real-time-systems-architect

Use SSE with a Next.js route handler that subscribes to BullMQ QueueEvents and forwards progress, completed, and failed events as SSE messages. On the client, consume the stream with EventSource in a React hook and reconnect with exponential backoff on errors.

Choose SSE when data flows server to client only, such as job progress or dashboard metrics, since it is simpler and works through reverse proxies. Use WebSocket when the client also sends events, such as presence heartbeats, chat, or collaborative editing.

Authenticate with JWT at connection time, track clients in a map keyed by connection ID, and broadcast join and leave events to room members. Run a reaper every 30 seconds to terminate connections that have not pinged within the timeout window.

Yes, but you must disable proxy buffering with the X-Accel-Buffering: no header and send keep-alive ping events every 15 seconds to prevent proxy timeouts. SSE also supports cookie-based auth, simplifying authentication compared to WebSocket.

Never trust local client state after a disconnect. Use version-stamped server state: on reconnect, fetch the latest server state and apply it whenever the server version is ahead of the client's last known version.

Use a bounded broadcaster where each subscriber has a capped queue. When a subscriber's queue is full, drop the oldest event and log a warning, then flush queued batches on each tick so one slow consumer never blocks producers.