Concurrency Control Mechanisms

1. Lock-Based Protocols

Lock-based protocols are a fundamental method for ensuring data consistency in concurrent database systems. These protocols use locks to control access to data items. There are two main types of locks:

• Shared Lock (S-Lock): Allows multiple transactions to read a data item simultaneously but prevents any transaction from writing to it.
• Exclusive Lock (X-Lock): Allows a transaction to read and write a data item but prevents other transactions from accessing it.

Example: Consider a bank database where multiple transactions are accessing the same account balance. A shared lock allows multiple transactions to read the balance simultaneously, but an exclusive lock is required if any transaction needs to update the balance.

2. Timestamp-Based Protocols

Timestamp-based protocols use timestamps to determine the order of transactions. Each transaction is assigned a unique timestamp when it starts. The protocol ensures that transactions are executed in the order of their timestamps, preventing conflicts and ensuring serializability.

Example: In an online shopping system, each customer's order is assigned a timestamp when placed. The system processes orders in the order of their timestamps, ensuring that no two orders for the same item are processed out of order.

3. Validation-Based Protocols

Validation-based protocols, also known as optimistic concurrency control, allow transactions to proceed without locking. However, before committing, each transaction is validated to ensure that it does not conflict with other transactions. If a conflict is detected, the transaction is rolled back and restarted.

Example: In a scientific research database, researchers might update their findings without locking the entire database. Before committing their changes, the system validates that no other researcher has updated the same data, ensuring data consistency.

4. Multi-Version Concurrency Control (MVCC)

Multi-Version Concurrency Control (MVCC) maintains multiple versions of data items, allowing transactions to access different versions based on their timestamps. This approach reduces the need for locking and improves concurrency by allowing readers and writers to operate simultaneously without blocking each other.

Example: In a version control system for software development, each commit creates a new version of the code. Developers can work on different versions simultaneously, and the system ensures that each developer sees the correct version of the code based on their timestamp.