Connection timeout and Command timeout in SQL Server

Hello all,

While working with SQL Server, one of the common issue application teams report is  timeouts. In this article, we are covering connection and Command timeouts and ways to isolate them.

There are 2 types of timeouts.

1. Connection timeout
2. Command Timeout

CONNECTION TIMEOUT:  

It is the time in seconds application waits while trying to create a connection with SQL Server before terminating the attempt. Default value of connection timeout is 15 seconds.

When you encounter Connection timeout issues, you should review:

1. Check if you are able to telnet SQL Server on SQL port
2. Check if the 3 way TCP handshake is working
3. The troubleshooting approach should be on fixing SQL Connectivity with application.

Additional information on troubleshooting SQL Connectivity issues is documented HERE.

Reviewing the exception and the stack trace  is a starting point to isolate connection timeout issues. Reviewing the thread stack trace, you would observe that application tries to create a connection and times out post encountering Connection timeout value (by default 15 seconds).

 Exception Details:  System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

Stack Trace:

 [SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)] 
System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling, SqlAuthenticationProviderManager sqlAuthProviderManager) +1431
System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions) +1085
System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnection owningObject, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) +70
System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +964
System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) +109
System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection) +1529
System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) +156
System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection) +258
System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions) +312
System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry) +202
System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry) +413
System.Data.SqlClient.SqlConnection.Open() +128
System.Data.Entity.Infrastructure.Interception.InternalDispatcher`1.Dispatch(TTarget target, Action`2 operation, TInterceptionContext interceptionContext, Action`3 executing, Action`3 executed) +104
System.Data.Entity.Infrastructure.Interception.DbConnectionDispatcher.Open(DbConnection connection, DbInterceptionContext interceptionContext) +503
System.Data.Entity.SqlServer.<>c__DisplayClass1.<Execute>b__0() +18
System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute(Func`1 operation) +234

COMMAND TIMEOUT:

This is the time in seconds to wait for the command to execute. This setting allows the cancellation of an ExecuteReader method call, due to delays from network traffic or heavy server use.  The default value is 30 seconds.

When you encounter Command timeout issues, you should review:

1. Check if there are any performance issues with SQL Server like blocking
2. Check in SQL Server why the query execution is taking more than Command Timeout value.
3. Collect Attention events in SQL profiler/Extended Events and track all queries timing out and review the waits.

Additional details on troubleshooting SQL query/command timeout is documented HERE:

Reviewing the stack trace closely, you would observe that application connects to SQL Server and it calls ExecuteReader method for retrieving data from database.

Exception Details: System.ComponentModel.Win32Exception: The wait operation timed out

Source Error:

Stack Trace:

 [Win32Exception (0x80004005): The wait operation timed out]
[SqlException (0x80131904): Execution Timeout Expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.] 
System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction) +3305692
System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) +736
System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady) +4061
System.Data.SqlClient.SqlDataReader.TryConsumeMetaData() +90
System.Data.SqlClient.SqlDataReader.get_MetaData() +99
System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted) +604
System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest) +3303
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry) +667
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) +83
System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) +301
System.Data.Entity.Infrastructure.Interception.InternalDispatcher`1.Dispatch(TTarget target, Func`3 operation, TInterceptionContext interceptionContext, Action`3 executing, Action`3 executed) +104
System.Data.Entity.Infrastructure.Interception.DbCommandDispatcher.Reader(DbCommand command, DbCommandInterceptionContext interceptionContext) +499
System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior) +36

SUMMARY:

To summarize, when an application reports timeout error, its essential to identify the type of timeout, if it is Connection/Command timeout. Exception reported and the stack trace is a starting point to isolate the type of timeout.  Accordingly, further data collection steps and actions can be performed to isolate the exact issue.

Hope this blog helps in identifying the timeouts and helps in isolating the timeout issues.

Please share your feedback, questions and/or suggestions.

Thanks,
Don Castelino | Premier Field Engineer | Microsoft

Disclaimer: All posts are provided AS IS with no warranties and confer no rights. Additionally, views expressed here are my own and not those of my employer, Microsoft.