cea3930b94b0f35a7f68ff16b9ef6dbb2b3a51ae
2 Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
 Joseph Doherty
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cea3930b94 |
feat(localdb): per-table needs_snapshot (0.3.0, on-disk schema v3)
Baselining one table used to set a single per-DATABASE needs_snapshot bit, and a snapshot streams every registered table in full — so opting one table into replication late re-sent all of them, in both directions at once on a pair enabling replication late. On a database holding a high-volume table (an event log at its retention cap) that is the largest transfer this library performs. 0.2.1 documented the cost and deferred the fix as needing an on-disk change. Schema v3 adds __localdb_snapshot_state (table_name PK, needs_snapshot), created by the shared DDL with IF NOT EXISTS so an older file gains it on open; the v2->v3 transition is a version stamp only. RegisterReplicated(..., baselineExistingRows: true) now flags that table alone. The database-wide flag survives unchanged for the paths that genuinely reach every table — a cap prune or an impossible peer ack loses delta continuity for the oplog as a whole. DeregisterReplicated drops a table's pending flag with its ledger rows, since nothing would ever stream (and so clear) it afterwards. Wire compatibility is additive and negotiated by CAPABILITY, not version: lib_schema_version stays 1 because ValidateHandshake compares it fail-closed for equality, so bumping it would stop 0.1.x/0.2.x pairs syncing outright. Handshake gains supports_partial_snapshot (field 5) and SnapshotBegin gains a tables scope (field 2) where EMPTY means all tables — precisely what a pre-0.3.0 sender emits and what a pre-0.3.0 receiver assumes. A peer that does not advertise the capability is served a full snapshot: a scoped one would make it advance last_applied_remote_seq to the snapshot's as-of and silently discard every delta below it for the tables left out. For the same reason a partial snapshot moves no watermark on either side. The receiver keeps last_applied_remote_seq (carrying only the HLC across, which is MAX-guarded), and the sender keeps _sentThruSeq, so the deltas the snapshot overlaps are still pumped and merged by LWW. Crash safety: snapshot debt now clears on the receiver's new SnapshotAck rather than on enqueue, so an ungraceful exit part-way through re-streams the same scope on reconnect. The ack rides the unbounded CONTROL lane like DeltaAck, so it composes with 0.2.1's bounded 64-message inbox and its out-of-handshake snapshot. Against a pre-0.3.0 peer the 0.2.x clear-on-enqueue behaviour is kept — there is no ack to wait for — with the handshake's horizon gap check as the net. Tests 199/199 (was 188): single-table baseline streams only that table, mixed- version interop (peer without the capability gets a full snapshot), debt standing until SnapshotAck, partial snapshot not advancing the receiver's watermark, no SnapshotAck emitted to an old peer, end-to-end over two real databases, plus v2->v3 upgrade and per-table flag isolation. README, ILocalDb remarks (which named this as the follow-up) and the umbrella CLAUDE.md index updated. |
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Joseph Doherty
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9377fa1678 |
perf(localdb): dereg API, O(1) backlog depth, byte-budget batching, prepare-once apply (0.2.0)
WP3.3 of the ScadaBridge architectural-review remediation. Six changes, all wire-compatible with 0.1.x peers — no proto change, no lib_schema_version bump. The on-disk bookkeeping schema goes to v2 and is upgraded in place when a file is opened. DeregisterReplicated(table). There was no way to stop capturing once a file had been registered, so a node that does not replicate kept paying three triggers inside every write transaction, appending to an oplog nothing would ever drain. Drops the triggers and prunes the table's oplog + row-version rows in one transaction; idempotent, and deliberately usable on a table this process never registered, since stale triggers left by an EARLIER build are the primary case. Deregistration must be symmetric across a pair, and already is enforced: the handshake compares registered-table digests fail-closed, so a node that drops a table its peer still replicates stops syncing loudly rather than diverging. RegisterReplicated(table, baselineExistingRows: true). Closes the late-opt-in hole: the snapshot streamer pages from __localdb_row_version, which only triggers populate, so a node that started replicating LATE never sent its pre-existing rows — silently, forever. Seeding alone would not have fixed it, because a node whose oplog starts at seq 1 presents no gap at the handshake and is told it needs no snapshot; the seed therefore also flags needs_snapshot, and the existing resync machinery carries the rows. The floor is HLC 0 stamped with this node's id: every genuine HLC is a UTC millisecond shifted left 16 bits and so is strictly greater, which makes a baselined row lose every comparison against a real remote write and win only where the peer has no version of that key at all. Two nodes baselining the same key both hold 0 and resolve by the node-id tie-break — convergent, and nothing better is available for two rows of unordered antiquity. Opt-in, so the documented CDC contract is unchanged unless asked. Pruning the ledger on deregister is safe precisely because re-registration can re-baseline; the two APIs are a pair. O(1) backlog depth. GetOplogDepth ran COUNT(*) over the entire unacked backlog, once a minute from maintenance AND on every metrics scrape — a full scan of a million rows exactly when a node is already behind. Now MAX(seq) - MAX(last_acked_seq, MIN(seq) - 1), floored at 0: three b-tree seeks, measured at 58 VM steps against 100,020 for the count on a 50k backlog. Exact, because seq is AUTOINCREMENT and every prune path removes a prefix (ack pruning by watermark, the row cap by count, the age cap by an HLC cutoff that rises with seq). The MIN clamp is what keeps it exact after a cap prune has moved the horizon above a stalled watermark — without it the caps would re-flag on every tick forever. Byte-budget batching. MaxBatchSize was the only cap on an outbound message, which made it a proxy for a byte budget that consumers had to tune against their widest row: ScadaBridge pinned it to 16 because 70 KB config_json times the 500 default is ~35 MB against a 4 MB gRPC limit. MaxBatchBytes (default 2 MB) now bounds delta and snapshot messages by summed serialized size, with the row count demoted to a secondary cap. A row larger than the budget is sent alone rather than stalling the stream on it. Since more, smaller messages would otherwise mean more apply transactions and more acks, inbound delta batches and acks ALREADY queued are coalesced — one transaction and one ack per burst, bounded by the sender's own caps, and a no-op under a trickle. LwwApplier prepares each statement once. It went through the generic transaction façade, which builds a fresh SqliteCommand per call: three SQL parses per applied row, on the path that drains a reconnect backlog. Statements are now held for the batch and rebound. Dropped __localdb_oplog_hlc. It indexed (table_name, pk_json, hlc), which no query has ever used — every oplog access path is by seq or a full hlc-predicate delete — while costing a b-tree insert inside every captured consumer write. 184/184 tests pass (was 149). New coverage: schema upgrade drops the index and preserves identity/watermarks/data, deregistration idempotency and trigger absence and cross-process cleanup, baseline seeding at the floor with LWW safety proved through the real applier, depth asserted equal to the counting query across every prune scenario plus a constant-time assertion over a 200k backlog, byte-budget chunking including the one-giant-row edge on both the delta and snapshot paths, prepare-once statement counting, and coalescing that must not swallow the message that ends its run. Version 0.2.0. |