Deadlocks in Azure SQL Database

Recently we were working with Azure Logic Apps to invoke Azure Functions.
By Default, Logic App runs parallel threads and we didn’t explicitly control the concurrency and left the default values.

So Logic App invoked several concurrent threads which in turn invoked several Azure Functions.
The problem was Azure Functions invoked Database Calls which caused Deadlocks. In Ideal world, Database should be able to handle numerous concurrent functions without deadlocks. Our process high percentage of shared data and we wanted to ensure the consistency , so we had Explicit Transactions in our Stored procedure calls. That’s the root cause of the problem and we didn’t want to remove the explicit Transaction.

The solution we implemented to alleviate this problem is to run this process in Sequence instead of parallel threads.

Log App Concurrency Control Behavior

For each loops execute in parallel by default. Customize the degree of parallelism, or set it to 1 to execute in sequence.

Deadlock Exception

Transaction (Process ID 166) was deadlocked on lock
| communication buffer resources with another process and has been chosen as the deadlock victim. Rerun the transaction.

Troubleshooting Deadlocks

So we have identified Deadlock happened in the database through our Application Insights. Next logical question is , what caused this deadlock.

Azure SQL Server Deadlock Count

These queries identifies the deadlock event time as well as the deadlock event details.

                
        SELECT * FROM sys.event_log   
        WHERE event_type = 'deadlock';
        WITH CTE AS (  
        SELECT CAST(event_data AS XML)  AS [target_data_XML]   
        FROM sys.fn_xe_telemetry_blob_target_read_file('dl', 
        null, null, null)  
        )  
        SELECT target_data_XML.value('(/event/@timestamp)[1]', 
        'DateTime2') AS Timestamp,  
        target_data_XML.query('/event/data[@name=''xml_report'']
        /value/deadlock') AS deadlock_xml,  
        target_data_XML.query('/event/data[@name=''database_name'']
        /value').value('(/value)[1]', 'nvarchar(100)') AS db_name  
        FROM CTE
                
        

You can save the Deadlock xml as xdl to view the Deadlock Diagram. This provides all the information we need to identify the root cause of the deadlock and take necessary steps to resolve the issue.

References

• https://blogs.msdn.microsoft.com/david_burgs_blog/2018/03/07/control-the-scale-of-a-logic-app/
• https://blogs.msdn.microsoft.com/azuresqldbsupport/2017/04/19/deadlock-analysis-for-sql-azure-database/
• https://blogs.msdn.microsoft.com/azuresqldbsupport/2017/01/21/lesson-learned-19-how-to-obtain-the-deadlocks-of-your-azure-sql-database/