MySQL Concurrent Transaction Control: Lost Update, Locking and MVCC Deep Dive

Concurrent Transaction Problems

Lost Update

• Rollback覆盖
• 提交覆盖

Dirty Read

A transaction reads data modified but not committed by another transaction

Non-repeatable Read

Multiple reads of the same row in a transaction are inconsistent

Phantom Read

Multiple queries with the same conditions in a transaction have inconsistent results

Locking Mechanism

Exclusive Lock

Exclusive lock is the most strict lock type in database concurrency control:

SELECT * FROM accounts WHERE id = 1 FOR UPDATE;

Read-Write Locks

• Shared read: Allows multiple threads to acquire read locks simultaneously
• Exclusive write: Only allows one thread to acquire write lock
• Read-write mutual exclusion

MVCC (Multi-Version Concurrency Control)

Working Principles

1. Version Storage Mechanism
2. Read Operation Processing
3. Concurrency Control Capability

Advantages

1. Improves concurrent performance
2. Reduces lock contention
3. Maintains consistency

Practical Applications

MySQL InnoDB’s MVCC Implementation:

• Records create and delete transaction IDs through hidden DB_TRX_ID field
• Uses undo logs to store old version data
• Implements visibility control under different isolation levels through ReadView mechanism

Global Queuing Mechanism for Database Transactions

Basic Concepts

Global queuing is the most basic transaction processing method, forcing all concurrent transactions to convert to serial execution.

Typical Characteristics

• Strong consistency guarantee
• Performance bottleneck
• Simple implementation