handling XMLHttpRequest abort on asp.net

Question

I use asynchronous XMLHttpRequest to call a function in ASP.net web service.
When I call an abort method on the XMLHttpRequest, after the server has received the request and processing it, the server continues processing the request.
Is there a way to stop the request processing on the server?

score 3 · Accepted Answer · edited May 23 '17 at 12:27

Generally speaking, no, you can't stop the request being processed by the server once it has started. After all, how would the server know when a request has been aborted?

It's like if you navigated to a web page but browsed to another one before the first one had loaded. That initial request will, at least to some extent (any client-side work will of course not take place), be fulfilled.

If you do wish to stop a long-running operation on the server, the service that is being invoked will need to be architected such that it can support being interrupted. Some psuedo code:

void MyLongRunningMethod(opId, args)
{
    work = GetWork(args)

    foreach(workItem in work)
    {
        DoWork(workItem)

        //Has this invocation been aborted?
        if(LookUpSet.Contains(opId))
        {
            LookUpSet.Remove(opId)
            return
        }
        //Or try this:
        if(Response.IsClientConnected)
        {
            HttpContext.Current.Response.End();
            return;
        }
    }
}

void AbortOperation(opId)
{
    LookUpSet[opId] = true
}

So the idea here is that MyLongRunningMethod periodically checks to see if it has been aborted, returning if so. It is intended that opId is unique, so you could generate it based on the session Id of the client appended with the current time or something (in Javascript, new Date().getTime() will get you the number of milliseconds since the epoch).

With this sort of approach, the server must maintain state (the LookUpSet in my example), so you will need some way of doing that, such as a database or just storing it in memory. The service will also need to be architected such that calling abort does not leave things in a non-working state, which of course depends very heavily on what it does.

The other really important requirement is that the data can be split up and worked on in chunks. This is what allows the service to be interruptable.

Finally, if some operation is to be aborted, then AbortOperation must be called - simply aborting the XMLHttpRequest invocation won't do help as the operation will continue until completion.

Edit

From this question: ASP.Net: How to stop page execution when browser disconnects?

You could also check the Response.IsClientConnected property to try and determine whether the invocation had been aborted.

Generally speaking, the server isn't going to know that a client has disconnected until it attempts to send data to it. See Best practice to detect a client disconnection in .NET? and Instantly detect client disconnection from server socket.

I don't understand something. XMLHttpRequest waits for a response from server, and that's why the connection isn't terminated during processing the request. When the request is aborted, the connection is terminated, so the server should know somehow about it. Am I missing something? — Andy, Nov 28 '12 at 21:23

score 0 · Answer 2 · answered Nov 29 '12 at 21:23

As nick_w wrote you can't stop the request being processed by the server once it has started. But there is ability to implement solution which will give you ability to cancel server task. Dino Esposito has several great articles about how such things can be implemented: Canceling Server Tasks with ASP.NET AJAX

And in the following articles to implement pooling to server Dino Esposito describes how to use SignalR library: Build a Progress Bar with SignalR; Long Polling and SignalR

So if you really need to cancel some task on server these articles can be used as starting point to implement required solution.

handling XMLHttpRequest abort on asp.net

2 Answers2