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EventSource refers to the technique of Server-Sent Events (SSE) implemented via the EventSource API in web browsers. It enables a client to receive automatic, real-time updates from a server over a single long-lived HTTP connection. It is designed for unidirectional server-to-client communication and complements techniques such as polling and WebSockets.

Communication with SSE uses the HTTP-based text/event-stream format. The server streams events as lines of text. Each event can include data lines prefixed with data:, an optional event type prefixed with event:, an optional identifier prefixed with id:, and an optional retry time prefixed with retry:. An empty line ends the current event. Data lines are concatenated to form the event payload, and if no event type is specified the default type is message. On the client side, events are delivered via an EventSource object, with handlers for onmessage and onerror, and the ability to listen for named events.

The transport remains a single open HTTP connection. The server can send events whenever available, and the client attempts automatic reconnection with a backoff strategy if the connection is lost, using the retry value if provided. The connection is typically established with a GET request to the stream URL and content is delivered with the text/event-stream MIME type.

Usage and compatibility considerations include browser support, cross-origin access via CORS, and security implications. SSE works well for live updates such as feeds or notifications and is generally simpler and lighter than WebSockets for server-to-client streams. Limitations include lack of binary data support, no built-in full-duplex communication, and reliance on HTTP streaming semantics. Standards are defined in the WHATWG HTML Living Standard, with wide implementation in major browsers.