High Performance Web Sites

Asynchronous = Instantaneous?

One of the cited benefits of Ajax is that it provides instantaneous feedback to the user because it requests information asynchronously from the backend web server. In the article referenced earlier, Jesse James Garrett uses Google Suggest and Google Maps as examples of web interfaces where "everything happens almost instantly."

Be careful! Using Ajax is no guarantee that the user won't be twiddling his thumbs waiting for those "asynchronous JavaScript and XML" responses to return. I'd hate to use Google Maps and Yahoo! Maps on a dial-up connection. In many applications, whether or not the user is kept waiting depends on how Ajax is used. Frontend engineers once again shoulder the responsibility of identifying and following the best practices required to ensure a fast user experience.

A key factor to whether the user might be kept waiting is whether the Ajax requests are passive or active. Passive requests are made in anticipation of a future need. For example, in a web-based email client, a passive request might be used to download the user's address book before it's actually needed. By loading it passively, the client makes sure the address book is already in its cache when the user needs to address an email message. Active requests are made based on the user's current actions. An example is finding all the email messages that match the user's search criteria.

The latter example illustrates that even though active Ajax requests are asynchronous, the ...