Owner decision on #107 (2026-11-13): enabling AuditLog:HashChain must not
chain the historical backlog. main chained every unchained row oldest-first.
- The first chaining pass on a pristine database (no head, no chained row)
writes a chain-start anchor: AuditLogChainTombstone (cutoff month,
ChainSeq 0), PurgedAtUtc = DB SYSUTCDATETIME(), RowHash =
SHA-256("ScadaBridge.AuditLog.ChainCutoff.v1" || YYYY-MM || ticks).
Written once under the exclusive chain lock, so both centrals agree
whatever their clocks/config. No migration: existing table + grants.
- Only rows with OccurredAtUtc >= cutoff are chained. The anchor hash is
the cutoff month's genesis, binding the cutoff into the chain.
- AuditLog:HashChain:ChainBacklog (default false) restores chain-everything,
read only on the very first enablement. Legacy chains (heads, no anchor)
keep chaining everything.
- verify-chain splits unchained rows into UnchainedRows vs PreCutoffRows
(+ ChainFromUtc); new breaks ChainedBeforeCutoff (row re-dated before the
cutoff, or cutoff moved forward) and CutoffAnchorInvalid (anchor hash /
month / multiplicity). Archive verify uses the anchor from its own
tombstone file or the DB.
- Partition switch-out keeps the anchor (deletes ChainSeq > 0 only); the
head store / export / verify-heads tombstone count excludes it.
- CLI verify-chain table prints Chain from + Pre-cutoff rows.
Tests: AuditLogHashChainCutoffTests (7, MSSQL), 6 verifier unit tests,
service/CLI/management wire tests; 9 mutations all killed.
Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi
Audit roadmap T1 (alog.md §11.4): one chain per UTC calendar month of
OccurredAtUtc. Each chained row carries ChainSeq (gap-free within its month)
and RowHash = SHA-256(prev.RowHash || canonical(row)); the v1 canonical form
(AuditChainHasher) is pinned by a known-answer vector computed independently.
- Migration AddAuditLogHashChain: nullable ChainSeq/RowHash on dbo.AuditLog
(metadata-only add), AuditLogChainHead (per-month chain end + partition-purge
checkpoint anchor), AuditLogChainTombstone, two partition-aligned filtered
indexes, scadabridge_audit_chainer role with a column-level UPDATE grant.
Production script docs/plans/sql/AddAuditLogHashChain.sql.
- AuditLogHashChain chains unchained rows oldest month first under an
exclusive sp_getapplock (zero timeout, so a dual-active central pair still
yields one chain), with an optimistic head check and a ChainSeq IS NULL
row-count check that roll a pass back instead of leaving a gap. The ingest
path is untouched.
- Purges keep the chain verifiable: the partition switch records the month's
checkpoint anchor and drops its tombstones; the per-channel DELETE writes a
tombstone per chained row in the same transaction (OUTPUT INTO). Both, and
the SourceNode backfill, hold the chain lock; the backfill skips chained
rows. The switch staging table gains the two new columns.
- AuditChainVerifier reports the first break (partition, ChainSeq, EventId,
kind) for edited, deleted, reordered rows and tail truncation.
- AuditLogHashChainService (central, default OFF via AuditLog:HashChain).
Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV
Joseph Doherty