An article introducing the implementation of locks in MySQL

MySQL lock implementation

MySQL is a very popular relational database management system that supports the processing of many concurrent users and transactions. In the case of high concurrency, the lock mechanism in MySQL is a crucial component. How to use the locking mechanism rationally and effectively improve concurrency performance is an important part of MySQL optimization. This article will introduce how locks are implemented in MySQL and how to choose different lock types to maximize performance.

Overview

In a concurrently accessed database, when multiple users try to modify shared data, various data consistency issues may occur. The most common problems are read and write conflicts and data race conditions. To solve these problems, MySQL provides different lock types to allow concurrent access while ensuring data consistency.

MySQL's locks are mainly divided into two categories: shared locks (Shared Locks) and exclusive locks (Exclusive Locks). Shared locks allow multiple users to access a resource at the same time. These users can only read the resource but not modify it. Therefore, when a user already holds a shared lock, other users can use the shared lock to read the same resource, but cannot use an exclusive lock to modify it. An exclusive lock only allows one user to access the same resource at a time, and multiple users cannot hold the lock at the same time.

MySQL lock implementation

MySQL lock implementation methods include: table lock, row lock, page lock, tuple lock (tuple lock) and gap lock (gap lock), etc.

1. Table lock

Table lock is the most basic locking method. During the modification operation, the entire table is locked. Therefore, other users will not be able to access the table, resulting in reduced concurrency performance. This locking mechanism can work properly if the table's data is small or there are few concurrent accesses. However, in large databases or high-concurrency databases, table locks can cause some concurrency problems and reduce system performance.

The syntax of table lock is as follows:

• Lock all tables: LOCK TABLES table_name [AS alias] lock_type[,table_name [AS alias] lock_type] …
• Unlock: UNLOCK TABLES

Among them, lock_type can be the following types:

• READ: shared read lock.
• WRITE: Exclusive write lock.

2. Row lock

Row lock is a more efficient locking mechanism in MySQL. For modifications to records in the table, MySQL only locks rows in the table that meet the conditions. Other accesses to rows that do not require locking are not affected. In the case of high concurrency and large data volume, using row locks can effectively reduce the scope and time of the lock and improve the performance of the system.

In the InnoDB storage engine, row locking is the default locking mechanism.

Row lock syntax:

• Open transaction: START TRANSACTION
• Row lock: SELECT… FOR UPDATE or UPDATE… WHERE…
• Submit Transaction: COMMIT or ROLLBACK

Among them, SELECT ... FOR UPDATE can lock the marked row to prevent the row from being modified by other transactions. The UPDATE ... WHERE ... statement can also lock one or more rows that meet the conditions.

3. Page lock

Page lock is a locking mechanism of MySQL. It locks in units of pages when scanning the table. Page locking is more effective than row locking when modifications are made to multiple records because the page is made up of more rows. Row locking, on the other hand, works individually on a per-row basis anyway, so works slower when modifying multiple rows.

As the name suggests, page lock is to lock the page. During the process of modifying a certain page, the page-level lock only protects the entire page and notifies the system that the locking of other pages will not hinder the execution of other transactions. . Locking the entire table or index of a page avoids locking too many rows, but may also cause concurrency issues.

How to use page locks:

• Change the mode of the InnoDB storage engine and set the commit mode (commit_mode) to 2, which means using page-level locks.
• In each stage of executing SQL commands, lock and unlock records.

You can use the following command to make the InnoDB engine use page-level locks:

• SET GLOBAL innodb_locks_unsafe_for_binlog=1;
• SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ ;

When using page locks, you must be careful not to use LIMIT in the SQL statement to limit the number of query rows, because this may cause the database to change from row-level locks to table-level locks.

4. Tuple lock

Tuple lock, also known as tuple lock, is a lock in the MySQL storage engine. In the InnoDB engine, tuple locks are only applied to "non-unique" secondary indexes to support concurrent operations. If the tuple is locked by another transaction, the other transaction can only complete the modification of the record with the same index value, not the modification of the entire data row.

Basic syntax of tuple lock:

• SELECT … FROM table_name WHERE key_column=user_input FOR UPDATE;
• UPDATE … WHERE key_column=user_input;

Among them, user_input represents the input of the end user, which can be a single value, conditional statement or range, etc.

Remember to be careful when using tuple locks. Unreasonable use will greatly reduce database concurrency performance.

5. Gap lock

Gap lock, also called Gap lock, is usually used to prevent non-repeated reads. Often appears in SQL operations that span multiple rows at a time.

For example:

SELECT c1 FROM tbl WHERE c1 BETWEEN 10 and 20 FOR UPDATE;

At this time, the database needs to lock all rows with c1 value between 10 and 20. The corresponding lock type is called gap lock, which locks values ​​that fall within the gap but do not exist.

How to use gap locks:

• Use phantom read view (MVCC) to track ongoing read operations and provide row-level read locks or gap locks for future read operations.
• Gap check: S's gap lock will check rows one by one to ensure that these rows will not be updated or deleted while maintaining "repeatable read".
• One of the most important issues with gap locks and their access is to be very careful about the concurrent operations of other things.

Summary

In actual development, we should choose the correct lock type and use appropriate locks according to actual needs to avoid deadlocks and improve the concurrency of the system. , try to avoid system performance degradation. The various locking functions provided by MySQL can solve the problem of transaction operations or simultaneous access by multiple users.

The correct selection of the appropriate MySQL lock mechanism can effectively improve the performance and concurrency of the system, making our applications more robust and reliable.

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