Primary Site Two-Phase Locking

Primary Site Two-Phase Locking (2PL) is a variant of the Two-Phase Locking protocol used in distributed databases. In this approach, one node is designated as the primary site for each data item, and it coordinates the locking and transaction processing to ensure serializability across the system.

Key Concepts

• Primary Site: A designated node that manages all lock requests for a specific data item.
• Global Serializability: Ensures that all transactions across distributed nodes maintain a serializable schedule.
• Coordination: The primary site acts as the single authority for granting or releasing locks, reducing complexity in lock management.

How Primary Site 2PL Works

The Primary Site 2PL protocol operates as follows:

1. Each data item in the distributed database is assigned to a primary site.
2. Transactions request locks from the primary site of the data item they wish to access.
3. The primary site uses the Two-Phase Locking protocol:
   • Growing Phase: Locks are granted to transactions but not released.
   • Shrinking Phase: Locks are released, but no new locks are granted.
4. Once the primary site grants a lock, the transaction can proceed to access the data at any replica site.

Advantages

• Centralized Coordination: Simplifies lock management by delegating responsibility to primary sites.
• Ensures Consistency: By centralizing lock control, Primary Site 2PL ensures consistency across distributed transactions.
• Scalability: Each primary site manages its own subset of data, reducing bottlenecks.

Limitations

• Single Point of Failure: If a primary site fails, the data it manages becomes unavailable.
• Communication Overhead: Lock requests and responses require network communication, increasing latency.
• Limited Concurrency: Centralized lock management may limit the level of parallelism.

Example of Primary Site 2PL

Consider a distributed database where:

• Node 1 is the primary site for data item A.
• Node 2 is the primary site for data item B.

Scenario

Transaction T1 and T2 execute as follows:

Step	Transaction T1	Transaction T2	Primary Site Action
1	BEGIN TRANSACTION	BEGIN TRANSACTION	Both transactions start.
2	READ(A)	-	T1 requests a shared lock for A from Node 1 (primary site for A).
3	-	WRITE(A)	T2 requests an exclusive lock for A from Node 1 but must wait as T1 holds a shared lock.
4	COMMIT	-	T1 releases the lock on A.
5	-	WRITE(A)	T2 acquires the exclusive lock on A and writes.
6	COMMIT	-	T2 releases the lock.

Applications

Primary Site 2PL is used in distributed systems where:

• Global Consistency: A strict guarantee of serializability is required.
• Low Update Rates: Environments with fewer updates to data items to minimize contention.
• Simplified Lock Management: Centralized control is beneficial for simpler coordination.

Comparison with Other 2PL Variants

Feature	Primary Site 2PL	Centralized 2PL	Distributed 2PL
Lock Management	Delegated to primary sites for each data item	Centralized at a single node	Fully distributed across all nodes
Fault Tolerance	Affected by primary site failure	Single point of failure	Higher fault tolerance
Communication Overhead	Moderate (lock requests to primary sites)	Low (single coordinator)	High (peer-to-peer communication)
Scalability	High (multiple primary sites)	Limited	High

See Also

• Two-Phase Locking
• Distributed Database
• Concurrency Control
• Deadlock Detection
• Centralized 2PL
• Distributed 2PL
• Primary-Replica Architecture