feat: bring the MemWAL LSM surface to parity across the SDKs (#3962)

## Why Four of the eight LSM methods are **remote-only in the core**. `impl BaseTable for NativeTable` implements only `set`/`unset`/`get_lsm_write_spec` and `close_lsm_writers`; `flush_lsm`, `compact_lsm` and `get_lsm_stats` fall through to trait defaults returning `NotSupported` (`rust/lancedb/src/table.rs:679,687,696`), and `checkpoint_lsm` is built on all three. That explains the state of the bindings: Node had bound the four that work against a local table and stopped, so a Cloud user could install an LSM write spec but had no way to observe fresh-tier state or drive a checkpoint. Java had none of it at all. | SDK | set/unset/get spec | closeWriters | flush | compact | getStats | checkpoint | |---|---|---|---|---|---|---| | Rust core | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | Python | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | Node *(before)* | ✅ | ✅ | — | — | — | — | | **Node (after)** | ✅ | ✅ | **new** | **new** | **new** | **new** | | Java *(before)* | — | — | — | — | — | — | | **Java (after)** | **new** | n/a | **new** | **new** | **new** | **new** | Go and C are separate repos and are out of scope here. `closeLsmWriters` drains cached in-process shard writers, so it has no meaning for Java, which is a pure REST client. ## Node Adds napi bindings for `flushLsm`, `compactLsm`, `checkpointLsm` and `getLsmStats`, plus typed `LsmStats` / `BucketStats` / `GenerationStats` / `MemtableStats` objects — typed rather than a JSON blob, matching the existing `LsmWriteSpec` object in the same file, with `u64` cast to `i64` per that file's convention. Because these four are remote-only, the new tests assert each binding reaches the core and surfaces `NotSupported` against a local table. That covers the wiring; behavior against a real endpoint stays covered by the mocked-endpoint tests in `rust/lancedb/src/remote/table.rs`. ## Python No new methods. All eight are on `LanceTable`, `AsyncTable` and `RemoteTable` — the last four landed on the sync `RemoteTable` in #3961, which is merged into this branch. What was missing here was reachability. `LsmWriteSpec` was importable only from the private `lancedb._lancedb`, appearing in `table.py` solely under `if TYPE_CHECKING:`, and `docs/src/python/python.md` had no mention of it, which per the repo's docs guidance means it rendered nowhere in the API reference. It is now `lancedb.LsmWriteSpec`, in `__all__`, and documented. ## Java Java reaches LanceDB purely over REST through the generated Lance Namespace client, and these routes are not in that spec, so they are issued through a small dedicated client rather than added to the spec. That call is revisitable — LSM is one of four unspecified route families alongside `multipart_write`, `page_cache/prewarm` and `branches/diff|merge`. If those are ever regularized into the spec as a group, `LanceDbTableLsm` is one file that gets deleted. `LsmWriteSpec` here is deliberately **not** `org.lance.memwal.InitializeMemWalParams`. That type defaults to maintaining *no* indexes where a spec here defaults to maintaining *every* index, and it cannot express the `null` that asks the server to resolve the set: | Value | On the wire | Meaning | |---|---|---| | unset (null) | `null` | Server resolves **every** maintainable index | | `Collections.emptyList()` | `[]` | Maintain **none** | | `Arrays.asList("id_idx")` | `["id_idx"]` | Exactly those | A dedicated test pins null and `[]` as distinct on the wire, since collapsing them is the failure mode that motivated a LanceDB-owned type. `checkpointLsm` is ported from `rust/lancedb/src/table/checkpoint.rs` with its constants and status semantics intact: 429/503 retried in place against an 8-budget, 421 restarting from flush against a 3-budget, 5s poll, and a target watermark fixed after the seal so it terminates under write load. `getLsmStats` returns typed `LsmStats` / `BucketStats` / `GenerationStats` / `MemtableStats`, mirroring the Rust structs in `rust/lancedb/src/table/lsm_stats.rs` and the objects Node exposes. Decoding is strict — see below. ## Review feedback Both gatekeeper findings were real. Each was reproduced against the scripted test server first, and each fix ships with the reproducer as a regression test. **The transport was doubling every checkpoint retry budget.** `HttpClients.createDefault()` installs Apache's default response retry strategy, whose retryable-status list is exactly 429 and 503 — the two statuses `isRetryable` owns. A 429 held against `flush_lsm` issued **18** wire requests where the loop intends 9, and `compact_lsm` was retried in place despite the loop being built to fall through to a fresh stats poll instead. Timing confirmed the mechanism: that run took 25.4s ≈ 16.3s of the loop's own backoff plus 9 × the transport's 1s retry interval. Automatic retries are now disabled, so the checkpoint loop is the sole owner of the 421/429/503 transitions. A side effect worth noting: `testCheckpointRetriesRetryableStatusInPlace` was passing on a transport-absorbed 429 and never reaching `issue()`'s retry branch at all. It now exercises the real path. **Stats decoding failed open.** `getLsmStats` read the response with Jackson's `path()`, which yields a missing node that iterates as an empty array — making "malformed" indistinguishable from "no buckets", which is indistinguishable from "drained". Four separate payloads made `checkpointLsm()` report convergence for a checkpoint that never ran: | Response | Before | Now | |---|---|---| | `{"lsm_stats": null}` or absent key | disabled ✓ | disabled ✓ | | `{"lsm_stats": {}}` | **reported success** | `IllegalStateException` | | empty response body | **reported success** | `IllegalStateException` | | bucket missing required fields | **reported success** | `IllegalStateException` | The empty-body row is the one to weight: a proxy 200 with no body is a realistic production event, and it silently reported a checkpoint that never happened. Decoding is now strict and fails closed, matching the serde contract on the Rust side exactly. One deliberate deviation from the review comment, which asked that *only* explicit JSON `null` count as disabled: Rust has `#[serde(default)]` on `lsm_stats`, so an **absent key** decodes to `None` there too. Java now matches that. It is an absent-or-malformed **`buckets`** that fails closed, which is the case the comment was actually protecting. ## Testing - Java: **33 passing** (8 existing + 25 LSM) against a scripted `com.sun.net.httpserver.HttpServer` — no new test dependency. Wire assertions mirror `rust/lancedb/src/remote/table.rs:6581-6748`; checkpoint tests cover convergence, not piling onto a latched bucket, 421 restart-from-flush, 429 retry-in-place, terminal-status propagation, reissue exhaustion, the exact wire-request count against the retry budget, and five malformed stats payloads. - Node: **19 LSM tests passing**; `cargo check`, `npm run build`, `npm run tsc`, `npm run lint`, `npm run docs` all clean. - Python: `ruff format --check` and `ruff check` clean. - Java formatting: `./mvnw -pl lancedb-core spotless:apply` and `spotless:check` both clean under a JDK 11 toolchain. ## Note: spotless needs a pre-16 JDK `./mvnw spotless:apply` fails on JDK 16+ with `JCTree$JCImport.getQualifiedIdentifier()` — google-java-format 1.7, pinned at `java/pom.xml:34`, predates JDK 16's compiler API change. **This is pre-existing** and reproduces on a pristine `main` checkout. It is not a blocker, just a toolchain requirement. Spotless was run against these sources under JDK 11 and both `spotless:apply` and `spotless:check` pass on the whole module: ```shell JAVA_HOME=/path/to/jdk11 ./mvnw -pl lancedb-core spotless:apply ``` Bumping the plugin so it works on modern JDKs is still worth doing, but separately from this PR. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
2026-08-22 14:08:21 +00:00 · 2026-08-19 11:44:46 -05:00
parent 11c1d81638
commit f1c4967eeb
25 changed files with 2581 additions and 16 deletions
@@ -213,6 +213,39 @@ version of the table.

 ***

+### checkpointLsm()
+
+```ts
+abstract checkpointLsm(): Promise<void>
+```
+
+Converge this table's LSM write path into its base table.
+
+Seals once, then triggers compaction and polls until the L0 that existed
+at the start is gone. The target set is fixed at the start, so
+generations created *during* the checkpoint are ignored — that is what
+lets it terminate under write load, and what makes it best-effort: it
+converges the fresh tier as of some instant. Idempotent, abandonable at
+any point, and safe to run on a cadence.
+
+There is no liveness bound — the compactor pool is shared across tables,
+so a checkpoint queued behind unrelated work looks exactly like one that
+is merging. The caller owns the deadline.
+
+#### Returns
+
+`Promise`&lt;`void`&gt;
+
+#### Example
+
+```ts
+const before = await table.getLsmStats();
+await table.checkpointLsm();
+const after = await table.getLsmStats();
+```
+
+***
+
 ### close()

 ```ts
@@ -250,6 +283,24 @@ It is a no-op when no writers are cached.

 ***

+### compactLsm()
+
+```ts
+abstract compactLsm(): Promise<void>
+```
+
+Trigger a background L0 → base compaction pass per bucket.
+
+Returns once the passes are *dispatched*, not once they finish — watch
+[Table#getLsmStats](Table.md#getlsmstats) for progress, or use
+[Table#checkpointLsm](Table.md#checkpointlsm) to wait for convergence.
+
+#### Returns
+
+`Promise`&lt;`void`&gt;
+
+***
+
 ### countRows()

 ```ts
@@ -448,6 +499,48 @@ Drop an index from the table.

 ***

+### flushLsm()
+
+```ts
+abstract flushLsm(): Promise<void>
+```
+
+Seal every bucket's active memtable into a new L0 generation.
+
+Returns once the seal is committed. Sealing an empty memtable is a no-op,
+so this is safe to call repeatedly.
+
+#### Returns
+
+`Promise`&lt;`void`&gt;
+
+***
+
+### getLsmStats()
+
+```ts
+abstract getLsmStats(includeGenerationRows?): Promise<undefined | LsmStats>
+```
+
+Read live per-bucket LSM state.
+
+Answers "how far behind is my fresh tier", "which bucket is hot", and
+"why is my fresh-tier vector search brute-force". Mutates no table state.
+
+Resolves to `undefined` only when the LSM write path is not enabled.
+
+#### Parameters
+
+* **includeGenerationRows?**: `boolean`
+    Also count rows per L0 generation.
+    Off by default because each count opens an uncached Lance dataset.
+
+#### Returns
+
+`Promise`&lt;`undefined` \| [`LsmStats`](../interfaces/LsmStats.md)&gt;
+
+***
+
 ### getLsmWriteSpec()

 ```ts
@@ -58,6 +58,7 @@
 - [BranchDiff](interfaces/BranchDiff.md)
 - [BranchIndexSummary](interfaces/BranchIndexSummary.md)
 - [BranchRowCountSummary](interfaces/BranchRowCountSummary.md)
+- [BucketStats](interfaces/BucketStats.md)
 - [ClientConfig](interfaces/ClientConfig.md)
 - [ColumnAlteration](interfaces/ColumnAlteration.md)
 - [ColumnOrdering](interfaces/ColumnOrdering.md)
@@ -81,6 +82,7 @@
 - [FtsToken](interfaces/FtsToken.md)
 - [FullTextQuery](interfaces/FullTextQuery.md)
 - [FullTextSearchOptions](interfaces/FullTextSearchOptions.md)
+- [GenerationStats](interfaces/GenerationStats.md)
 - [HnswPqOptions](interfaces/HnswPqOptions.md)
 - [HnswSqOptions](interfaces/HnswSqOptions.md)
 - [IndexConfig](interfaces/IndexConfig.md)
@@ -94,7 +96,9 @@
 - [JobInfo](interfaces/JobInfo.md)
 - [ListNamespacesOptions](interfaces/ListNamespacesOptions.md)
 - [ListNamespacesResponse](interfaces/ListNamespacesResponse.md)
+- [LsmStats](interfaces/LsmStats.md)
 - [LsmWriteSpec](interfaces/LsmWriteSpec.md)
+- [MemtableStats](interfaces/MemtableStats.md)
 - [MergeBlocker](interfaces/MergeBlocker.md)
 - [MergeBranchResult](interfaces/MergeBranchResult.md)
 - [MergePreview](interfaces/MergePreview.md)
@@ -0,0 +1,116 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / BucketStats
+
+# Interface: BucketStats
+
+Live state of one bucket. A table is N buckets on one node; flattening to a
+single number hides the one hot bucket that is usually why someone opened
+this endpoint.
+
+## Properties
+
+### compacting
+
+```ts
+compacting: boolean;
+```
+
+Whether a pass owns this bucket's compaction latch right now. Says *a*
+driver is running, not *whose*, and the latch is held from dispatch —
+including while the pass queues for a pod-wide compactor permit. Read it
+as "do not pile on", never as "mine is progressing".
+
+***
+
+### currentGeneration
+
+```ts
+currentGeneration: number;
+```
+
+The generation the active memtable will become.
+
+***
+
+### generations
+
+```ts
+generations: GenerationStats[];
+```
+
+Flushed L0 generations not yet merged into the base table.
+
+***
+
+### manifestVersion
+
+```ts
+manifestVersion: number;
+```
+
+Version of the shard manifest these numbers were read from.
+
+***
+
+### memtables?
+
+```ts
+optional memtables: MemtableStats[];
+```
+
+Oldest first, active last. Absent for a `"Sealed"` bucket, whose
+in-memory state is torn down.
+
+***
+
+### replayAfterWalEntryPosition
+
+```ts
+replayAfterWalEntryPosition: number;
+```
+
+WAL position replay resumes from.
+
+***
+
+### shardId
+
+```ts
+shardId: string;
+```
+
+The shard this bucket writes.
+
+***
+
+### status
+
+```ts
+status: string;
+```
+
+`"Active"` or `"Sealed"` (drop-table 2PC in flight).
+
+***
+
+### walEntryPositionLastSeen
+
+```ts
+walEntryPositionLastSeen: number;
+```
+
+Highest WAL position the writer has seen. The difference against
+`replayAfterWalEntryPosition` is the WAL lag.
+
+***
+
+### writerEpoch
+
+```ts
+writerEpoch: number;
+```
+
+Epoch of the writer that currently owns the shard.
@@ -0,0 +1,40 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / GenerationStats
+
+# Interface: GenerationStats
+
+One flushed L0 generation.
+
+## Properties
+
+### bytes
+
+```ts
+bytes: number;
+```
+
+On-disk size of the generation.
+
+***
+
+### generation
+
+```ts
+generation: number;
+```
+
+The generation number. Increases as memtables are sealed into L0.
+
+***
+
+### rows?
+
+```ts
+optional rows: number;
+```
+
+Present only when `includeGenerationRows` was requested. Off by default
+because each count opens an uncached Lance dataset.
@@ -0,0 +1,22 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / LsmStats
+
+# Interface: LsmStats
+
+Live per-bucket LSM state, as returned by `Table#getLsmStats`.
+
+Nothing here is derived: sums and differences (total L0 bytes, WAL lag) are
+the caller's to compute.
+
+## Properties
+
+### buckets
+
+```ts
+buckets: BucketStats[];
+```
+
+One entry per bucket backing this table.
@@ -0,0 +1,60 @@
+[**@lancedb/lancedb**](../README.md) • **Docs**
+
+***
+
+[@lancedb/lancedb](../globals.md) / MemtableStats
+
+# Interface: MemtableStats
+
+One in-memory memtable.
+
+## Properties
+
+### batches
+
+```ts
+batches: number;
+```
+
+Record batches currently buffered.
+
+***
+
+### bytes
+
+```ts
+bytes: number;
+```
+
+Estimated in-memory size.
+
+***
+
+### generation
+
+```ts
+generation: number;
+```
+
+The generation this memtable will become once sealed.
+
+***
+
+### indexes
+
+```ts
+indexes: string[];
+```
+
+Names of the indexes this memtable carries. An absent name is the whole
+answer to "why is my fresh-tier search on that column brute-force".
+
+***
+
+### rows
+
+```ts
+rows: number;
+```
+
+Rows currently buffered.
@@ -52,6 +52,8 @@ listing a storage directory.

 ::: lancedb.table.Branches

+::: lancedb.LsmWriteSpec
+
 ## Expressions

 Type-safe expression builder for filters and projections. Use these instead