Can the client send data back over an SSE connection?

No, the stream is strictly server to client. Clients send data with ordinary separate requests, usually POSTs, which is fine for occasional actions like sending a chat message or acknowledging a notification. If the client needs to send many messages per second, that is the signal to consider WebSocket instead.

Does SSE work through load balancers and proxies?

Yes, better than WebSocket, because it is a normal HTTP response. The two things that break it are response buffering, fixed with proxy_buffering off or an X-Accel-Buffering: no header, and idle timeouts, handled by sending a comment line as a heartbeat every 15 to 30 seconds. Serve it over HTTP/2 to avoid the browser's 6-connection-per-domain limit on HTTP/1.1.

How many SSE connections can one server hold?

On an event loop runtime, tens of thousands per box is routine, since an idle connection costs a few tens of kilobytes and a file descriptor. The practical limits are the same as any push fleet: fan-out architecture for getting events to the right server, file descriptor limits, and handling the reconnect wave when a server restarts.

Real-Timechapter 3 of 4 · 3 lessons

Server-Sent Events (SSE)

One HTTP connection; server streams events to client indefinitely.

← All chapters View in cheatsheet

Server-Sent Events is push with the smallest possible surface: one long-lived response that your server keeps writing events into.

It is the least famous of the four techniques here and arguably the best default for the most common need, a server that talks and a client that listens. Being the least famous is most of why teams skip it, reaching instead for the option they have heard of, then spending a quarter building reconnection logic that was already in the box.

Lessons

3 in this chapter

The Event Stream ProtocolOne GET response that never ends, carrying plain-text events separated by blank lines.2 min
Reconnection and Last-Event-IDThe browser reconnects and resumes by itself, if your server can answer the replay question.2 min
SSE vs WebSocketIf the client rarely talks, SSE does the same job with a fraction of the operational surface.2 min