guide	https://lance.org/guide/object_store/

Data types

Touching schema-language scalar mappings, blob columns, JSON, list columns.

Topic	URL
Data types overview	https://lance.org/guide/data_types/
Arrays / list types	https://lance.org/guide/arrays/
Blobs (LargeBinary)	https://lance.org/guide/blob/
JSON	https://lance.org/guide/json/

Performance & tuning

Optimizing scans, fragment counts, cache behavior, memory pool sizing.

Topic	URL
Performance guide	https://lance.org/guide/performance/

Compaction & cleanup

Touching omnigraph optimize / cleanup, the underlying compact_files / cleanup_old_versions.

Topic	URL
Read-and-write guide (covers compact_files, cleanup_old_versions)	https://lance.org/guide/read_and_write/
Performance (compaction tradeoffs)	https://lance.org/guide/performance/
Fragment-reuse index	https://lance.org/format/index/system/frag_reuse/

DataFusion integration

The runtime substrate that may carry our query execution. See docs/dev/invariants.md: we don't rebuild relational machinery.

Topic	URL
DataFusion integration	https://lance.org/integrations/datafusion/

SDK reference

Looking up a specific Rust API (signature, return type, error variant).

Topic	URL
SDK docs landing	https://lance.org/sdk_docs/

What's not in this index (and why)

• Namespace REST API model surface (/format/namespace/client/operations/models/...) — hundreds of REST schema docs for the Lance Namespace catalog API. Omnigraph does not run a Lance Namespace server, so these are not reachable from our problem space.
• Spark / Trino / Databricks / Dataproc / Hive / Glue / Polaris / Iceberg / Unity / OneLake / Gravitino integrations — not part of OmniGraph's deployment surface.
• Python / TF / PyTorch / Hugging Face / Ray integrations — OmniGraph is Rust-only; Python notebooks aren't relevant.
• Community / governance / release / voting / PMC pages — meta, not technical.

If a future need pulls one of these into scope, add a row to the matching domain section above and link it from AGENTS.md's topic index.

Maintenance

When Lance ships a major release that changes any of the above (file format bump, new index type, transaction semantics change, new branching primitive), refresh this index in the same change as the omnigraph upgrade. Stale Lance pointers are worse than no pointers.

Last alignment audit: 2026-05-22 (Lance 6.0.1 upstream; omnigraph pinned at 6.0.1)

Migration from Lance 4.0.0 → 6.0.1 landed in this cycle (DataFusion 52 → 53, Arrow 57 → 58, lance-tokenizer 6.0.1 added, tantivy* removed). Direct 4 → 6 jump; v5.x was not used as an intermediate (rationale in ~/.claude/plans/shimmering-percolating-duckling.md). Behavior-affecting findings:

• DatasetIndexExt moved from lance-index to lance::index (Lance PR #6280, v5.0). Six import sites updated. lance-index::IndexType and lance-index::is_system_index stayed in lance-index. omnigraph-cli and omnigraph-server gained lance = { workspace = true } in their dev-dependencies.
• DescribeTableResponse gained is_only_declared: Option<bool> (lance-namespace 6.0+, v5.0 PR #6186). Set to Some(false) in both BranchManifestNamespace::describe_table and StagedTableNamespace::describe_table — every table we return is physically materialized via Dataset::open, never "declared-only."
• MergeInsertBuilder execute_reader return shape preserved (Arc<Dataset>, MergeStats); the publisher CAS chain at db/manifest/publisher.rs:370-391 works unchanged. Pinned by tests/lance_surface_guards.rs::_compile_merge_insert_builder_method_chain.
• LanceError::TooMuchWriteContention variant retained in v6.0.1 (no rename). The typed publisher translation at db/manifest/publisher.rs:417-430 continues to apply. Pinned by lance_surface_guards.rs::lance_error_too_much_write_contention_variant_exists.
• ManifestLocation field shape stable: .path: object_store::path::Path, .size: Option<u64>, .e_tag: Option<String>, .naming_scheme: ManifestNamingScheme. Pinned by lance_surface_guards.rs::manifest_location_field_shape.
• LanceFileVersion::default() flipped V2_0 → V2_1 (v5.0). No effect — every data_storage_version callsite explicitly pins Some(LanceFileVersion::V2_2) (load-bearing for blob v2: Blob v2 requires file version >= 2.2 enforced in lance/src/dataset/write.rs:748).
• Dataset::checkout_version(N).await?.restore().await?: restore() takes &mut self and returns Result<()> (mutates in place, does not consume + return a new dataset). The recovery rollback hammer at db/manifest/recovery.rs:505-522 continues to work. Pinned by lance_surface_guards.rs::_compile_checkout_version_then_restore_signature.
• DatasetBuilder::from_namespace(...).with_branch(...).with_version(...).load() surface preserved (the namespace builder chain at db/manifest/namespace.rs:162-174). Pinned by lance_surface_guards.rs::_compile_dataset_builder_from_namespace_signature.
• compact_files(&mut ds, CompactionOptions::default(), None) signature stable. CompactionOptions still does not expose data_storage_version; compact_files builds its own WriteParams { ..Default::default() }. Note: LanceFileVersion::default() is now V2_1 in v6, so optimize-rewritten fragments come out at V2_1 by default (was V2_0 in v4). Existing explicit V2_2 pins on creates/appends still apply.
• Dataset::delete(predicate) returns DeleteResult { new_dataset: Arc<Dataset>, num_deleted_rows: u64 } — unchanged shape. Pinned by lance_surface_guards.rs::_compile_delete_result_field_shape. MR-A will repurpose this guard to the staged two-phase variant once DeleteBuilder::execute_uncommitted migration lands.
• File reader read methods now async (Lance PR #6710, v6.0). No effect — omnigraph reaches Lance exclusively through Dataset::scan and the staged-write API.
• Tokenizer vendored as lance-tokenizer (Lance PR #6512, v6.0). No effect — no direct tokenizer imports.
• Lance #6658 closed (2026-05-14) but DeleteBuilder::execute_uncommitted did not ship in v6.0.1 — binary search across the release stream shows it first appears in v7.0.0-beta.10 (the closing commits landed on main but didn't backport to the 6.x line). Tracked as MR-A: migrate delete_where to staged, retire the parse-time D2 mutation rule, extend recovery sidecar coverage. Gated on the Lance v7.x bump, not this PR. v7.0.0-rc.1 dropped 2026-05-21.
• Lance #6666 still open (build_index_metadata_from_segments public): vector-index two-phase blocked; inline create_vector_index residual retained.
• Lance #6877 still open (MergeInsertBuilder dup-rowid): PR #109's SourceDedupeBehavior::FirstSeen + check_batch_unique_by_keys precondition stay load-bearing.
• Dataset::force_delete_branch (branches().delete(name, force=true), dataset.rs:524) tolerates a missing branch-contents ref (vs plain delete_branch's RefNotFound), but on the local store still errors NotFound if the branch tree/ directory is fully absent (remove_dir_all's NotFound is not caught for Lance's native error variant, refs.rs:526-549). Both variants still refuse a branch with referencing descendants (RefConflict). TableStore::force_delete_branch wraps this to be fully idempotent (tolerates already-absent). The single-authority branch-delete redesign uses it for orphan reclamation (eager best-effort reclaim + cleanup reconciler). Pinned by lance_surface_guards.rs::force_delete_branch_semantics. Branch delete is "flip the ref atomically, then remove_dir_all(tree/{branch})"; branch-exclusive data lives under tree/{branch}/ so a drop reclaims it immediately without touching main.
• Lance blob-v2 compact_files bug (no public issue found as of 2026-06): compact_files disables binary-copy for blob datasets and forces BlobHandling::AllBinary on the read side; the v2.1+ structural decoder then mis-counts column infos for the blob-v2 struct and fails with Invalid user input: there were more fields in the schema than provided column indices / infos (lance-encoding/src/decoder.rs::ColumnInfoIter::expect_next). This fails even a pristine uniform-V2_2 multi-fragment blob table; vector/list/scalar/ragged columns and mixed file versions all compact fine. Reads/queries use descriptor handling (BlobHandling::default()) and are unaffected. optimize skips blob-bearing tables behind LANCE_SUPPORTS_BLOB_COMPACTION = false (db/omnigraph/optimize.rs), reporting SkipReason::BlobColumnsUnsupportedByLance. Pinned by lance_surface_guards.rs::compact_files_still_fails_on_blob_columns, which turns red when the bug is fixed → flip the gate, remove the skip branch + the maintenance.rs::optimize_skips_blob_table_and_reports_skip skip assertions.

Surface guards added: crates/omnigraph/tests/lance_surface_guards.rs (10 named guards; 5 runtime + 5 compile-only). Future Lance bumps re-run this file first as the smoke check. Two additional guards from the original plan deferred to follow-up (manifest_cas_returns_row_level_contention_variant needs full publisher-race harness; table_version_metadata_byte_compatible_with_v4 needs pub(crate) reach extension).

Bump this date stanza on the next alignment pass.