Blocking, Deadlocks, and Troubleshooting

Certainly! Here’s a comprehensive guide on Blocking, Deadlocks, and Troubleshooting in SQL Server, covering definitions, causes, detection methods, resolution strategies, and best practices.

1. Introduction

In SQL Server, blocking and deadlocks are common concurrency issues that can significantly impact database performance. Understanding their differences, causes, and how to troubleshoot them is essential for maintaining a healthy SQL Server environment.

2. Understanding Blocking

What is Blocking?

Blocking occurs when one session holds a lock on a resource (like a row, page, or table), and another session attempts to access that same resource in a conflicting mode. The second session is forced to wait until the first session releases the lock.

Common Causes of Blocking

Long-running transactions: Transactions that take a long time to complete can hold locks for extended periods, causing other sessions to wait.
High contention for resources: Multiple sessions attempting to access the same resource simultaneously can lead to blocking.
Inappropriate isolation levels: Using higher isolation levels like SERIALIZABLE can increase the likelihood of blocking.

Detecting Blocking

To identify blocking sessions, you can query the sys.dm_exec_requests and sys.dm_exec_sessions dynamic management views (DMVs):

SELECT
    blocking_session_id AS BlockingSessionID,
    session_id AS BlockedSessionID,
    wait_type,
    wait_time,
    wait_resource,
    wait_time_ms,
    wait_type,
    percent_complete,
    estimated_completion_time,
    command,
    sql_text.text AS SQLText
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text(sql_handle) AS sql_text
WHERE blocking_session_id <> 0;

This query provides information about the blocking session, the blocked session, and the SQL text being executed.

3. Understanding Deadlocks

What is a Deadlock?

A deadlock occurs when two or more sessions are each waiting for the other to release a resource, creating a cycle of dependencies that cannot be resolved without intervention. SQL Server automatically detects deadlocks and terminates one of the sessions (the “victim”) to break the cycle.

Common Causes of Deadlocks

Circular locking: Transactions acquiring locks in different orders can lead to circular dependencies.
High contention: Multiple sessions competing for the same resources can increase the likelihood of deadlocks.
Inconsistent transaction patterns: Different applications or users accessing the same resources in varying sequences can cause deadlocks.

Detecting Deadlocks

SQL Server provides several methods to detect deadlocks:

SQL Server Profiler: Use the “Deadlock Graph” event to capture deadlock information.
Extended Events: The system_health extended event session captures deadlock information by default. You can query it as follows: SELECT XEvent.value('(event/@name)[1]', 'varchar(50)') AS event_name, XEvent.value('(event/@timestamp)[1]', 'datetime') AS timestamp, XEvent.value('(event/data[@name="duration"]/value)[1]', 'bigint') AS duration, XEvent.value('(event/data[@name="lockMode"]/value)[1]', 'varchar(50)') AS lock_mode, XEvent.value('(event/data[@name="lockResource"]/value)[1]', 'varchar(50)') AS lock_resource, XEvent.value('(event/data[@name="sqlText"]/value)[1]', 'varchar(max)') AS sql_text FROM (SELECT CAST(target_data AS XML) AS TargetData FROM sys.dm_xe_sessions AS s JOIN sys.dm_xe_session_targets AS t ON s.address = t.event_session_address WHERE s.name = 'system_health' AND t.target_name = 'event_file') AS Data CROSS APPLY TargetData.nodes('declare namespace p="http://schemas.microsoft.com/sqlserver/2004/07/showplan"; //p:event') AS XEvent(XEvent);

This query extracts deadlock information from the system_health extended event session.

4. Resolving Blocking and Deadlocks

Strategies for Resolving Blocking

Identify and terminate blocking sessions: Use the KILL command to terminate sessions causing blocking: KILL <BlockingSessionID>; Replace <BlockingSessionID> with the actual session ID of the blocking session.
Optimize long-running transactions: Review and optimize queries and transactions that take a long time to complete.
Implement appropriate indexing: Ensure that tables have appropriate indexes to speed up query execution and reduce lock contention.

Strategies for Resolving Deadlocks

Analyze deadlock graphs: Review the deadlock graph to understand the resources involved and the order in which locks were acquired.
Modify application logic: Ensure that transactions access resources in a consistent order to prevent circular dependencies.
Set deadlock priority: Use the SET DEADLOCK_PRIORITY command to influence which session SQL Server chooses as the victim: SET DEADLOCK_PRIORITY LOW; This command sets the deadlock priority to low for the current session.

5. Preventing Blocking and Deadlocks

Best Practices for Prevention

Use appropriate isolation levels: Choose the isolation level that balances data consistency and concurrency requirements. For example, READ COMMITTED is the default and generally sufficient for most applications.
Keep transactions short: Minimize the duration of transactions to reduce the time locks are held.
Access resources in a consistent order: Ensure that all transactions access resources in the same order to prevent circular dependencies.
Use NOLOCK hint judiciously: The NOLOCK hint allows reading data without acquiring locks, but it can result in dirty reads. Use it only when appropriate.

6. Monitoring and Troubleshooting Tools

SQL Server provides several tools to monitor and troubleshoot blocking and deadlocks:

SQL Server Profiler: Capture and analyze events related to blocking and deadlocks.
Extended Events: Use the system_health extended event session to capture deadlock information.
Dynamic Management Views (DMVs): Query DMVs like sys.dm_exec_requests and sys.dm_exec_sessions to monitor active sessions and blocking.
Activity Monitor: Use SQL Server Management Studio’s Activity Monitor to view current activity and identify blocking sessions.

7. Conclusion

Blocking and deadlocks are common concurrency issues in SQL Server that can impact performance. By understanding their causes, detecting them using the appropriate tools, and implementing strategies to resolve and prevent them, you can maintain a healthy and efficient SQL Server environment.

If you need further assistance or have specific scenarios you’d like to discuss, feel free to ask!