Why is my async ASP.NET Web API controller blocking the main thread?

Question

I have an ASP.NET Web API controller that I would have thought would operate asynchronously. The controller is designed to sleep 20 seconds for the first request, but service any subsequent requests immediately. So my anticipated timeline would be something like:

Raise request 1.
Raise request 2.
Raise request 3.
Request 2 returns.
Request 3 returns.
Wait ~20 seconds.
Request 1 returns.

Instead, no requests return until request 1 is finished.

I can confirm (based on the debug outputs), that the entry thread and sleepy thread id are different. I've intentionally used TaskCreationOptions.LongRunning to force the sleep onto a separate thread, but still the application refuses to service any new requests until that sleep has finished.

Am I missing something fundamental about how async Web API controllers really work?

public class ValuesController : ApiController
{
    private static bool _firstTime = true;

    public async Task<string> Get()
    {
        Debug.WriteLine("Entry thread id: {0}. Sync: {1}",
            Thread.CurrentThread.ManagedThreadId,
            SynchronizationContext.Current);
        await LongWaitAsync();
        return "FOOBAR";
    }

    private Task LongWaitAsync()
    {
        return Task.Factory.StartNew(() =>
            {
                if (_firstTime)
                {
                    _firstTime = false;
                    Debug.WriteLine("Sleepy thread id: {0}. Sync: {1}",
                        Thread.CurrentThread.ManagedThreadId,
                        SynchronizationContext.Current);
                    Thread.Sleep(20000);
                    Debug.WriteLine("Finished sleeping");
                }
            },
            CancellationToken.None,
            TaskCreationOptions.LongRunning,
            TaskScheduler.Default);
    }
}

As a first step, I'd mark `_firstTime` as `volatile`, to make sure that separate threads can see it changing consistently. — Damien_The_Unbeliever, Feb 21 '13 at 07:17
Marking `_firstTime` as `volatile` makes no difference. I've tried hosting in IIS Express 8.0 and IIS 7.5, both on Win 7 Pro. — Snixtor, Feb 21 '13 at 07:20
@Snixtor - it may make no difference, but it's something that ought to be done anyway. — Damien_The_Unbeliever, Feb 21 '13 at 07:20
@Damien_The_Unbeliever Duly noted. I'll admit I had to look up its meaning and it's definitely relevant to my example code. — Snixtor, Feb 21 '13 at 07:22
Does Task Get() call Result or Wait()? Those are blocking operations. — Alex, Feb 21 '13 at 08:17

score 14 · Accepted Answer · answered Feb 21 '13 at 08:22

This actually has nothing to do with the server, and everything to do with the client. Both Chrome and Firefox don't appear to want to send what they deem a "duplicate" request until the first one has its response. A separate "private" session of either browser will return from the second request immediately. Internet Explorer 9 doesn't seem to exhibit this behaviour.

To isolate from client implementations, I put together the following client.

class Program
{
    static void Main(string[] args)
    {
        var t1 = Task.Run(() => FetchData(1));
        var t2 = Task.Run(() => FetchData(2));
        var t3 = Task.Run(() => FetchData(3));

        var index = Task.WaitAny(t1, t2, t3);
        Console.WriteLine("Task {0} finished first", index + 1);

        Task.WaitAll(t1, t2, t3);
        Console.WriteLine("All tasks have finished");

        Console.WriteLine("Press any key");
        Console.ReadKey(true);
    }

    static void FetchData(int clientNumber)
    {
        var client = new WebClient();
        string data = client.DownloadString("http://localhost:61852/api/values");
        Console.WriteLine("Client {0} got data: {1}", clientNumber, data);
    }
}

It's output goes:

Client 2 got data: "FOOBAR" (within milliseconds of startup)
Client 3 got data: "FOOBAR"
Task 2 finished first
(long wait here)
Client 1 got data: "FOOBAR"
All tasks have finished

Thought I was going crazy. Thanks for the revelation! – NovaJoe Oct 01 '14 at 14:19 — NovaJoe, Oct 01 '14 at 14:19

score 0 · Answer 2 · answered Feb 21 '13 at 10:26

In my case here is output (it switches to another thread from 5 to 10):

Entry thread id: 5. Sync: System.Web.LegacyAspNetSynchronizationContext
Sleepy thread id: 10. Sync: 
Finished sleeping
The thread '<No Name>' (0x2c84) has exited with code 0 (0x0).
Entry thread id: 7. Sync: System.Web.LegacyAspNetSynchronizationContext
The thread '<No Name>' (0x1818) has exited with code 0 (0x0).
Entry thread id: 5. Sync: System.Web.LegacyAspNetSynchronizationContext
The thread '<No Name>' (0xd4c) has exited with code 0 (0x0).
The thread '<No Name>' (0x2c30) has exited with code 0 (0x0).

This could be due to environment and machine running (single core) that makes the runtime decide how to run it.

Why is my async ASP.NET Web API controller blocking the main thread?

2 Answers2

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