4-3 Locking Protocols

Key Concepts

Locking protocols are mechanisms used in database management systems to ensure data consistency and integrity during concurrent transactions. The 4-3 Locking Protocols are a set of rules that define when and how locks should be acquired and released to prevent conflicts and ensure serializability.

1. Two-Phase Locking (2PL)

Two-Phase Locking (2PL) is a fundamental locking protocol that ensures serializability by dividing the transaction into two phases: the growing phase and the shrinking phase.

• Growing Phase: During this phase, a transaction can acquire locks but cannot release any locks.
• Shrinking Phase: During this phase, a transaction can release locks but cannot acquire any new locks.

Example: Imagine a transaction that needs to read and write data from multiple tables. In the growing phase, the transaction acquires read locks on the tables it needs to read and write locks on the tables it needs to update. Once all locks are acquired, the transaction enters the shrinking phase and releases the locks as it completes its operations.

2. Strict Two-Phase Locking (Strict 2PL)

Strict Two-Phase Locking (Strict 2PL) is a stricter version of 2PL where all exclusive locks (write locks) are held until the transaction commits or aborts. This ensures that no other transaction can read or write the locked data until the transaction is complete.

Example: Consider a banking transaction where a user transfers money from one account to another. The transaction acquires write locks on both accounts during the growing phase. In Strict 2PL, these write locks are not released until the transaction commits, ensuring that no other transaction can access the accounts until the transfer is complete.

3. Rigorous Two-Phase Locking (Rigorous 2PL)

Rigorous Two-Phase Locking (Rigorous 2PL) is an even stricter version of 2PL where all locks, both shared (read) and exclusive (write), are held until the transaction commits or aborts. This ensures that no other transaction can access the locked data until the transaction is complete.

Example: In a stock trading system, a transaction might need to read stock prices and update the user's portfolio. In Rigorous 2PL, both read and write locks are held until the transaction commits, ensuring that no other transaction can access the stock prices or the user's portfolio until the transaction is complete.

Conclusion

Understanding the 4-3 Locking Protocols is crucial for ensuring data consistency and integrity in concurrent database transactions. By applying these protocols, database systems can prevent conflicts and ensure that transactions are executed in a serializable manner, maintaining the integrity of the data.