Concurrency Control Techniques
Concurrency control in a Database Management System (DBMS) ensures that multiple transactions can occur simultaneously without leading to data inconsistency or integrity issues. Here are the primary techniques used for concurrency control:
1. Locking
Locking is a fundamental technique that restricts access to data to ensure consistency:
- Types of Locks: Shared Lock (S-lock), Exclusive Lock (X-lock), Update Lock (U-lock)
- Lock Granularity: Database-level, Table-level, Page-level, Row-level, Column-level
- Two-Phase Locking (2PL): Ensures serializability with growing and shrinking phases
- Deadlock Handling: Prevention, Detection, Avoidance
2. Timestamp-Based Protocols
Timestamp-based protocols use timestamps to order transactions and ensure serializability:
- Timestamp Ordering (TO): Assigns each transaction a timestamp and orders operations based on these timestamps to avoid conflicts.
- Thomas's Write Rule: An optimization of the basic TO protocol that allows overwriting of obsolete data values to improve concurrency.
3. Optimistic Concurrency Control (OCC)
OCC assumes conflicts are rare and allows transactions to proceed without restrictions, validating at the end:
- Read Phase: Transactions read from the database without acquiring locks.
- Validation Phase: Checks if the transaction can be committed without conflicts.
- Write Phase: Commits the transaction and updates the database if validation is successful.
4. Multiversion Concurrency Control (MVCC)
MVCC keeps multiple versions of data to manage read and write operations concurrently:
- Snapshot Isolation: Each transaction sees a consistent snapshot of the database, ensuring it does not interfere with other transactions.
- Version Maintenance: Maintains different versions of data items to serve different transactions without conflicts.
5. Validation-Based Protocols
Similar to OCC, validation-based protocols perform checks at the end of transactions:
- Validation: Ensures that a transaction does not violate serializability before committing.
- Commit or Rollback: Commits if validation is successful, otherwise rolls back.
6. Two-Phase Commit Protocol (2PC)
2PC is used in distributed databases to ensure all or none of the operations are committed across different sites:
- Prepare Phase: Coordinator sends a prepare request to all participants and waits for a vote.
- Commit Phase: If all participants vote to commit, the coordinator sends a commit request; otherwise, it sends a rollback request.
7. Summary
Concurrency control techniques are crucial for maintaining data consistency and integrity in DBMS. Each technique has its advantages and is suited for different scenarios. Understanding these techniques helps in designing robust and efficient database systems.