AJAX callback locks the page

Question

I have a long-running database query that I've placed in an asynchronous AJAX callback (or so I think) to allow the user to navigate to another page if they're not interested in the results.

Whether I call the query automatically after page load or on click, the page always locks up until it returns, i.e links and buttons don't work. Internet Explorer 6's own menus are fine however, so it's not that IE itself is overworked.

What could I be doing wrong? It seems as though my request isn't really asynchronous. Here is a code snippet:

this.XmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
this.XmlHttp.open("POST", url, true);
this.XmlHttp.send(data);

Answer 1

It's not the browser that blocks the page, it's the server.

IIS only handles one request at a time from each user. As the user already has a request from the AJAX call that the server is working on, the new request will be queued until the first request is handled.

To keep the long running request from blocking the user on the server you have to make the page that the AJAX call gets session-less. That way it's not associated to the user.