How to do long polling properly in MVC 3

Question

I am trying to wire up an AsyncController so that when a user clicks save on an order on the order page, all users viewing the same order should get a notification that the order has changed. My approach to implement this is to do long polling ajax requests on the order page, however how to make a scalable AsyncController to deal with this is not obvious to me.

So this is what I have so far, the ID is the ID of the order that is signaled as changed or polled for changes.

public class MessageController : AsyncController
{
    static readonly ConcurrentDictionary<int, AutoResetEvent> Events = new ConcurrentDictionary<int, AutoResetEvent>();

    public ActionResult Signal(int id)
    {
        AutoResetEvent @event;
        if (Events.TryGetValue(id, out @event))
            @event.Set();

        return Content("Signal");
    }

    public void WaitAsync(int id)
    {
        Events.TryAdd(id, new AutoResetEvent(false));

        // TODO: This "works", but I should probably not block this thread.
        Events[id].WaitOne();
    }

    public ActionResult WaitCompleted()
    {
        return Content("WaitCompleted");
    }
}

I have had a look at How to do long-polling AJAX requests in ASP.NET MVC? . I am trying to understand all details about this code but as far as I understand this code it is blocking each worker thread in the thread pool which, as far as I understand would eventually lead to thread starvation.

So, how should I implement this in a nice, scalable way? Bear in mind that I do not wish to use any more third party components, I want to get a good understanding of how to implement this scenario properly.

Deepsea

Answer 1

Actually I was able to implement this without blocking the worker threads, the thing I was missing was ThreadPool.RegisterWaitForSingleObject.

public class ConcurrentLookup<TKey, TValue>
{
    private readonly Dictionary<TKey, List<TValue>> _lookup = new Dictionary<TKey, List<TValue>>();

    public void Add(TKey key, TValue value)
    {
        lock (_lookup)
        {
            if (!_lookup.ContainsKey(key))
                _lookup.Add(key, new List<TValue>());

            _lookup[key].Add(value);
        }
    }

    public List<TValue> Remove(TKey key)
    {
        lock (_lookup)
        {
            if (!_lookup.ContainsKey(key))
                return new List<TValue>();

            var values = _lookup[key];
            _lookup.Remove(key);

            return values;
        }
    }
}

[SessionState(SessionStateBehavior.Disabled)]
public class MessageController : AsyncController
{
    static readonly ConcurrentLookup<int, ManualResetEvent> Events = new ConcurrentLookup<int, ManualResetEvent>();

    public ActionResult Signal(int id)
    {
        foreach (var @event in Events.Remove(id))
            @event.Set();

        return Content("Signal " + id);
    }

    public void WaitAsync(int id)
    {
        AsyncManager.OutstandingOperations.Increment();

        var @event = new ManualResetEvent(false);

        Events.Add(id, @event);

        RegisteredWaitHandle handle = null;
        handle = ThreadPool.RegisterWaitForSingleObject(@event, (state, timeout) => 
        {
            handle.Unregister(@event);
            @event.Dispose();

            AsyncManager.Parameters["id"] = id;
            AsyncManager.Parameters["timeout"] = timeout;
            AsyncManager.OutstandingOperations.Decrement();
        }, null, new TimeSpan(0, 2, 0), false);
    }


    public ActionResult WaitCompleted(int id, bool timeout)
    {
        return Content("WaitCompleted " + id + " " + (timeout? "Timeout" : "Signaled"));
    }
}

Deepsea

Answer 2

Now days it is much easier to implement long polling with async/await.

public class MessageController : ApiController
{
    private static readonly ConcurrentDictionary<int, ManualResetEventAsync> ManualResetEvents = new ConcurrentDictionary<int, ManualResetEventAsync>();

    [HttpGet]
    public IHttpActionResult Signal(int id)
    {
        if (ManualResetEvents.TryGetValue(id, out var manualResetEvent) == false)
        {
            return Content(HttpStatusCode.OK, "Signal: No one waiting for signal");
        }

        manualResetEvent.Set();

        return Content(HttpStatusCode.OK, "Signaled: " + id);
    }

    [HttpGet]
    public async Task<IHttpActionResult> Wait(int id)
    {
        var manualResetEvent = ManualResetEvents.GetOrAdd(id, _ => new ManualResetEventAsync());

        var signaled = await manualResetEvent.WaitAsync(TimeSpan.FromSeconds(100));

        var disposed = manualResetEvent.DisposeIfNoWaiters();
        if (disposed)
        {
            ManualResetEvents.TryRemove(id, out var _);
        }

        return Content(HttpStatusCode.OK, "Wait: " + (signaled ? "Signaled" : "Timeout") + " " + id);
    }
}

internal class ManualResetEventAsync
{
    private readonly SemaphoreSlim semaphore = new SemaphoreSlim(0, int.MaxValue);
    private int waiters;

    public void Set()
    {
        semaphore.Release(int.MaxValue);
    }

    public async Task<bool> WaitAsync(TimeSpan timeSpan)
    {
        lock (semaphore)
        {
            waiters++;
        }

        var task = await semaphore.WaitAsync(timeSpan);

        lock (semaphore)
        {
            waiters--;
        }

        return task;
    }

    public bool DisposeIfNoWaiters()
    {
        lock (semaphore)
        {
            if (waiters != 0)
            {
                return false;
            }

            semaphore.Dispose();
            return true;
        }
    }
}

Deepsea