Identify sessions that cause blocking

Introduction

When working with Microsoft Azure SQL Solutions, it is important to monitor and manage sessions effectively. Blocking occurs when one session holds a lock on a resource that another session needs, causing it to wait. This article will guide you through the process of identifying sessions that cause blocking in Azure SQL solutions. You will learn about various methods and techniques to detect and resolve blocking.

1. Monitoring Blocked Sessions

To identify blocking sessions, you need to actively monitor your Azure SQL solution. Follow these steps:

1. Use the sys.dm_exec_requests view to identify sessions that are currently blocked. Execute the following query:

   SELECT *
FROM sys.dm_exec_requests
WHERE blocking_session_id > 0;

2. Analyze the results to identify the blocking session and the session(s) being blocked.

2. Gathering Session Details

Once you have identified the blocking session, gather more details about it using the sys.dm_exec_sessions view. Execute the following query, replacing with the actual identifier:

SELECT *
FROM sys.dm_exec_sessions
WHERE session_id = ;


This query will provide information about the blocking session, including login details and execution context.

3. Collecting Resource and Lock Information

To gain a deeper understanding of the resources being locked and the associated locks, utilize the sys.dm_tran_locks view. Use the following query to select locks related to the blocking session:

SELECT *
FROM sys.dm_tran_locks
WHERE request_session_id = ;


This query will return information regarding the resources and locks associated with the blocking session.

4. Investigating Query Execution Plans

Query execution plans can shed light on the cause of blocking by identifying expensive queries or missing indexes. Follow these steps:

1. Capture the query plan of the blocking session using the query:

   SELECT *
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_query_plan (qs.plan_handle) AS qp
WHERE qs.session_id = ;


2. Analyze the execution plan for any inefficiencies, such as long-running queries or table scans. Consider optimizing these queries or adding missing indexes.

5. Resolving Blocking Issues

To resolve blocking, consider the following approaches:

1. Kill the blocking session using the KILL statement. Execute the query:

   KILL ;


   Note: Killing a session forcefully may lead to data inconsistencies, so use this option with caution.

2. Identify and resolve the root cause of blocking. This may involve optimizing queries, redesigning data models, or restructuring transactional logic.

3. Implement appropriate isolation levels and locking hints in your queries to minimize blocking scenarios.

4. Consider implementing read replicas to offload read workloads and reduce contention.

Conclusion

Detecting and resolving blocking sessions is crucial for maintaining optimal performance in Azure SQL Solutions. By monitoring blocked sessions, gathering detailed information, analyzing execution plans, and applying appropriate resolution strategies, you can mitigate and prevent blocking issues. Strengthen your knowledge and skills in identifying sessions that cause blocking to excel in the Examining Administering Microsoft Azure SQL Solutions exam.