Pessimistic Locking vs Optimistic Locking

Pessimistic Locking and Optimistic Locking are two common concurrency control strategies used to handle data consistency issues when multiple threads or processes access shared resources. The main difference between them lies in their approach to handling resource contention.

1. Pessimistic Locking

• Definition: Pessimistic locking assumes that concurrency conflicts will happen, so it locks the resource before accessing or modifying the data to prevent other threads or processes from accessing it during the operation.
• Characteristics:
  • Locks the resource before accessing it.
  • Other threads or processes must wait until the lock is released.
  • Often implemented through row locks or table locks in databases.
• Use Cases:
  • Useful when the likelihood of data conflicts is high and data consistency must be guaranteed.
  • In high-concurrency scenarios, it can lead to performance issues as the lock is held for a longer time, blocking other threads.
• Example: A bank account transfer operation. If two threads attempt to modify the same account at the same time, pessimistic locking ensures that only one thread can access the account at a time.

2. Optimistic Locking

• Definition: Optimistic locking assumes that concurrency conflicts are rare, so it does not lock the resource during data access. Instead, it checks if the data has changed before committing the operation. Typically, it uses version control or validation to detect conflicts.
• Characteristics:
  • No locking is done before accessing the data.
  • Before committing the data, it checks if it has been modified by another thread. If modified, the operation is usually aborted or retried.
  • Implemented using version numbers or timestamps in databases.
• Use Cases:
  • Ideal for scenarios where data conflicts are rare, or the system can tolerate retries in case of conflict.
  • Reduces the overhead of locking and generally performs better in high-concurrency situations.
• Example: An e-commerce system for inventory management. When multiple users attempt to purchase the same product, optimistic locking uses a version number to ensure that the stock count hasn't been modified by others before the update.

Summary

Feature	Pessimistic Locking	Optimistic Locking
Locking Strategy	Locks before operation	No locking before operation, checks after
Performance	High overhead due to long lock hold times	Low overhead, better for high concurrency
Conflict Handling	Blocks on conflict	Retries or aborts on conflict
Use Cases	High conflict probability, strict consistency	Low conflict probability, can tolerate retries

The choice between pessimistic locking and optimistic locking depends on the system's concurrency requirements, the likelihood of data conflicts, and the need for performance optimization.