Pub/sub between servers
Redis, NATS, or Kafka. Publisher writes once; every server delivers to its local SSE clients.
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Client → server sticky? No
Each SSE stream lives on one server. On reconnect, LB may route to a different server. That's fine — Last-Event-ID makes it stateless.
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Per-user fan-out
Every server subscribes to a channel per online user. Publisher publishes once. Delivering server has O(1) local clients per channel.