Server-Sent Events
Also known as:
SSE
EventSource
Definition
Server-Sent Events (SSE) stream data one way, server to client, over a single long-lived HTTP connection using the text/event-stream content type. The browser’s EventSource API handles the connection, auto-reconnects when it drops, and sends a Last-Event-ID header so the server can resume from where it left off. SSE is simpler than WebSockets when you only need server-to-client updates, not full duplex.
Key Takeaways
- SSE is one-way streaming from server to client over ordinary HTTP, no special protocol upgrade needed.
- The EventSource API auto-reconnects (default about 3 seconds) and replays from Last-Event-ID.
- It is simpler than WebSockets when you do not need the client to stream back.
- Because it is HTTP, it works with standard infrastructure, though some proxies buffer the stream.
How It Works
- The client opens an EventSource to a URL that responds with Content-Type text/event-stream.
- The server keeps the connection open and writes events as they happen, each as a small text frame.
- The client receives events through onmessage handlers as they arrive.
- If the connection drops, EventSource reconnects and sends Last-Event-ID so the server can resume.
Where It Is Used
- Live dashboards, notification feeds, and sports scores commonly use SSE.
- Many LLM chat UIs stream tokens to the browser over SSE.
- When the client also needs to send a steady stream, teams reach for WebSockets instead.