SQL SERVER – Locking, Blocking, and Deadlocking: Differences, Similarities, and Best Practices

I have recently written Understanding SQL Server Deadlocks: A Beginner’s Guide, and after reading it, lots of people have asked me the difference between Locking, Blocking, and Deadlocking. In this blog post, we will delve into three essential concepts in database management: locking, blocking, and deadlocking. We’ll explore their differences, similarities, and real-world examples and wrap up with best practices to avoid potential issues.


SQL SERVER - Locking, Blocking, and Deadlocking: Differences, Similarities, and Best Practices 3oking-800x673 Locking is a mechanism used by databases to ensure that multiple transactions can access shared resources without compromising data integrity. When a transaction wants to access a resource (e.g., a row in a table), it requests a lock. If the lock is granted, the transaction can proceed with its operation. Otherwise, the transaction must wait until the lock is released.

There are two primary types of locks: shared locks and exclusive locks. Shared locks allow multiple transactions to read a resource, but none can modify it. On the other hand, exclusive locks allow only one transaction to access a resource for reading or writing.

Real-world example: Imagine a library with a shared book inventory system. When a patron wants to borrow a book, the system will request a shared lock on the book’s record. This allows other patrons to see the book’s details, but they cannot borrow it until the current patron returns it. When the book is returned, the shared lock is released, and another patron can request an exclusive lock to borrow the book.


Blocking occurs when one transaction is waiting for another transaction to release a lock on a resource. This can cause performance issues and delays in processing transactions. Blocking is a natural consequence of locking, as transactions must wait for locks to ensure data consistency.

Real-world example: Continuing with the library example, suppose there is a book with a single copy available. Two patrons try to borrow it simultaneously. The system grants an exclusive lock to one patron while the other patron’s request is blocked until the lock is released.


A deadlock occurs when two or more transactions are waiting for each other to release locks, creating a circular dependency and preventing any of the transactions from proceeding. Deadlocks can severely impact database performance and often require manual intervention to resolve.

Real-world example: In our library scenario, imagine two patrons, Alice and Bob, who want to borrow two books, X and Y. Alice has an exclusive lock on book X and is waiting for a lock on book Y, while Bob has an exclusive lock on book Y and is waiting for a lock on book X. Neither can proceed until the other releases their lock, resulting in a deadlock.

Similarities and Differences

While locking, blocking, and deadlocking are related concepts, they have distinct differences:

Locking is a necessary mechanism to ensure data consistency and integrity in multi-transaction environments. It is neither inherently good nor bad but required for proper database functioning.

Blocking is a consequence of locking. While it can cause performance issues, it is a normal part of database operations and ensures that transactions wait for locks to maintain data consistency.

Deadlocking is an undesirable situation where transactions are stuck in a circular dependency, waiting for each other to release locks. Deadlocks can severely impact database performance and should be avoided or resolved quickly.

Despite these differences, all three concepts are interconnected and play a role in maintaining data consistency and integrity in database systems.

Best Practices

To minimize the impact of locking, blocking, and deadlocking, consider the following best practices:

Keep transactions short: Longer transactions increase the likelihood of blocking and deadlocking. Design your transactions to be as short as possible, committing or rolling back changes promptly.

Acquire locks in a consistent order: Deadlocks can often be avoided by acquiring locks in a consistent order across all transactions. This reduces the chances of circular dependencies between transactions.

Use appropriate lock types: Use shared locks for read operations and exclusive locks for write operations. Avoid using higher-level locks than necessary, as this can increase blocking and deadlocking.

Implement deadlock detection and resolution mechanisms: Database systems often include built-in deadlock detection mechanisms. Make use of these features to identify and resolve deadlocks quickly. In some cases, manual intervention may be needed to terminate one or more transactions to break a deadlock.

Optimize query performance: Slow or poorly optimized queries can exacerbate locking and blocking issues. Regularly monitor and optimize query performance to minimize the impact of locking and blocking on database performance.

Consider using lock-free or optimistic concurrency control techniques: Some databases support lock-free or optimistic concurrency control mechanisms, which allow transactions to proceed without acquiring locks. This can help minimize locking and blocking but may require additional application logic to handle conflicts when multiple transactions attempt to modify the same resource simultaneously.

By understanding the differences and similarities between locking, blocking, and deadlocking and applying best practices to minimize their impact, you can ensure that your database system remains performant, consistent, and reliable.


Locking, blocking, and deadlocking are essential concepts in database management systems. Locking is necessary to maintain data consistency and integrity while blocking is a natural consequence of locking. Deadlocking, on the other hand, is an undesirable situation that can lead to severe performance issues.

Understanding these concepts and their real-world implications is crucial for designing and maintaining efficient and reliable database systems. By following best practices such as keeping transactions short, acquiring locks in a consistent order, using appropriate lock types, and implementing deadlock detection and resolution mechanisms, you can minimize the impact of these issues and ensure a smooth and consistent experience for your users.

If, after following everything, you still face issues with deadlocks or SQL Server performance tuning, you can always engage in Comprehensive Database Performance Health Check and reach out to me on Twitter.

Reference: Pinal Dave (http://blog.SQLAuthority.com)

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