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Published on

March 22, 2022

Understanding and Resolving Deadlocks in SQL Server

Dealing with deadlocks in SQL Server is crucial for maintaining the performance and reliability of database applications. This comprehensive article aims to equip you with the knowledge and practices you need to handle deadlocks effectively.

What Are Deadlocks in SQL Server?

A deadlock occurs when two or more processes hold exclusive locks on resources and each process is preventing the other from completing their task. In SQL Server, this scenario causes an impasse where none of the processes can proceed, leading to a halt in system performance.

Common Causes of Deadlocks

  • Different locking orders by concurrent transactions
  • Mixture of read and write locks on resources that are accessed by multiple processes
  • Complex transactions that involve many different resources
  • Resource scarcity due to high concurrency levels

Deadlock Detection and Resolution in SQL Server

SQL Server has an in-built deadlock detection system which constantly monitors for deadlocks and resolves them by choosing one process as the ‘deadlock victim.’ This chosen process is terminated and its transactions are rolled back to release the resources for other processes.

Strategies for Handling and Preventing Deadlocks

  • Minimize Transaction Scope: Keep transactions as short and as focused as possible. This reduces the chance of conflicting resource requests.
  • Lock Order Consistency: Ensure that all transactions acquire locks in the same order. This avoids circular wait conditions.
  • Indexing: Appropriate indexing can reduce the number of locks required and the time held.
  • Use ROWLOCK or NOLOCK hints judiciously to minimize locking resources.
  • Optimize Query Performance: Poorly performing queries can increase the risk of deadlocks by holding onto resources for longer periods.
  • Isolation Levels: Use the correct isolation level for the transaction to ensure that there is a balance between accuracy and performance.
  • Analyze Deadlock Graphs to Understand Specific Scenarios.
  • Implement Retry Logic in Application Code: Write your application logic to gracefully handle being chosen as a deadlock victim and retry the operation.

Monitoring Deadlock Events

Monitor deadlocks using SQL Server Profiler or Extended Events. Analyzing the deadlock graph provided can help pinpoint the cause and aid in resolution.

Reviewing and Optimizing Code

Proactively review code for potential deadlock scenarios and seek to refactor or optimize where necessary.

Troubleshooting and Analysis

When deadlocks occur, it is essential to collect the necessary data to analyze the deadlock scenario. This data is integral in preventing recurrent problems.

  • SQL Server Error Logs: Check the error logs of SQL Server to see information about deadlocks.
  • Trace Flags: Enable trace flags like 1222 to capture deadlock information in the error logs.
  • System Health Session: The default system health session in SQL Server captures deadlock events.
  • DMVs and DMFs: Use dynamic management views and functions like
    sys.dm_tran_locks

    and

    sys.dm_os_waiting_tasks

    to analyze current locks and wait chains.

Best Practices for Avoiding Deadlocks

  • Maintain a consistent coding approach across the application.
  • Design databases with consideration for concurrency and locking scenarios.
  • Continually monitor and audit system performance, identifying potential deadlock hazards.
  • Test for deadlocks during the development phase using various concurrency levels.
  • Utilize trace flags to provide additional information for diagnosis only when needed.

In conclusion, handling deadlocks in SQL Server involves being proactive, understanding the root causes, careful monitoring, and employing a set of strategies aimed at prevention and resolution. By applying the principles highlighted in this article, database administrators and developers can mitigate the challenges posed by deadlocks and ensure smooth operation of their database systems.

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code review, Deadlock Detection, Deadlock Graphs, Deadlocks, DMFs, DMVs, error logs, Extended Events, indexing, Isolation Levels, lock order, monitoring, performance auditing, Query Optimization, SQL Server, SQL Server Profiler, sys.dm_os_waiting_tasks, sys.dm_tran_locks, trace flags, Transaction Scope, troubleshooting

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