Question

or do Server-Sent Events and WebSocket replace Comet techniques?

Answer 1

Comet is a set of technology principles/communication patterns that are typically implemented using HTTP long-poll. It enables a server to send data to the browser on demand (i.e. server push). Current comet implementations require some complex Javascript on the client side and support from the server-side (for long-held requests).

Server-Sent Events is a standard (HTML5) browser API for enabling this sort of on demand server push. You can think of Server-Sent Events as taking what has been done with complex Javascript and pushing it down into the browser itself.

WebSockets allows a browser to establish a persistent full-duplex/bi-directional connection to a server with WebSocket support. It does not require the client to keep making periodic HTTP requests to the server in order to maintain the connection as with AJAX/long-poll. Once the connection is established the overhead per message is very low (a few bytes) compared to the overhead with normal HTTP/HTTP long-poll. You can use WebSockets for efficient server push, but this is just one application.

There are also libraries that build on the AJAX/comet/WebSockets transport layer to provide things like session management, channels, broadcast, pubsub, etc. CometD is an example of this. Another popular example is Socket.IO. Both support WebSockets if it is available for the underlying transport but also support standard AJAX/long-poll if WebSockets is not available.