buzz/perf/RELAY_BUS_SCALING.md

Buzz relay bus scaling harness

This harness gives reproducible evidence for the rewrite's Redis fan-out scaling claim:

• old/global bus: every relay pod receives every community's event;
• new/community-scoped bus: each pod retains only the server-resolved community topics for which it has local subscribers (buzz:{community_id}:global or buzz:{community_id}:channel:{channel_id}).

Run:

./perf/relay_bus_scaling.py

Baseline scenario used for the PR summary:

64 communities × 100 events/s, one subscribed community, all pods interested in that community

Current output:

pods	old global cluster ingress/s	old avg pod ingress/s	new scoped cluster ingress/s	new avg pod ingress/s	reduction	old irrelevant/pod	new irrelevant/pod
1	6,400	6,400	100	100	64.0×	98.44%	0.00%
2	12,800	6,400	200	100	64.0×	98.44%	0.00%
4	25,600	6,400	400	100	64.0×	98.44%	0.00%

The code path this corresponds to is buzz_pubsub::EventTopicKey::redis_channel() (crates/buzz-pubsub/src/topic.rs), with retain_topic / release_topic driving dynamic local Redis SUBSCRIBE interest.

This isolates the bus-bound scaling property. Live relay latency, DB capacity, and client rendering should be measured separately with a full stack because they include unrelated bottlenecks.