Connection problems with SQL Server in ASP.NET applications using out-of-process session state

Question

I have several ASP.NET applications deployed in a farm of 4 Windows 2003 machines. Each application uses a separate App Pool and Virtual Directory in IIS. They rely heavily on sessions which are persisted out of process on a single SQL Server 2000 (<sessionstate mode="sqlserver" ... />). Applications are compiled against .NET 3.0 but .NET 3.5 SP1 is installed on servers.

Each web server receives approximately 10 requests/second. Every once in a while I get some exceptions in logs:

System.Data.SqlClient.SqlException: A transport-level error has occurred when receiving results from the server. (provider: TCP Provider, error: 0 - The semaphore timeout period has expired.)    at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)    at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)    at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)    at System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error)    at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)    at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()    at System.Data.SqlClient.TdsParserStateObject.ReadBuffer()    at System.Data.SqlClient.TdsParserStateObject.ReadByte()    at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)    at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)    at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)    at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)    at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe)    at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()    at System.Web.SessionState.SqlSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) 

or another:

System.Data.SqlClient.SqlException: A transport-level error has occurred when sending the request to the server. (provider: TCP Provider, error: 0 - An existing connection was forcibly closed by the remote host.)    at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection)    at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)    at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)    at System.Data.SqlClient.TdsParserStateObject.WriteSni()    at System.Data.SqlClient.TdsParserStateObject.WritePacket(Byte flushMode)    at System.Data.SqlClient.TdsParserStateObject.ExecuteFlush()    at System.Data.SqlClient.TdsParser.TdsExecuteRPC(_SqlRPC[] rpcArray, Int32 timeout, Boolean inSchema, SqlNotificationRequest notificationRequest, TdsParserStateObject stateObj, Boolean isCommandProc)    at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)    at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)    at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe)    at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()    at System.Web.SessionState.SqlSessionStateStore.SetAndReleaseItemExclusive(HttpContext context, String id, SessionStateStoreData item, Object lockId, Boolean newItem) 

or yet another:

System.Data.SqlClient.SqlException: Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.    at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)    at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)    at System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error)    at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)    at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()    at System.Data.SqlClient.TdsParserStateObject.ReadBuffer()    at System.Data.SqlClient.TdsParserStateObject.ReadByte()    at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)    at System.Data.SqlClient.SqlDataReader.ConsumeMetaData()    at System.Data.SqlClient.SqlDataReader.get_MetaData()    at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)    at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)    at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)    at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)    at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)    at System.Data.SqlClient.SqlCommand.ExecuteReader()    at System.Web.SessionState.SqlSessionStateStore.DoGet(HttpContext context, String id, Boolean getExclusive, Boolean& locked, TimeSpan& lockAge, Object& lockId, SessionStateActions& actionFlags) 

These errors occur a couple of times a day for a period of about 1-2 minutes and then disappear. Has anyone encountered such problems? What could you suggest me to do in order to further track down the problem? To me it looks more like network issues than application. Could it be some settings on the SQL Server which cannot handle so many concurrent connections? Any suggestions would be greatly appreciated.

---

UPDATE:

I've solved the problem by performing major updates to the application in order to reduce the number and size of the objects being stored in the session.

Answer 1

Transport level errors are often linked to the connection to sql server being broken ... usually network.

Timeout Expired is usually thrown when a sql query takes too long to run.

So I would troubleshoot the link to your Sql Server and then monitor to see what queries are timing out.

Sounds like a SQL job is running, backup? That might be locking tables or restarting the service.

Answer 2

In my case the problem was related with host TCP configuration (virtual machine in VMWare). After quick research (some article found in Google and MSDN Blogs) I've turned off in system registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\synattackprotect (0) and HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\EnableTCPChimney (0). Now it works just fine. Of course this was only test machine (corporate network not visible from Internet) and I wouldn't do that on production environment ;-)