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Blocking Detection for ASP.NET Core

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Blocking Detection for ASP.NET Core

NuGet version (Ben.Demystifier) Build status

Detect Blocking Calls in ASP.NET Core Applications

Blocking calls can lead to ThreadPool starvation. Ouputs a warning to the log when blocking calls are made on the ThreadPool.


Add early to your ASP.NET Core pipeline (see: samples)



Doesn't detect everything... so its not a panacea; you should actively try to avoid using blocking calls.

  1. For
    methods with occurances of
    , detection won't alert for blocking
    calls after occurances in the case where the returned
    wasn't already completed
  2. Won't alert for blocking calls that don't block, like on precompleted
    s (e.g. a single small
  3. Won't alert for blocking that happens in syscalls (e.g.

Will detect CLR initiated waits

etc; if they do block.


If you had a method like:

public static int BlockingTask()
    // Detected blocking
    return MethodAsync().Result;

private static async Task MethodAsync() { await Task.Delay(1000); return 5; }

It would output to your log

warn: Ben.Diagnostics.BlockingMonitor[6]
  Blocking method has been invoked and blocked, this can lead to threadpool starvation.
    at System.Threading.Tasks.Task.InternalWait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
    at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
    at mvc.HomeController.BlockingTask()                d__12.MoveNext()
    at System.Runtime.CompilerServices.AsyncTaskMethodBuilder.Start[TStateMachine](TStateMachine& stateMachine)
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.d__10.MoveNext()
    at Microsoft.AspNetCore.Hosting.Internal.RequestServicesContainerMiddleware.Invoke(HttpContext httpContext)
    at Microsoft.AspNetCore.Hosting.Internal.HostingApplication.ProcessRequestAsync(Context context)
    at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.Frame`1.d__2.MoveNext()
    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
    at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c__DisplayClass4_0.b__0()
    at Microsoft.AspNetCore.Server.Kestrel.Internal.System.IO.Pipelines.Pipe.<>c.<.cctor>b__67_3(Object o)
    at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.LoggingThreadPool.<>c__DisplayClass6_0.b__0()
    at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.LoggingThreadPool.b__3_0(Object o)
    at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
    at System.Threading.ThreadPoolWorkQueue.Dispatch()

Also catches held locks

120: public Task LockedMethod(HttpContext httpContext)
121: {
122:     var response = httpContext.Response;
123:     response.StatusCode = 200;
124:     response.ContentType = "text/plain";
125:     lock (obj) // **********
126:     {
127:         // locked outside for 1 sec
128:     }
129:     var s = new string('n', 16);
130:     response.ContentLength = s.Length;
131:     return response.WriteAsync(s);
132: }

Outputs (with some extra formatting)

warn: Microsoft.AspNetCore.Diagnostics.DetectBlocking[6]
 Blocking method has been invoked and blocked, this can lead to threadpool starvation.
     at Task AoA.Gaia.Startup.LockedMethod(HttpContext httpContext)
       in C:\Work\AoA\src\AoA.Gaia\Startup.cs:line 125 ********
     at IApplicationBuilder Microsoft.AspNetCore.Builder.UseExtensions.Use(IApplicationBuilder app, Func, Task> middleware)+() => { }
     at void Microsoft.AspNetCore.Server.Kestrel.Internal.System.IO.Pipelines.Pipe._scheduleContinuation(object o)
     at void Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.LoggingThreadPool.Schedule(Action action, object state)+() => { }
     at void Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.LoggingThreadPool.RunAction()+(object o) => { }
     at bool System.Threading.ThreadPoolWorkQueue.Dispatch()

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