Connections from the pool are not reused when using async methods in parallel

Whem I’m using async methods (like FirstOrDefaultAsync) to get a result from the Database inside a Parallel.For, the connections are not correctly reused from the connection pool.

The connections will rise to the configured Max Pool Size and after some time (around 1-2 minutes on my machine using localdb), exceptions will be thrown.

Using either async methods and a normal for-loop or non-async methods an a Parallel.For-loop, the connections are properly reused and I can observe that a normal for-loop uses 1 connection and a Parallel.For-loop uses 4 connections which corresponds to MaxDegreeOfParallelism.

Exception message:
System.InvalidOperationException occurred
  HResult=0x80131509
  Message=Timeout expired.  The timeout period elapsed prior to obtaining a connection from the pool.  This may have occurred because all pooled connections were in use and max pool size was reached.
Stack trace:
   at System.Data.Common.ADP.ExceptionWithStackTrace(Exception e)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.<OpenAsync>d__31.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<BufferlessMoveNext>d__9.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Storage.ExecutionStrategy.<ExecuteImplementationAsync>d__33`2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.EntityFrameworkCore.Storage.ExecutionStrategy.<ExecuteImplementationAsync>d__33`2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable.AsyncEnumerator.<MoveNext>d__8.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.<MoveNext>d__4.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.<_FirstOrDefault>d__82`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.TaskResultAsyncEnumerable`1.Enumerator.<MoveNext>d__3.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.SelectAsyncEnumerable`2.SelectAsyncEnumerator.<MoveNext>d__4.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at EfPoolingTest.UnitTest1.<>c.<NormalContextParallel>b__4_0(Int32 i) in C:\Users\xxx\documents\visual studio 2017\Projects\EfPoolingTest\EfPoolingTest\UnitTest1.cs:line 70
   at System.Threading.Tasks.Parallel.<>c__DisplayClass19_0`1.<ForWorker>b__1(RangeWorker& currentWorker, Int32 timeout, Boolean& replicationDelegateYieldedBeforeCompletion)

Steps to reproduce

• Clone https://github.com/wertzui/EfPoolingTest and open the project
• Run the “ViewConnections.sql” script to see that there are no connections open (beside the 2 used for the script)
• Run the test “NormalContextParallelAsyncConfigureAwaitFalse” (I suggest running in Debug mode, so you can see the output)
• Run the “ViewConnections.sql” script multiple times to see the connections increase.
• After some time you will also get an exception
• Run the other tests and observe the connection count

Further technical details

EF Core version: 2.0.0 (also tested with 1.1.3) Database Provider: Microsoft.EntityFrameworkCore.SqlServer/localdb Operating system: Windows 10 FCU IDE: Visual Studio 2017 15.4.1

[ralmsdeveloper]

Hello @wertzui, Seeing better, I made this little change in the Context and it was more satisfying, until it reaches 100, but then reduce!

using Microsoft.EntityFrameworkCore;
using System.Data.SqlClient;

namespace EfCoreLongStringTest
{
    public class MyContext : DbContext
    {
        public DbSet<MyTable> MyTable { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseSqlServer("Server=(localdb)\\mssqllocaldb;Initial Catalog=EfCorePoolingTest;Integrated Security=true;",
                    options => options.EnableRetryOnFailure());
            }
            base.OnConfiguring(optionsBuilder);
        }

        public override void Dispose()
        {
            SqlConnection.ClearPool((SqlConnection)Database.GetDbConnection());  
            base.Dispose(); 
        }
    }
}

If you can take a look at this it would be good, because the EF team would have a better direction!

[wertzui]

I tried your example and the behavior does not change (100 connections and exception after some time). I added that test case to my repo.

Afaik calling context.Database.CloseConnection() does not really close the connection, but returns it to the pool.