What is SSE
Key idea:
SSE (Server-Sent Events, EventSource API) — a standard for streaming data from server to client over plain HTTP. Client opens a GET to the endpoint with Accept: text/event-stream, the server keeps the connection open and sends messages as they are ready. Unlike WebSocket: unidirectional (server→client only), works through any HTTP proxy, native auto-reconnect, no Upgrade handshake.
Below: details, example, related terms, FAQ.
Details
- Content-Type: text/event-stream
- Format:
data: {json}\n\n - event: custom event name, id: for resume after disconnect
- Reconnection: automatic with
Last-Event-IDheader - 6 connections per-domain limit in Chrome (HTTP/1.1). HTTP/2 removes the limit
Example
const es = new EventSource('/stream');
es.onmessage = e => console.log(JSON.parse(e.data));
es.addEventListener('user-login', e => {...});Related Terms
Learn more
Frequently Asked Questions
When to use SSE over WebSocket?
Server-only push (notifications, live feed). Simpler code, works through proxies. For chat/games you need WebSocket.
Automatic reconnect?
Yes. Browser reconnects with exponential backoff + sends <code>Last-Event-ID</code>.
HTTP/1.1 6-connection limit — a problem?
For a dashboard with 10+ SSE feeds — yes. Use a single multiplex endpoint or migrate to HTTP/2/3.