Multiversion Concurrency Control

Multiversion Concurrency Control (MVCC) is a concurrency control method used in database systems that allows multiple versions of a data item to exist simultaneously. It ensures consistent reads without locking and provides high concurrency by maintaining transaction isolation.

Key Concepts

• Data Versioning: MVCC creates a new version of a data item for every write operation, enabling transactions to access consistent snapshots.
• Timestamps: Each transaction and data version is associated with timestamps, which determine visibility and consistency.
• Read and Write Operations:
  • Reads access the latest version of data visible to the transaction, avoiding conflicts.
  • Writes create a new version of the data item without blocking other transactions.

How MVCC Works

MVCC operates based on the following principles:

1. Each transaction is assigned a unique timestamp at the start.
2. Data items maintain multiple versions, each with associated timestamps for creation and expiration.
3. Read and write operations follow these rules:
   • A transaction reads the most recent version of a data item whose timestamp is less than or equal to the transaction's start timestamp.
   • A transaction creates a new version of a data item during writes, ensuring isolation.

Advantages

• Non-Blocking Reads: Readers do not block writers, and vice versa, improving concurrency.
• Consistent Snapshots: Transactions operate on consistent snapshots of data without interfering with other transactions.
• Deadlock-Free: MVCC avoids deadlocks by not requiring locks for reads.

Limitations

• Storage Overhead: Maintaining multiple versions of data increases storage requirements.
• Garbage Collection: Older versions of data must be periodically removed to free space.
• Write Amplification: Frequent writes can generate a large number of versions, impacting performance.

Example of MVCC

Consider a scenario with two transactions accessing a shared data item A:

Step	Transaction T1	Transaction T2	Explanation
1	BEGIN TRANSACTION (TS=1)	BEGIN TRANSACTION (TS=2)	T1 and T2 start with timestamps 1 and 2, respectively.
2	READ(A)	-	T1 reads the current version of A (value=100, TS=0).
3	-	WRITE(A, value=200)	T2 writes a new version of A with TS=2.
4	READ(A)	-	T1 still sees the version of A with TS=0 (value=100), ensuring a consistent snapshot.
5	COMMIT	COMMIT	Both transactions commit successfully.

In this example, T1 reads the old version of A while T2 writes a new version, demonstrating how MVCC prevents conflicts.

MVCC in Databases

MVCC is widely implemented in modern database systems:

• PostgreSQL: Provides MVCC for high-performance and consistent transactions.
• MySQL (InnoDB): Uses MVCC to support isolation levels like Repeatable Read and Read Committed.
• Oracle Database: Implements MVCC to manage consistent reads and undo data.

Variants of MVCC

• Snapshot Isolation:
  • Transactions work with a snapshot of the database at their start time.
  • Prevents dirty reads and non-repeatable reads but allows write skew.
• Serializable Snapshot Isolation (SSI):
  • Extends snapshot isolation to detect and prevent write conflicts, ensuring serializability.

See Also

• Concurrency Control
• Timestamp-Based Protocol
• Transaction Management
• Snapshot Isolation
• Serializable
• Database Anomalies