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## Summary Implements **M1 from #1115**: batches observation/transmission INSERTs into a single SQLite `BEGIN/COMMIT` window instead of fsyncing per packet. At ~250 obs/sec this drops WAL fsync rate from ~20/s to ~1/s and eliminates the `obs-persist skipped` / `SQLITE_BUSY` log spam that the issue documents. This is a **partial fix** — it ships the group-commit mechanism. Acceptance items 6–7 (measured fsync rate / measured `obs-persist skipped` rate at staging steady-state) require post-deploy observation, and M2 (per-`tx_hash` observation buffering) is intentionally deferred. The issue stays open for the user to verify on staging. > Partial fix for #1115 — does not auto-close. Refs #1115. ## Mechanism - `Store` gains an active `*sql.Tx`, `pendingRows` counter, `gcMu`, and the `groupCommitMs` / `groupCommitMaxRows` knobs. `SetGroupCommit(ms, maxRows)` enables the mode; `FlushGroupTx()` commits the in-flight tx. - `InsertTransmission` lazily opens a tx on the first call after each flush, then issues all writes through `tx.Stmt()` bindings of the existing prepared statements. With `MaxOpenConns(1)` the connection is already serialized; `gcMu` serializes group-commit state without contention. - A goroutine in `cmd/ingestor/main.go` calls `FlushGroupTx()` every `groupCommitMs` ms. `pendingRows >= groupCommitMaxRows` triggers an eager flush. `Close()` flushes before the WAL checkpoint so no rows are lost on graceful shutdown. - `groupCommitMs == 0` short-circuits to the legacy per-call auto-commit path (statements bound to `s.db`, no tx) — current behavior preserved byte-for-byte for operators who opt out. ## Config Two new optional fields (ingestor-only), both documented in `config.example.json`: | Field | Default | Effect | |---|---|---| | `groupCommitMs` | `1000` | Flush window in ms. `0` disables batching (legacy per-packet auto-commit). | | `groupCommitMaxRows` | `1000` | Safety cap; when exceeded the queue flushes immediately to bound memory and the crash-loss window. | No DB schema change. No required config change on upgrade. ## Tests (TDD red → green visible in commits) `cmd/ingestor/group_commit_test.go` — three assertions, written first as the red commit: - `TestGroupCommit_BatchesInsertsIntoOneTx` — 50 `InsertTransmission` calls inside a wide window produce **0** commits until `FlushGroupTx`, then exactly **1**; all 50 rows visible after flush. (This is the spec's "50 observations → 1 SQLite write transaction" assertion.) - `TestGroupCommit_Disabled` — `groupCommitMs=0` keeps every insert immediately visible and `GroupCommitFlushes` never advances. (Spec's "groupCommitMs=0 reverts to per-packet behavior" assertion.) - `TestGroupCommit_MaxRowsForcesEarlyFlush` — cap=3, 7 inserts → 2 auto-flushes from the cap + 1 final manual flush = 3 total. Red commit: `e2b0370` (stubs `SetGroupCommit` / `FlushGroupTx` so the tests compile and fail on **assertions**, not import errors). Green commit: `73f3559`. Full ingestor suite (`go test ./...` in `cmd/ingestor`) stays green, ~49 s. ## Performance This PR is the perf change itself. Local micro-test (the new `TestGroupCommit_BatchesInsertsIntoOneTx`) shows the structural property: 50 inserts → 1 commit. The fsync-rate measurement called out in the M1 acceptance criteria (`~20/s → ~1/s` at 250 obs/sec) requires staging deployment to confirm — that's the remaining open item that keeps #1115 open after this merges. No hot-path regressions: when `groupCommitMs > 0` we acquire one mutex per insert (uncontended in the steady state — the connection was already single-threaded via `MaxOpenConns(1)`). When `groupCommitMs == 0` the code path is identical to before plus one nil-tx check. ## What this PR does NOT do (per spec) - Does not collapse "30 observations of one packet" into 1 row write — that's M2. - Does not eliminate dual-writer contention with `cmd/server`'s `resolved_path` writes. - Does not change observation ordering or live broadcast latency. --------- Co-authored-by: corescope-bot <bot@corescope.local>