What is Optimistic Locking?
Optimistic locking is a concurrency control mechanism used in multi-user environments to manage concurrent access to data. It is called “optimistic” because it assumes that conflicts between users accessing the same data concurrently are rare. Instead of blocking other users from modifying the data, optimistic locking allows multiple users to work concurrently, and it detects conflicts only at the time of updating the data.
In the context of databases and data access frameworks like Entity Framework, optimistic locking is typically implemented using a versioning mechanism, such as a timestamp column or a concurrency token. Here’s how optimistic locking works:
- Versioning Mechanism: When an entity is retrieved from the database, it includes a version indicator, either a timestamp or a specific concurrency token property. This version value represents the state of the data when it was fetched.
- Reading Data: When a user reads data, the current version value is stored in memory along with the data.
- Updating Data: When a user wants to modify the data and save the changes, the system checks whether the current version value in memory matches the version value in the database.
- Concurrency Check: If the version values match, it means that the data hasn’t been modified by any other user since it was retrieved. The update is allowed to proceed, and the version value in the database is incremented or changed to reflect the modification.
- Conflict Detection: If the version values don’t match, it indicates that the data has been modified by another user since it was read. A concurrency exception is raised, indicating a conflict.
- Conflict Resolution: The application can handle the concurrency exception and decide how to resolve the conflict, typically by refreshing the data from the database and reapplying the changes or notifying the user about the conflict.
Optimistic locking provides several advantages, including better performance and scalability compared to pessimistic locking. It allows multiple users to work concurrently without blocking each other, improving the application’s responsiveness and user experience. However, it does require careful handling of concurrency exceptions and proper conflict resolution strategies to ensure data consistency.
In Entity Framework, optimistic locking is the default concurrency control mechanism when you use a timestamp column or a
ConcurrencyCheck attribute (with a concurrency token property) on the entity classes. It is essential to understand how optimistic locking works to handle concurrency conflicts properly and ensure data integrity in multi-user applications.