Exploring the role of pessimistic concurrency control in maintaining isolation in ACID transactions

31 Oct 2023

In the context of database systems, concurrency control plays a critical role in ensuring the isolation property of ACID transactions. One common approach to concurrency control is pessimistic concurrency control, which relies on locking to enforce exclusive access to data items. In this blog post, we will explore the role of pessimistic concurrency control in maintaining isolation in ACID transactions.

What is pessimistic concurrency control?

Pessimistic concurrency control is a technique used in database systems to ensure that transactions do not interfere with each other. It assumes that conflicts are likely to occur and therefore resorts to locking mechanisms to prevent multiple transactions from accessing or modifying the same data concurrently. This approach maintains strict isolation by granting exclusive access to a transaction while it is performing its operations.

Ensuring isolation through locking

Locking is the fundamental mechanism employed in pessimistic concurrency control. When a transaction needs to read or modify a data item, it requests a lock on that item. The lock prevents other transactions from accessing or modifying the data item until the lock is released. This ensures that conflicting operations do not occur concurrently, maintaining the isolation property of ACID transactions.

Types of locks in pessimistic concurrency control

Pessimistic concurrency control typically uses two types of locks: shared locks and exclusive locks. A shared lock allows multiple transactions to read a data item simultaneously but prevents any transaction from modifying it. On the other hand, an exclusive lock grants exclusive access to a transaction, preventing other transactions from both reading and modifying the data item.

Blocking and deadlocks

While pessimistic concurrency control helps maintain isolation, it can also lead to blocking and deadlocks if not managed properly. Blocking occurs when a transaction is blocked from proceeding due to another transaction holding a conflicting lock. Deadlocks occur when two or more transactions are trapped in a cyclic dependency, where each transaction is waiting for a lock held by another transaction.

To mitigate blocking and deadlocks, several techniques can be employed, such as lock timeout, deadlock detection, and deadlock avoidance algorithms. These techniques help to ensure that transactions can make progress even in the presence of conflicts, while still maintaining isolation.

Conclusion

Pessimistic concurrency control is a vital technique in maintaining isolation in ACID transactions. By using locks to enforce exclusive access to data items, it prevents concurrent operations that can result in inconsistencies. While there are challenges such as blocking and deadlocks associated with pessimistic concurrency control, they can be mitigated through various techniques. Understanding the role of pessimistic concurrency control is crucial for building robust and high-performance database systems.

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