Synchronous Concurrent HttpClient Usage

Question

I am trying to use HttpClient synchronously but when I make many concurrent requests it stops working. I wrote two tests, one for asynchronous usage and one for synchronous usage. TestMethod always returns response after 4 seconds. Asynchronous test works fine. Almost all requests are timed out in synchronous test, only ~20 last requests are successful. I tried both using a single HttpClient for requests and creating new HttpClient instance for each new requests. No difference. Maybe it has something to do with this deadlock.

I am using VS2013 with .NET Framework 4.5.1 targeting Framework 4.5. I get HttpClient via NuGet: <package id="Microsoft.Net.Http" version="2.2.15" targetFramework="net45" />

I don't want to use HttpClient asynchronously just yet because it means that I have to rewrite my whole app. Any ideas what am I doing wrong here?

// all 800 requests complete successfully
[Test]
public async void Asyncmethod()
{
    var sw = new Stopwatch();
    sw.Start();

    var client = new HttpClient();
    client.Timeout = TimeSpan.FromSeconds(15);

    var tasks = Enumerable.Range(0, 800).Select(x => Task.Run(async () =>
    {
        try
        {
            var swx = new Stopwatch();
            swx.Start();
            var response = await client.GetStringAsync("http://localhost:7002/TestMethod").ConfigureAwait(false);
            swx.Stop();
            Console.WriteLine(x + " " + response + " in " + swx.ElapsedMilliseconds + " ms.");
        }
        catch (Exception e)
        {
            Console.WriteLine(x + " Exception: " + e.Message);
        }
    })).ToArray();

    await Task.WhenAll(tasks);

    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
}

// almost all of 800 requests time out
[Test]
public void Syncmethod()
{
    var sw = new Stopwatch();
    sw.Start();

    var client = new HttpClient();

    var tasks = Enumerable.Range(0, 800).Select(x => Task.Run(() =>
    {
        try
        {
            var swx = new Stopwatch();
            swx.Start();
            var response = client.GetStringAsync("http://localhost:7002/TestMethod");
            if (response.Wait(15000))
            {
                swx.Stop();
                Console.WriteLine(x + " " + response.Result + " in " + swx.ElapsedMilliseconds + " ms.");
            }
            else
            {
                swx.Stop();
                Console.WriteLine(x + " timed out.");
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(x + " Exception: " + e.Message);
        }
    })).ToArray();

    foreach (var task in tasks)
        task.Wait(60000);

    sw.Stop();
    Console.WriteLine(sw.ElapsedMilliseconds);
}

Answer 1

I do recommend that you use await for HttpClient instead of Wait. If you truly want synchronous requests, consider using WebClient which supports synchronous methods.

That said, I believe the reason you're seeing that behavior is due to ServicePointManager.DefaultConnectionLimit, which will throttle the number of requests to a given server. If you want to have large numbers of concurrent requests to the same server (synchronous or asynchronous), then you'll need to increase that limit (it is 2 by default for UI applications).