Two-Phase Locking

Two-Phase Locking (2PL) is a concurrency control protocol used in database systems to ensure serializability of transactions. It operates in two distinct phases where locks are acquired and released in a specific order, preventing conflicts between concurrent transactions.

Key Concepts

• Locking Protocol: Transactions acquire locks on data items before accessing them and release locks when no longer needed.
• Serializability: 2PL guarantees that the resulting transaction schedule is equivalent to a serial execution.
• Phases of 2PL: The protocol divides transaction execution into two phases:
  • Growing Phase: Locks are acquired but not released.
  • Shrinking Phase: Locks are released but not acquired.

Phases of Two-Phase Locking

1. Growing Phase:
   • The transaction requests and acquires locks as needed.
   • No locks are released in this phase.
2. Shrinking Phase:
   • The transaction releases locks as operations complete.
   • No new locks are acquired after the first lock is released.

Example of Two-Phase Locking

Consider two transactions, T1 and T2, accessing a shared data item:

Step	Transaction T1	Transaction T2	Explanation
1	BEGIN TRANSACTION	BEGIN TRANSACTION	Both transactions start.
2	READ(A)	-	T1 acquires a shared lock on A and reads its value.
3	-	WRITE(A)	T2 waits because T1 holds a shared lock on A.
4	WRITE(A)	-	T1 upgrades to an exclusive lock and writes to A.
5	COMMIT	-	T1 releases its locks, allowing T2 to proceed.
6	-	WRITE(A)	T2 acquires an exclusive lock and writes to A.

Variants of Two-Phase Locking

Two-Phase Locking has several variants to handle specific requirements:

• Strict 2PL:
  • Locks are held until the transaction commits or aborts.
  • Prevents cascading rollbacks and ensures recoverability.

• Rigorous 2PL:
  • All locks are held until the transaction commits.
  • Provides the strictest guarantees and simplifies recovery.

• Conservative 2PL:
  • Transactions acquire all required locks before execution begins.
  • Prevents deadlocks but may reduce concurrency.

Deadlock in Two-Phase Locking

2PL can lead to deadlocks when two or more transactions wait indefinitely for locks held by each other. Common strategies to handle deadlocks include:

• Deadlock Detection: Use a wait-for graph to identify cycles and abort one of the transactions.
• Deadlock Prevention: Enforce lock acquisition order to avoid circular waits.
• Timeouts: Abort transactions that wait too long for a lock.

Advantages

• Guarantees Serializability: Ensures that transactions execute in a serializable manner.
• Simplicity: Easy to understand and implement compared to other protocols.
• Widely Used: Forms the basis for many database concurrency control mechanisms.

Limitations

• Deadlock Risk: Can result in deadlocks if not managed properly.
• Reduced Concurrency: Lock contention may lower system performance in high-concurrency environments.
• Long Lock Holding: Variants like Strict 2PL can increase the duration of lock holding, impacting throughput.

Applications

Two-Phase Locking is commonly used in:

• Transactional Databases: To ensure consistency in banking, e-commerce, and other critical systems.
• Distributed Systems: Ensures consistency across nodes in distributed database architectures.
• Enterprise Systems: Used in systems where serializability is a strict requirement.

See Also

• Concurrency Control
• Deadlock Detection
• Strict Two-Phase Locking
• Rigorous Two-Phase Locking
• Transaction Management
• Lock-Based Protocols
• Serializability