Question

I have an ASP.NET MVC web application that makes REST style web service calls to other servers. I have a scenario where I am making two HttpWebRequest calls to two separate services. I need them both to complete to continue, but their order doesn’t matter. They may take 1-2 seconds each and I am running them in sequence now. Running them in parallel would decrease the user response time, but what is the best way?

In researching this, I can think of several options:

• Execute one request on the main thread, and spin up a second thread for the other request. Should created new threads or use a thread pool? If I use a pool, how do I size it? Also, not sure how I can join the threads back together (e.g. use ThreadPool.RegisterWaitForSingleObject)?
• Try and leverage the built-in IAsyncResult support for one or both of the requests. Again, not sure on which threads the async request execute, so not sure how to size the thread pool. How do I join IAsyncResult back into my main thread? All the examples I find process information in the callback, but I can just wait in my main thread and use the IsCompleted property?

I need to find a solution that will both function, and perform at scale. That is why I am worried about thread pool sizing. I would hate to have requests blocking because they are waiting for available threads.

Answer 1

One of the answers to Multithreading WebRequests, a good and stable approach? : CSharp uses a ManualResetEvent event = new ManualResetEvent(), a reference counter equaling the number of in flight requests and Interlocked.Decrement to control the event.Set(). The main thread then waits by calling event.WaitOne().

However, WaitHandles – Auto/ManualResetEvent and Mutex mentions that ManualResetEvent “can be significantly slower than using the various Monitor methods” like Wait, Pulse and PulseAll.

I ended up basing my code off of this Noah Blumenthal blog post: Run generic tasks async (fluent-ly). I did make two changes: implement IDisposable and call .Close() on the ManualResetEvent and switch from using a lock() to Interlocked .Increment() and .Decrement().

public class AsyncQueueManager : IDisposable {
    private readonly ManualResetEvent waitHandle = new ManualResetEvent(true);
    private int count = 0;

    public AsyncQueueManager Queue(Action<object> action) {
        Interlocked.Increment(ref count);
        waitHandle.Reset();
        Action<object> actionWrapper = CreateActionWrapper(action);
        WaitCallback waitCallback = new WaitCallback(actionWrapper);
        ThreadPool.QueueUserWorkItem(waitCallback);
        return this;
    }

    private Action<object> CreateActionWrapper(Action<object> action) {
        Action<object> actionWrapper = (object state) =>
        {
            try {
                action(state);
            } catch (Exception ex) {
                // log
            } finally {
                if (Interlocked.Decrement(ref count) == 0) {
                    waitHandle.Set();
                }
            }
        };
        return actionWrapper;
    }

    public void Wait() {
        waitHandle.WaitOne();
    }
    public void Wait(TimeSpan timeout) {
        waitHandle.WaitOne(timeout);
    }

    public void Dispose() {
        waitHandle.Close();
    }
}