Verifies the FilterIndex path in LiveEventStore: each event matches
exactly ONE of N subscribers, so without an index the relay walks
all N subs per event (O(N)); with the author-keyed index the lookup
is O(1) and per-event latency stays flat as N grows.
Different from fanoutLatency (broadcast: 1 event reaches every
sub). Here per-event work is lookup-bound rather than
delivery-bound. Configurable via -DfanoutScalingSubs (comma list)
and -DfanoutScalingEvents.
Measured (laptop, JDK 21, full sweep):
100 subs, 2k events: p50=1.35ms p99=5.71ms
1000 subs, 2k events: p50=1.05ms p99=3.05ms
5000 subs, 1k events: p50=1.01ms p99=2.96ms
p50 ~1ms across N — the predicted O(1) scaling. Default events count
adapts downward at high N to stay below WebSocketSessionPump's
8192-frame outbound cap (test-client read side is the bottleneck
above ~5k subs, not the relay).
Plan doc updated with the measured numbers in the "How to verify"
section.
Claude
0d0fbf36c9