Suppose you are building a live dashboard. The server needs to push updates whenever a job finishes, a metric changes, or a new alert appears, but the browser does not need to send messages back over the same connection.

WebSockets work, but full bidirectional messaging may be more complexity than you need. Long polling works, but every response closes the request and forces the client to reconnect.

Server-Sent Events (SSE) sit between those two. The browser opens one connection and the server streams text events over a single long-lived HTTP response.

This chapter covers what SSE provides and what it does not, how the event stream format works, how browser reconnection and Last-Event-ID work, how to implement SSE safely on the server, and when to choose SSE instead of long polling or WebSockets.