CentralCommunicationActor.HandleHeartbeat was forwarding each incoming
HeartbeatMessage to Context.Parent, which resolves to the /user
guardian — a non-actor. Every site heartbeat went straight to dead
letters (~1026 per central node per 30 minutes at the default ~2s
interval across three sites).
The aggregator now exposes MarkHeartbeat(siteId, receivedAt) which
bumps LastReportReceivedAt on already-known sites (and clears IsOnline
if it had flipped) without touching LatestReport. Heartbeats from
unregistered sites are dropped — first registration still happens on
the first full report. CentralCommunicationActor calls this in place
of the no-op Tell.
The result: heartbeats now serve their stated health-monitoring
purpose (per CLAUDE.md) by keeping a site marked online between the
30s full reports if a single report is briefly delayed, and the dead
letter noise disappears entirely.
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