Say I am creating a webchat, where the chat messages are stored in a SQL database (I don't know how else to do it), what benefits does using AJAX to long-poll instead of simply polling every x seconds?
Since PHP runs only when you open the page, the long-polled PHP script will have to check for new messages every second as well. What benefits does long-polling have then? Either way, I'm going to have a latency of x seconds, only with long-polling the periodic checking happens on the server.
Long polling, in your case, has two advantages:
First, long polling allows clients to receive message updates immediately after they become available on the server, increasing the responsiveness of your webchat.
The second advantage is that almost no change is required in the client application in order to work in this mode. From the client’s point of view, a blocked poll request looks like a network delay, the only difference is that the client doesn't need to wait between sending poll requests, as it would if you were simply polling every x seconds.
However, making a server hold requests increases server load. Usual web servers with synchronous request handling use one thread per each request, this means that waiting request blocks the thread by which it is handled. Thus, 100 chat clients which use long polling to get message updates from the server, will block 100 threads.
Most of these threads will be in the waiting state, but every thread still uses a considerable amount of resources. This problem is solved in Comet by asynchronous request processing, a technique allowing request blocking without blocking a thread, which is now supported by several web servers including Tomcat.
Reference for my answer: oBIX Watch communication engine reference document
