Eve Martin:

Sometimes we need information from our servers instantaneously. For this sort of use case our usual AJAX request/response doesn’t cut it. Instead, we need a push-based method like WebSockets, Long Polling, Server-Sent Events (SSE) and - more recently - HTTP2 push. In this article, we compare two methods for implementing realtime - WebSockets and Server-Sent Events, with information on which to use and when.

This is a nice, reasoned comparison of the two technologies: advantages, stumbling blocks, open source resources, etc.