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.
This commit is contained in:
Joseph Doherty
@@ -57,7 +57,13 @@ internal sealed class SnapshotStreamer(SqliteLocalDb db, OplogStore store, Repli
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break;
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lastPk = rows[^1].PkJson;
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// One DB page may span several messages: the page is bounded by ROW COUNT, but a
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// message is bounded by BYTES (ReplicationOptions.MaxBatchBytes) — the same budget
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// the delta pump honours, and for the same reason. Wide rows are exactly why a
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// snapshot is the easiest way to exceed the transport's message limit: it ships
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// whole rows, all of them, back to back.
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var batch = new SnapshotBatch { TableName = table.Name };
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var bytes = 0L;
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foreach (var row in rows)
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{
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if (!row.IsTombstone && row.RowJson is null)
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@@ -79,11 +85,25 @@ internal sealed class SnapshotStreamer(SqliteLocalDb db, OplogStore store, Repli
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};
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if (row.RowJson is not null)
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snapshotRow.RowJson = row.RowJson;
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var size = RowSize(snapshotRow);
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// Flush before adding, never after: a message keeps at least one row however
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// large, so an oversized row goes out alone rather than stalling the snapshot.
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if (batch.Rows.Count > 0 && bytes + size > options.MaxBatchBytes)
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{
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await send(new SyncMessage { SnapshotBatch = batch }, ct);
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totalRows += batch.Rows.Count;
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batch = new SnapshotBatch { TableName = table.Name };
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bytes = 0;
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}
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batch.Rows.Add(snapshotRow);
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bytes += size;
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}
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if (batch.Rows.Count > 0)
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{
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await send(new SyncMessage { SnapshotBatch = batch }, ct);
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totalRows += batch.Rows.Count;
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totalRows += batch.Rows.Count;
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}
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if (rows.Count < options.MaxBatchSize)
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break;
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@@ -104,11 +124,23 @@ internal sealed class SnapshotStreamer(SqliteLocalDb db, OplogStore store, Repli
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return asOfSeq;
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}
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/// <summary>
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/// Serialized bytes one row contributes to its batch message: the row's own encoding plus the
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/// repeated field's tag and length prefix, as protobuf itself computes them.
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/// </summary>
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private static long RowSize(SnapshotRow row) =>
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1 + Google.Protobuf.CodedOutputStream.ComputeMessageSize(row);
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// Pages one table's rows by joining the row-version ledger to the live table on the PK columns
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// extracted from pk_json. Rows present in the live table but absent from __localdb_row_version
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// (data written BEFORE the table was registered for replication) are intentionally NOT snapshotted:
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// they were never captured, consistent with CDC semantics — a full re-registration is required to
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// baseline them.
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// (data written BEFORE the table was registered for replication) are NOT snapshotted: they were
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// never captured, consistent with CDC semantics.
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//
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// That is a real hole for a node that starts replicating LATE — its whole pre-existing dataset is
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// invisible to the peer, silently and permanently — and the supported way to close it is to
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// register with `ILocalDb.RegisterReplicated(table, baselineExistingRows: true)`, which seeds the
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// ledger at an LWW-safe floor and flags the resync that carries the rows here. Nothing changes on
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// this side: baselined rows are ordinary ledger rows and page like any other.
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private static string BuildPageSql(ReplicatedTable table)
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{
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var name = Ident(table.Name);
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