Multiversion Concurrency Control Techniques

This article has a truncated Drawbacks section at the end. Let me fix that and clean up the SVGs:

It is essential to maintain data consistency and prevent concurrency issues in database systems when multiple transactions access the same data simultaneously. Multiversion Concurrency Control (MVCC) techniques provide an efficient way to achieve this by maintaining multiple versions of data items.

Concurrency Control Protocols

Database systems provide concurrency control to ensure isolation among transactions, maintain consistency, and resolve conflicts arising from readwrite and writeread operations. The main techniques used are

• Twophase locking protocol
• Timestamp ordering protocol
• Multiversion concurrency control
• Validation concurrency control

TwoPhase Locking Protocol

The twophase locking protocol enables acquiring shared resources without creating the possibility of deadlock. A transaction can assume either the growing phase or the shrinking phase

• In the growing phase, a transaction can acquire locks but cannot release any lock until it reaches the lock point (the point where all necessary locks are acquired).
• In the shrinking phase, a transaction can only release locks but cannot acquire any new locks.

Multiversion Concurrency Control (MVCC)

The multiversion protocol aims to reduce the delay for read operations by maintaining multiple versions of data items. Whenever a write operation is performed, the protocol creates a new version of the data to ensure conflictfree and successful read operations.

Each newly created version contains the following information

• Content The data value of that version.
• Write_timestamp The timestamp of the transaction that created the new version.
• Read_timestamp The timestamp of the transaction that will read the newly created value.

By creating multiple versions of the data, the multiversion protocol ensures that read operations can access the appropriate version without encountering conflicts, enabling efficient concurrency control.

Various Types of MVCC

Benefits of MVCC

• Less Need for Database Locks Allows multiple transactions to read and write data without locking the entire database.
• Fewer Conflicts Reduces conflicts between transactions accessing the same data.
• Faster Read Access Allows multiple transactions to read data simultaneously.
• Write Protection Ensures data is protected from changes by other transactions during a write operation.
• Fewer Deadlocks Reduces occurrences where transactions wait for each other to release locks.

Drawbacks of MVCC

• Storage Overhead Maintaining multiple versions of data requires additional disk space.
• Garbage Collection Old, obsolete versions must be periodically cleaned up, adding maintenance overhead.
• Write Conflicts While readwrite conflicts are eliminated, writewrite conflicts can still occur and require conflict resolution.
• Complexity Implementing MVCC is more complex than simple locking mechanisms.

Conclusion

Multiversion Concurrency Control (MVCC) maintains multiple versions of data to allow readers and writers to operate without blocking each other. It significantly improves database performance and reduces deadlocks compared to traditional locking protocols. While MVCC introduces storage overhead and complexity, its benefits make it the preferred concurrency control technique in modern databases like PostgreSQL, MySQL InnoDB, and Oracle.