Clustering

Set a strategy and a shared cookie, and nodes find each other:

What crosses nodes

• Publishes. An event published on one node reaches subscribers on all of them. Each node fans out into its own registry, and the frame is encoded once per broadcast rather than once per subscriber.
• Presence. Membership is a CRDT, so it converges after a partition without any node coordinating. A node that dies has its members removed elsewhere within about five seconds.
• App configuration. Credential changes are published to every node, so a rotated secret stops verifying everywhere as soon as the dashboard returns.
• Channel queries. Occupancy asks every node and sums the answers.

What stays local

• Rate limits. Token buckets are per node, so a cluster-wide limit is approximate: N nodes allow up to N times the rate. That is deliberate — an exact global limiter would put a coordination hop in the publish path.
• Cache channel retention. Each node keeps the last event it saw for a cache channel. A subscriber may get a cache miss on a node that has not seen the event yet.
• Metrics. Each node reports its own numbers; the dashboard sums them.

Sizing

Connections are cheap: one process and one socket each. What moves first under load is the run queue, so watch vm_total_run_queue_lengths_total alongside connection counts. Add nodes when a single one is saturating its schedulers, and remember that every node holds a full copy of presence.