EventSource is a new HTML5 API specified in the WHATWG draft on Server Sent Events.
EventSource works by sending a stream of text based events over a normal HTTP connection that never closes.
EventSource is designed to handle network problems well and to fall back to traditional long-polling gracefully.
The server maintains a buffer of events and if the event-stream connection gets disconnected the browser will retry it with an added "Last-Event-Id" header that the server can use to play back any lost events.
This makes Server Sent events a very reliable delivery mechanism.
HTTP/1.1 200 OK Content-Type: text/event-stream data: This is an event event: notification data: This is a notification id: 12345 data: This is an event with an id data: Multiline event data: Line 2