<p>We are running a .NET Core 3.1 application in a Kubernetes cluster. The application connects to an Azure SQL Database using EF Core 3.1.7, with Microsoft.Data.SqlClient 1.1.3.</p>
<p>At seemingly random times, we would receive the following error.</p>
<pre><code> ---&gt; System.Data.SqlClient.SqlException (0x80131904): Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
 ---&gt; System.ComponentModel.Win32Exception (258): Unknown error 258
   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.TdsParserStateObject.ThrowExceptionAndWarning(Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error)
   at System.Data.SqlClient.TdsParserStateObject.ReadSniSyncOverAsync()
   at System.Data.SqlClient.TdsParserStateObject.TryReadNetworkPacket()
   at System.Data.SqlClient.TdsParserStateObject.TryPrepareBuffer()
   at System.Data.SqlClient.TdsParserStateObject.TryReadByte(Byte&amp; value)
   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, SqlDataReader ds)
   at System.Data.SqlClient.SqlCommand.ExecuteScalar()
</code></pre>
<p>Even though it seems random, it definitely happens more often under heavier loads. From my research, it appears as if this specific timeout is related to the connection timeout rather than the command timeout. I.e. the client is not able to establish a connection at all. This is not a query that is timing out.</p>
<p>Potential root causes we've eliminated:</p>
<ul>
<li><strong>Azure SQL Server Capacity:</strong> The behaviour is observed whether we run on 4 or 16 vCPUs. Azure Support also confirmed that there are no issues in the logs. This includes the number of open connections, which is only around 50. We also ran load tests from other connections and the server held up fine.</li>
<li><strong>Microsoft.Data.SqlClient Versions:</strong> We've been running on version 1.1.3 and this behaviour only started a week ago (2021-03-16).</li>
<li><strong>Network Capacity:</strong> We are maxing out at around 1-2MB/s at this stage, which is pretty pedestrian.</li>
<li><strong>Kubernetes Scaling:</strong> There is no correlation between the occurrence of the events and when we scale up more pods.</li>
<li><strong>Connection String Issues:</strong> Our system used to work fine, but regardless we changed a few settings mentioned in other articles to see if the issue would not resolve itself. Mars is disabled. We cannot disable connection pooling. We have <code>TrusServerCertificate</code> set to true. Here is the current connection string: <code>Server=tcp:***.database.windows.net,1433;Initial Catalog=***;Persist Security Info=False;User ID=***;Password=***;MultipleActiveResultSets=False;Encrypt=True;Connection Timeout=60;TrustServerCertificate=True;</code></li>
</ul>
<p><strong>Update 1:</strong>
As requested, an example of two timeouts that just occurred. It is a Sunday, so traffic is extremely low. Database utilization (CPU, Mem, IO) is sitting between 2-6%.</p>
<pre><code>Microsoft.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
 ---&gt; System.ComponentModel.Win32Exception (258): Unknown error 258
   at Microsoft.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at Microsoft.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at Microsoft.Data.SqlClient.TdsParserStateObject.ThrowExceptionAndWarning(Boolean callerHasConnectionLock, Boolean asyncClose)
   at Microsoft.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error)
   at Microsoft.Data.SqlClient.TdsParserStateObject.ReadSniSyncOverAsync()
   at Microsoft.Data.SqlClient.TdsParserStateObject.TryReadNetworkPacket()
   at Microsoft.Data.SqlClient.TdsParserStateObject.TryPrepareBuffer()
   at Microsoft.Data.SqlClient.TdsParserStateObject.TryReadByte(Byte&amp; value)
   at Microsoft.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean&amp; dataReady)
   at Microsoft.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at Microsoft.Data.SqlClient.TdsParser.TdsExecuteTransactionManagerRequest(Byte[] buffer, TransactionManagerRequestType request, String transactionName, TransactionManagerIsolationLevel isoLevel, Int32 timeout, SqlInternalTransaction transaction, TdsParserStateObject stateObj, Boolean isDelegateControlRequest)
   at Microsoft.Data.SqlClient.SqlInternalConnectionTds.ExecuteTransactionYukon(TransactionRequest transactionRequest, String transactionName, IsolationLevel iso, SqlInternalTransaction internalTransaction, Boolean isDelegateControlRequest)
   at Microsoft.Data.SqlClient.SqlInternalConnection.BeginSqlTransaction(IsolationLevel iso, String transactionName, Boolean shouldReconnect)
   at Microsoft.Data.SqlClient.SqlConnection.BeginTransaction(IsolationLevel iso, String transactionName)
   at Microsoft.Data.SqlClient.SqlConnection.BeginDbTransaction(IsolationLevel isolationLevel)
   at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.BeginTransaction(IsolationLevel isolationLevel)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
</code></pre>
<p>We are also receiving errors on our database health checker when using this command: <code>Microsoft.EntityFrameworkCore.Infrastructure.DatabaseFacade.CanConnect()</code></p>
<p>The above stack trace is the issue we are trying to solve versus this stack trace below of the SQL query timing out.</p>
<pre><code>Microsoft.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
 ---&gt; System.ComponentModel.Win32Exception (258): Unknown error 258
   at Microsoft.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at Microsoft.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at Microsoft.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean&amp; dataReady)
   at Microsoft.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at Microsoft.Data.SqlClient.SqlDataReader.get_MetaData()
   at Microsoft.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted)
   at Microsoft.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean isAsync, Int32 timeout, Task&amp; task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest)
   at Microsoft.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, TaskCompletionSource`1 completion, Int32 timeout, Task&amp; task, Boolean&amp; usedCache, Boolean asyncWrite, Boolean inRetry, String method)
   at Microsoft.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)
</code></pre>

<p>The problem was an infrastructure issue at <a href="https://github.com/Azure/aks-engine/issues/4341" rel="noreferrer">Azure</a>.</p>
<blockquote>
<p>There is a known issue within Azure Network where the dhcp lease is
lost whenever a disk attach/detach happens on some VM fleets. There is
a fix rolling out at the moment to regions. I'll check to see when
Azure Status update will be published for this.</p>
</blockquote>
<p>The problem disappeared, so it appears as if the fix has been rolled out globally.</p>
<p>For anyone else running into this issue in the future, you can identify it by establishing an <a href="https://learn.microsoft.com/en-us/azure/aks/ssh" rel="noreferrer">SSH connection into the node</a> (not the pod). Do an <code>ls -al /var/log/</code> and identify all the <code>syslog</code> files and run the following grep on each file.</p>
<pre><code>cat /var/log/syslog | grep 'carrier'
</code></pre>
<p>If you have any <code>Lost carrier</code> and <code>Gained carrier</code> messages in the log, there is a some sort of a network issue. In our case it was the DHCP lease.</p>
<p><a href="https://i.stack.imgur.com/gcCeZ.png" rel="noreferrer"><img src="https://i.stack.imgur.com/gcCeZ.png" alt="enter image description here" /></a></p>
