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Document the central alarm monitor and fan-out reversal

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gateway.md describes the always-on GatewayAlarmMonitor, the session-less
StreamAlarms feed, and session-less AcknowledgeAlarm. DesignDecisions.md
records that the v1 "one subscriber per session / no fan-out" rule is
superseded for the alarm subsystem: alarm state is gateway-wide, so the
gateway monitors it centrally and fans it out to all clients.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Joseph Doherty
2026-05-21 16:32:23 -04:00
parent ac12c150c3
commit 0361dc1817
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docs/DesignDecisions.md
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@@ -82,6 +82,18 @@ fan-out may be added later with explicit backpressure semantics.
Rationale: one subscriber preserves simple event ordering and failure behavior
while parity is being proven.
### Alarms — superseded for the alarm subsystem
The single-subscriber rule above no longer applies to alarms. The gateway runs
an always-on central alarm monitor (`GatewayAlarmMonitor`) that owns one
gateway-managed worker session, caches the active-alarm set, and fans it out to
any number of clients through the session-less `StreamAlarms` RPC. Per-session
alarm auto-subscribe is removed; `AcknowledgeAlarm` is session-less and routes
through the monitor. Data-side `StreamEvents` remains one subscriber per
session. Rationale: alarm state is gateway-wide, not session-scoped — every
client wants the same current set plus updates, and forcing each to own a
worker would multiply AVEVA polling load for no benefit.
## Authentication
Decision: API key authentication for the public gateway.