ASP.NET AjaxAsynch request has separate session

We are writing a search application that saves the search criteria to session state and executes the search inside of an updatepanel. Sometimes when we execute multiple searches successively the 2nd or 3rd search will sometimes return results from the first set of search criteria.

Example: our first search we do a look up on "John Smith" -> John Smith results are displayed. The second search we do a look up on "Bob Jones" -> John Smith results are displayed.

We save all of the search criteria in session state as I said, and read it from session state inside of the ajax request to format the DB query. When we put break points in VS everything behaves as normal, but without them we get the original search criteria and results.

My guess is because they are saved in session, that the ajax request somehow gets its own session and saves the criteria to that, and then retrieves the criteria from that session every time, but the non-async stuff is able to see when the criteria is modified and saves the changes to state accordingly, but because they are from two different sessions there is a disparity in what is saved and read.

To elaborate more, there was a suggestion of appending the search criteria to the query string which normally is good practice and I agree thats how it should be but following our requirements I don't see it as being viable. They want it so the user fills out the input controls hits search and there is no page reload, the only thing they see is a progress indicator on the page, and they still have the ability to navigate and use other features on the current page. If I were to add criteria to the query string I would have to do another request causing the whole page to load, which depending on the search criteria can take a really long time. This is why we are using an ajax call to perform the search and why we aren't causing another full page request..... I hope this clarifies the situation.

There are not multiple sessions between normal ASP.NET page loads, postbacks, and ASP.NET AJAX partial postbacks. I can tell you that with certainty.

Rather than storing the search string in the session, how about just using the search TextBox's contents directly? I can't think of any reason why you'd need to shuffle it around, since it will be available throughout the entire page lifecycle anyway.

Finally, concerning your requirements... Using an UpdatePanel does not fulfill the requirement that your users should be able to use other functionality on the page if that functionality also raises partial postbacks. Only one partial postback can be in progress at a time. If another event is raised while your search is in progress, the search request will be canceled without any notification.

Using a page method or web service for the search would be a much faster, easier, and more robust way of doing it. I don't usually plug my own site, but I think a couple of my posts are exactly relevant to what you're doing:

You could use a user control to render the search results through a web service (very much faster than an UpdatePanel):

Or, you could return the search results as JSON and render that on the client side (even faster):

Either of those methods could take your search functionality out of the partial postback paradigm, so that it runs faster, uses less bandwidth and server resource, and doesn't preclude other UpdatePanel activity from occurring concurrently.

Just another thought, I've always run into problems with updatepanel and prefer to write my atlas ajax requests through the library directly, using PageMethods. You have more control over what you send and receive. UpdatePanel sends the entire page, and receives the entire page control heirarchy, then it parses out what is 'fresh' and displays that.

Edit: What is the code you're using to save the criteria to the session? And do you have code in the method that actually checks to see if the session has some saved criteria, and passes that back instead? Maybe that's why the 2nd/3rd updatepanel postbacks are returning the first set of criteria instead of the expected results? As an aside, I know from doing some heavy atlas ajax things that there is definitely not two sessions (one for normal postback, one for async) Is there any chance you're using a webfarm?

Edit #2: I wouldn't have been able to write what I've written above (first para) if I hadn't been a fan of someone who replied as well:

You need to set the EnableSession property of the WebMethod attribute for the function you are calling.

[WebMethod( EnableSession=true )]
public static void DoSomething(){
    /// ....

If you use generic handlers .ashx, just derive from IRequiresSessionState interface

public class ActionRequest : IHttpHandler, IRequiresSessionState

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