The #687/F4 crowding re-read doubled its room up to 8 re-reads with no cap, so a pathological single-ms
cluster on a streaming gateway was streamed several times over for one page. The room is now capped at
ServerHistorian:MaxTieClusterOverfetch (plumbed into GatewayHistorianDataSource as maxPreWindowRows,
default 65536); past it, or once a read has already seen more, the read is BadTooManyOperations with
PreWindowRows set.
Gateway HistoryRead-AtTime passed AVEVA's own interpolation through: live it
answered the previous value as Good at a stored NULL's own time, null + Bad at
a Good sample right after a NULL, a drifted 99.99999999944 for a stored 100,
and no Interpolated bit. It is now served from raw gateway reads and evaluated
with Sql.Common's AtTimeEvaluator, as the SQL backends do (Part 11 §6.5.4).
GatewayAtTimeReader: one <=1,024-row forward window read [first-1ms, last+1ms]
for the batch, a walk-back (density-seeded, x4 growth, whole-remainder shortcut,
bisects dense history) for the first time's prev, and an 18-row probe for the
last time's next; truncated window reads continue or jump with a density-sized
margin. All bounds whole ms; a row counts only strictly inside its request
(AVEVA's boundary row never a sample); Bad rows value-less (#690); never past
now + 1 day; strict-gateway ceiling learned (#691.3) without re-probing.
10 dense times = 1 read (exact) / 3 (interpolated); 10 hourly = 12.
Kill switch ServerHistorian:GatewayAtTimeFromRaw (default true; false = the
old AVEVA passthrough). Known limit: the wire carries no data type, so numeric
tags interpolate as Double (Boolean/integer nodes are not stepped/rounded as on
the SQL backends).
Gate record: docs/plans/2026-07-20-localdb-phase2-live-gate.md.
Checks 1, 2, 5, 6 pass. Checks 3 and 4 are NOT satisfied — the defect they
were meant to confirm turned out to be the opposite of what the plan assumed.
Three defects crash-looped every driver node before check 1 could even run:
1. An empty ServerHistorian:ApiKey kills the host. ServerHistorianOptions-
Validator exists to turn exactly that class of failure into a named
OptionsValidationException, but its documented fail tier explicitly excluded
ApiKey on the reasoning that a keyless client "degrades — the gateway rejects
calls". It does not: the client validates its own options at construction, so
the process dies during Akka startup and never makes a call.
2. UseTls disagreeing with the endpoint scheme kills the host too, in both
directions (both messages confirmed in the shipped client assembly). Moving an
endpoint from https to http without clearing UseTls is an ordinary migration
slip.
3. Plaintext h2c was UNREACHABLE. HistorianGatewayClientAdapter forwarded the
TLS-only options unconditionally, and AllowUntrustedServerCertificate defaults
to false, so it always sent RequireCertificateValidation=true — which the
client rejects outright when UseTls=false. Every http:// deployment crashed,
though the scheme is documented as the supported way to select h2c, and the
only workaround was to assert a certificate posture for a connection that has
no certificate.
The fourth was the blocker, and it is Phase 2's own:
4. The drain gate deferred to a Primary that cannot deliver. Redundancy roles are
elected CLUSTER-WIDE; the alarm queue is PAIR-LOCAL. On the rig the elected
driver Primary is central-1 — it carries the driver Akka role, replicates
nobody's LocalDb and does not even run the alarm historian — so every driver
node logged "Historian drain suspended", including the two site-b nodes that
have no peer at all. Nothing drained anywhere, where before Phase 2 it drained
fine. The cost is not a duplicate; it is the buffer growing to the capacity
wall and evicting the audit trail it exists to protect.
Fixed in three layers: a separate ShouldDrainAlarmHistory policy (unknown role
drains; the two gates now deliberately disagree, and a test pins that); peer-
host matching in DriverHostActor so a node stands down only for a Primary
holding its rows; and AddAlarmHistorian short-circuiting the gate when
replication is unconfigured — testing BOTH Replication:PeerAddress and
SyncListenPort, since only the dialing half sets the former while both halves
share the queue.
Every one of these follows from the asymmetry: a false allow costs a duplicate
row, which at-least-once delivery already accepts and payload-hash ids
collapse; a false deny loses data silently.
A third vacuous test, caught by the same delete-the-guard discipline: the
role-view tests stayed green with the guard removed, because AwaitAssert polls
until an assertion passes and the assertion was "reads open" — which is the
SEEDED value, satisfied at the first poll before the actor processed anything.
They now assert the sequence of published values through a recording view; the
control then goes red for exactly the cases that matter.
Migration evidence: 11 legacy rows across two deliberately overlapping files
converged to exactly 9 identical rows on both nodes, proving D-6's payload-hash
identity on real nodes rather than in a fixture.
Open design fork, recorded in the gate doc rather than decided here: a pair
cannot currently identify its own Primary, so both halves drain. Safe in every
topology — nothing loses data — but the gate's de-duplication benefit is
unrealised until roles are scoped per pair.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Joseph Doherty