Asp.NET Concurrent AJAX Calls

In Asp.NET "if two concurrent requests are made for the same session (by using the same SessionID value), the first request gets exclusive access to the session information. The second request executes only after the first request is finished." (ASP.NET Session State Overview)

The keywords here are "same session".

When an Ajax call to an ASP.NET Web method is made:

[WebMethod(true)] // EnableSession = true

public static void MethodName()

{

}

If the "EnableSession" is set to true there can be only one ajax call at a time. Further calls will wait in pending state until the call is finished. Once it's finished the order of previous calls is not guaranteed.

If EnableSession is set to false you can make concurrent calls but will not have access to the Session. The number of calls that can be made depends on each browser.

However, if you're user ASP.NET MVC 3 and above there is an attribute named SessionState that enables you to make concurrent calls and still access your session (in read-only mode)

. (Stefan's Tech Notes)

Concurrent calls :

[SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]

Sequencial calls:

[SessionState(System.Web.SessionState.SessionStateBehavior.Required)]