Server-Sent Events (SSE)

What Is SSE?

Server-Sent Events (SSE) allow a server to push updates to the browser over a single long-lived HTTP connection.

SSE is unidirectional (server -> client only).

If the client needs to send data back, it must use a separate HTTP request.

Why SSE Was Created

Traditional HTTP:

Client → Request → Server → Response → Close

Problem:

• Server cannot push updates on its own.
• Polling solution:
  • Client keeps asking.
• WebSocket solution:
  • Persistent bi-directional connection
• SSE solution:
  • Persistent server -> client streaming over HTTP

It is simpler than WebSockets when you only need push notifications.

How SSE Works

1 Client Opens a Connection

Browser creates an EventSourc:

const source = new EventSource("/events");

This sends a regular HTTP request:

GET /events HTTP/1.1
Accept: text/event-stream

2 Server Responds With Special Content-Type

Server responds:

HTTP/1.1 200 OK
Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive

Important header:

Content-Type: text/event-stream

This tells the browser:

Keep this connection open. Data will stream.

3 Server Streams Messages

Messages are sent in a special format:

data: Hello World

data: Another message

Each message ends with a blank line.

4 Browser Receives Events Automatically