Web API 2 Asp.Net - setting explicit Timeout value

I think setting a timeout on the request is the wrong approach as you will have no visibility of what is going on during the 40 seconds.

Rather make a ajax web request and then subsequent web requests to see if the process has completed.

For example,

1. Queue the request somehow with the initial request.
2. Write something to pick up and process the item from the queue. This also means if something goes wrong, you can just roll back at this point. You also need to store the status of the item somewhere.
3. Write a periodic poll in Javascript that makes another ajax request every 5 seconds to see if the request has been processed or not.

Depending on what kind of method is running on your WebApi service you could try the following:

1. Start a StopWatch at the start of your action
2. Check periodically if the elapsed time is greater than your arbitrary limit. When that happens throw an Exception (I called mine CalculationTimeLimitExceededException)
3. Catch the exception and perform a rollback (assuming you want to do a rollback on the server)
4. Return a response (e.g. HTTP 500 with some useful information, e.g. server timeout)
5. Since the client gets a response within your time limit you can then handle the error on the client side.

Update (added code for PerformanceWatcher):

public class PerformanceWatcher : IDisposable
{
    private System.Diagnostics.Stopwatch _sw;
    private Timer _timer;
    private int _maxSeconds;

    public bool TimeLimitExceeded { get; private set; }

    public PerformanceWatcher(int maxSeconds)
    {
        _maxSeconds = maxSeconds;
        // start the StopWatch
        _sw = System.Diagnostics.Stopwatch.StartNew();
        // check every second
        _timer = new Timer(1000);
        _timer.AutoReset = true;

        // set event-handler
        _timer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
        _timer.Enabled = true;
    }

    private void OnTimedEvent(object source, ElapsedEventArgs e)
    {
        // check if time limit was reached
        if (this._sw.Elapsed.TotalSeconds > _maxSeconds)
        {
            this.TimeLimitExceeded = true;
        }
    }

    public void Dispose()
    {
        this._timer.Dispose();
    }
}

You can use this class in your action:

[HttpGet]
public HttpResponseMessage GetResultFromLongRunningMethod()
{
    using (PerformanceWatcher watcher = new PerformanceWatcher(10))
    {
        try
        {
            // begin long-running operation
            for (int i = 0; i < 20; i++) 
            {
                if (watcher.TimeLimitExceeded) 
                {
                    throw new TimeLimitExceededException();
                }
                Thread.Sleep(1000);
            }
            // end long-running operation
        } catch (TimeLimitExceededException e)
        {
            return this.Request.CreateErrorResponse(HttpStatusCode.InternalServerError, "Time limit exceeded");
        }
    }
    return this.Request.CreateResponse(HttpStatusCode.OK, "everything ok");
}

The above code isn't tested; I just copied some elements from my own classes.

Update 2: Fixed a bug in the code (Exception thrown from event handler wasn't caught)