C# Connection pool issue: The timeout period elapsed prior to obtaining a connection from the pool

Question

OrmLiteConnectionFactory is responsible for creating a new SqlConnection . MaxConnectionPoolSize = 100 . PoolSize < 50 .

Code that creates a new connection:

services.AddSingleton<IDbConnectionFactory>((IDbConnectionFactory)Activator.CreateInstance(typeof(OrmLiteConnectionFactory), connectionString));
services.AddScoped<IDbConnection>((provider) =>
            {
                var factory = provider.GetRequiredService<IDbConnectionFactory>();
                var connection = factory.OpenDbConnection();
                return connection;
            });

Sometimes while opening a new connection SqlClient throws:

System.InvalidOperationException: '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.

Call Stack:

at System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)
   at System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)
   at System.Data.SqlClient.SqlConnection.Open()
   at ServiceStack.OrmLite.OrmLiteConnection.Open()

So, the max connection pool size is not reached but a new connection can't be established. Really appreciate any thoughts.

Answer 1

The connection pool is machine wide. Are there other apps or processes running that use database connections? Maybe your SQL Server has a 'max worker threads' limit?

Otherwise an answer is to look at why so many connections are in use for so long. Maybe the connections can be released earlier.

Answer 2

What you must do every time your app does db work is to make sure that SqlDataReaders , SqlCommand , SqlConnection objects are closed and disposed of as soon as possible. The easiest way to do that is to use the Using keyword on objects that implement IDisposable . Otherwise, these types of objects remain in memory until the garbage collector gets around to destroying them. Check source code in your entire app to implement the above method.

I had to do this when the exact same issue reared its head when my company's customers were calling our APIs!