ReachabilityChanged

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Defined in: src/cluster/ClusterEvents.ts:103

This node’s own failure detector changed its verdict about one peer (#161).

Two things separate it from MemberUnreachable / MemberReachable, and both matter when the question is what can this node actually see:

It is always a local observation. Member status travels in gossip, so MemberUnreachable also fires for a peer that someone else has stopped hearing from while this node’s own heartbeats to it arrive normally.
It does not depend on the member’s status. MemberUnreachable is only emitted for a member that was up; a peer that falls silent while joining, weakly-up or leaving produces no reachability event at all, it simply gets downed later.

Emitted on transition only, and never for a peer that has been healthy since this node first saw it — the interesting edges are the fall and every recovery after it. The verdict is recomputed once per detector tick (heartbeatIntervalMs) and turns negative at unreachableAfterMs, so this fires well ahead of any downing decision.

It carries no observer set. “Which other nodes also cannot reach X” would need an observer→subject table on the wire, and gossip carries a flat member list — see #161 for the follow-up that would have to add one.