EventSource

EventSource is a web API that enables web applications to receive automatic, real-time updates from a server using server-sent events (SSE). It opens a single long-lived HTTP connection through which the server can push text streams to the client. The API is designed for unidirectional communication from server to client, making it well suited for live feeds, notification streams, and real-time dashboards.

The server sends data in the text/event-stream format. Each event can include fields such as data, id, event, and retry. The data field contains the event payload, which may span multiple lines; the event field designates a named event type, while id assigns a unique identifier that the client can reuse when reconnecting. The retry field, if sent by the server, instructs the client on how long to wait before attempting to reconnect after a disruption. Lines beginning with a colon are treated as comments. An empty line signals the end of the current event.

On the client side, the API provides an EventSource object. The constructor, typically new EventSource(url, options), creates the connection. The object exposes readyState and supports event handlers such as onopen, onmessage, and onerror. A MessageEvent carries the event data and may expose lastEventId and the event type. The connection can be closed programmatically with close(). If the connection drops, the browser automatically attempts to reconnect using the last known URL, honoring the server-provided retry interval when available.

Use cases include live news updates, stock price feeds, and real-time monitoring. EventSource is supported by most major browsers but is not available in older ones like Internet Explorer, though polyfills exist. It differs from WebSocket by providing a simpler, server-to-client channel without full duplex communication. Security considerations include content type checks (text/event-stream) and proper handling of reconnection logic in servers.