Professional Ajax, 2nd Edition

3.2. Fallback Patterns

The previous section dealt with when to send or receive data from the server, which presupposes that everything goes according to plan on the server side: the request is received, the necessary changes are made, and the appropriate response is sent to the client. But what happens if there's an error on the server? Or worse yet, what if the request never makes it to the server? When developing Ajax applications, it is imperative that you plan ahead for these problems and describe how your application should work if one of these should occur.

3.2.1. Cancel Pending Requests

If an error occurs on the server, meaning that a status of something other than 200 or 304 is returned, you need to decide what to do. Chances are that if a file is not found (404) or an internal server error occurred (302), trying again in a few minutes isn't going to help, since both of these require an administrator to fix the problem. The simplest way to deal with this situation is to simply cancel all pending requests. You can set a flag somewhere in your code that says, "don't send any more requests." This clearly has the highest impact on solutions using the Periodic Refresh pattern.

The comment notification example can be modified to take this into account. This is a case where the Ajax solution provides additional value to the user but is not the primary focus of the page. If a request fails, there is no reason to alert the user; you can simply cancel any future requests to prevent ...