This article was originally published on Jo4 Blog.
We replaced our notification-bell polling with Server-Sent Events. On a single pod, this is easy — Spring WebFlux + a Flux of events, done in 30 lines. On multiple pods, it gets interesting fast: a write that happens on pod A needs to reach a subscriber on pod B, and naively, it doesn't.
Here's the full pattern: how the SSE endpoint is shaped, how Redis pub/sub fans events across pods, and the subtle ordering and lifecycle gotchas we hit along the way.
The Endpoint
A /notifications/stream SSE endpoint that emits the current unread count on subscribe, then live updates whenever the count changes:
