AJAX: Asynchronous?

Asynchronous?

So AJAX stands for Asynchronous JavaScript and XML. As a technology, it sounds perfect for supporting client-server communications for fat-client web applications. But is it really? I mean, it is 'asynchronous' and it's HTTP, right? Well, not totally...

While the XMLHTTPRequest object does make an asynchronous call to the back-end server, these calls in themselves are not asynchronous on the TCP layer, only at thread-level are they asynchronous. Therefor, the calls themselves are synchronized and prone to being queued.

According to HTTP 1.1 specifications, only two connections are supported per process. Internet Explorer sticks to this limitation, while other browser such as Firefox support a higher connection limit by default. The means that requests get queued if there is more than the browser limit already active.

When requests queue up, long requests will block other requests, including images, javascript and css file calls. So, if you really want an asynchronous call system, AJAX is not the solution. But most likely a piece of the total solution.