7

As traffic to our Azure website has increased, we've been seeing an increasing number of "A task was canceled" errors. A typical trace might look like this:

Fatal web api error: Controller: CustomerUserEventsController; 
Url: http://app.payboard.com/api/organizations/9ddf55d1-e0c1-4a8f-9327-eef38682e090/addcustomeruserevent?callback=jQuery210035782216349616647_1398442710964&cookieId=05be2755-dc0d-414d-b0d2-ea1986a929c3&customerId=&customerName=&customerUserId=&customerUserFirstName=&customerUserLastName=&eventName=hr-index-GET&_=1398442710965; 
Error: A task was canceled. ( at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext())

We used to maybe get one a day; it's been increasing gradually, so that now we're getting a couple dozen.

You'll notice that there's only framework code in the stack trace - none of our code is in there at all. So I'm a bit at a loss for how to troubleshoot it. Any suggestions on how to approach tracking these down?

ahmelsayed
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Ken Smith
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  • We also met this issue before, that exception occurs when the server is very busy handling the requests or it's not available at that time. What we did is scaled out Azure websites, and at least now we don't see this exception anymore. But I am not quite sure of this. – Jerry Bian Jul 10 '14 at 09:32

2 Answers2

3

See the answer here:

ASP.NET Web API OperationCanceledException when browser cancels the request

And the WebAPI bug here:

http://aspnetwebstack.codeplex.com/workitem/1797

Basically, it's expected when an HTTP request is canceled, for instance, if the user closes the page that initiated the request. To keep it from showing up in your logs, basically, you just need to create a custom message handler that suppresses the error:

/// <summary>
/// See https://stackoverflow.com/questions/22157596/asp-net-web-api-operationcanceledexception-when-browser-cancels-the-request
/// </summary>
public class CancelledTaskBugWorkaroundMessageHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var response = await base.SendAsync(request, cancellationToken);

        // Try to suppress response content when the cancellation token has fired; ASP.NET will log to the Application event log if there's content in this case.
        if (cancellationToken.IsCancellationRequested)
        {
            return new HttpResponseMessage(HttpStatusCode.InternalServerError);
        }

        return response;
    }
}
Community
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Ken Smith
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0

Had the exact same problem. In my case the solution was to change the authority (an https URL) from https://login.windows.net/... to https://login.microsoftonline.com/... as it could not be reached anymore. I am not publishing the full authority as it is a specific one for our organisation. Theoretically the first one is more direct, so I used that based on advise from a Microsoft specialist. But if it cannot be reached then there is a problem. Will need to go figure out nw why it is unreachable.

Hope this will be a lead to resolve your issues. Regards, Coen.

Coen
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