SQL Timeout to Principle Server Error
I am currently investigating an occurrence of the same timeout exception with the same key wording:


  This failure occured while attempting to connect to the Principle server.


The environment is SQL Server 2012 with mirroring, so I suspect that the problem was not directly related to a long-running query, but rather to a temporary network/sync issue, either between the application and the SQL servers or between the SQL servers themselves. Stack trace:

System.Data.SqlClient.SqlException (0x80131904): Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.  This failure occured while attempting to connect to the Principle server. ---&gt; System.ComponentModel.Win32Exception (0x80004005): The wait operation timed out
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean&amp; dataReady)
   at System.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   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, Int32 timeout, Task&amp; task, Boolean asyncWrite)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task&amp; task, Boolean asyncWrite)
   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.ExecuteDbDataReader(CommandBehavior behavior)
   at System.Data.Common.DbCommand.ExecuteReader()
   at System.Data.Linq.SqlClient.SqlProvider.Execute(Expression query, QueryInfo queryInfo, IObjectReaderFactory factory, Object[] parentArgs, Object[] userArgs, ICompiledSubQuery[] subQueries, Object lastResult)
   at System.Data.Linq.SqlClient.SqlProvider.ExecuteAll(Expression query, QueryInfo[] queryInfos, IObjectReaderFactory factory, Object[] userArguments, ICompiledSubQuery[] subQueries)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.ChangeDirector.StandardChangeDirector.DynamicUpdate(TrackedObject item)
   at System.Data.Linq.ChangeDirector.StandardChangeDirector.Update(TrackedObject item)
   at System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode)
   at System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode)
   ...


Update: evidently, this wording is included in the exception message whenever a command times out against a mirrored/failover partner environment. It does indeed reflect a command that exceeded its timeout, and has nothing to do with a failure to attempting to connect. You can verify this by running a simple WAITFOR DELAY '00:00:31' command, which exceeds the default 30-second SqlCommand timeout, and triggers the same exception message.

I have opened a Connect bug to ask Microsoft to correct the misleading wording in the message.