Server-Sent Events (SSE)

Server-Sent Events (SSE) is a browser-native technology for streaming real-time updates from server to client over a standard HTTP connection. Unlike WebSockets, SSE is unidirectional (server to client only), uses regular HTTP (simplifying infrastructure -- no special proxy configuration needed), and includes built-in reconnection with automatic retry and last-event-ID tracking. The EventSource browser API makes consuming SSE streams trivial: const es = new EventSource('/api/events') with onmessage callbacks.

The SSE protocol uses a simple text-based format with event, data, id, and retry fields. Each event is separated by a double newline. Named events (event: orderUpdate) allow multiplexing different event types on a single connection, and the id field enables resumable streams -- on reconnection, the browser sends the Last-Event-ID header and the server can replay missed events. The retry field controls the reconnection interval in milliseconds.

SSE is ideal for use cases like live dashboards, notification feeds, progress updates for long-running operations, and AI/LLM streaming responses. It's simpler than WebSockets when bidirectional communication isn't needed and works through HTTP/2 with connection multiplexing. Limitations include a maximum of 6 concurrent connections per domain in HTTP/1.1 (resolved by HTTP/2), text-only data (binary requires base64 encoding), and no built-in client-to-server channel (use regular HTTP requests for that direction).