Optimistic and pessimistic locking are two different concurrency control mechanisms used in databases and multi-user systems to handle concurrent access to shared resources, such as database records, files, or data structures. Both mechanisms aim to prevent conflicts and maintain data consistency when multiple users or processes attempt to access and modify the same resource simultaneously. Let’s explore the differences between them:
Optimistic Locking:
- In optimistic locking, the system assumes that conflicts between concurrent transactions are infrequent, and it allows multiple transactions to access the shared resource simultaneously without blocking or locking it immediately.
- When a transaction wants to modify the resource, it first reads the resource’s state and notes the version or timestamp associated with it.
- Before committing the changes, the transaction rechecks the resource’s state to ensure that no other transaction has modified it in the meantime.
- If no conflicts are detected, the transaction commits its changes. However, if a conflict is found, the transaction is typically rolled back, and the user or application must decide how to handle the situation (e.g., retry the operation, show an error message, etc.).
- Optimistic locking is useful when conflicts are rare, and the overhead of locking is higher than the cost of handling occasional conflicts.
Pessimistic Locking:
- In pessimistic locking, the system assumes that conflicts between concurrent transactions are more likely to occur, and it ensures that only one transaction can access the resource exclusively at a time.
- When a transaction wants to modify the resource, it requests a lock on the resource, which prevents other transactions from accessing it until the lock is released.
- Pessimistic locking can be implemented using various types of locks, such as shared locks (allowing multiple users to read but not write) and exclusive locks (preventing any other user from accessing the resource).
- Once a transaction acquires the lock, it can perform its operations on the resource safely, knowing that no other transaction can modify it simultaneously.
- Pessimistic locking is suitable when conflicts are common, and the cost of handling conflicts is higher than the overhead of using locks.
In summary, optimistic locking is based on the assumption that conflicts are infrequent, and it allows multiple transactions to operate concurrently until they attempt to commit changes. On the other hand, pessimistic locking assumes that conflicts are more likely and ensures exclusive access to the resource, potentially leading to blocking and queuing of other transactions. The choice between optimistic and pessimistic locking depends on the specific use case, the frequency of conflicts, and the desired trade-offs between concurrency and potential contention.