Concurrency Control Mechanisms in NewSQL Databases

1. Introduction

Concurrency control is crucial in NewSQL databases to ensure data integrity and consistency in a multi-user environment. NewSQL databases combine the scalability of NoSQL with the ACID properties of traditional SQL databases, making concurrency control mechanisms essential for high-performance and reliable transactions.

2. Key Concepts

• **ACID Properties**: Essential for ensuring reliable transactions.
• **Isolation Levels**: Determines how transaction integrity is visible to other transactions.
• **Deadlocks**: A situation where transactions are waiting for each other to release locks.

3. Concurrency Control Mechanisms

3.1 Pessimistic Concurrency Control

Pessimistic concurrency control assumes that conflicts will occur, thus it locks resources before accessing them.

BEGIN TRANSACTION;
LOCK TABLE my_table IN EXCLUSIVE MODE;
-- Perform operations
COMMIT;
            

3.2 Optimistic Concurrency Control

Optimistic concurrency control assumes that conflicts are rare and allows transactions to proceed without locking resources, checking for conflicts before committing.

BEGIN TRANSACTION;
-- Perform operations
IF (SELECT version FROM my_table WHERE id = @id) = @version THEN
    UPDATE my_table SET value = @newValue WHERE id = @id;
    COMMIT;
ELSE
    ROLLBACK;
END IF;
            

3.3 Multiversion Concurrency Control (MVCC)

MVCC allows multiple versions of data to exist, thus providing greater concurrency. Readers never block writers and vice versa.

-- No explicit locking is required
SELECT * FROM my_table AS OF TIMESTAMP '2023-10-01 12:00:00';
            

4. Best Practices

• Choose the right concurrency control mechanism based on application requirements.
• Regularly monitor and tune database performance related to concurrency.
• Implement deadlock detection and resolution strategies.
• Use isolation levels judiciously to balance performance and consistency.

5. FAQ