overview
MySQL is a very popular relational database used by many systems to store and manage data. In a high-concurrency environment, in order to ensure data consistency and reliability, the database needs to implement concurrency control, including a lock mechanism. MySQL provides two types of locks, one is table-level locks and the other is row-level locks. This article will discuss in depth the row-level locks in MySQL, including the concept of row locks, the implementation principles, types, and usage scenarios of row locks.
Basic concept of row lock
1. The concept of row lock
The row lock in MySQL is a fine-grained lock, which can determine which record the data to be locked is, thus ensuring the isolation of access to the same data by different threads. Row locks can control data consistency during concurrent read and write access, but their granularity is small, requiring more frequent locking and unlocking, which consumes more resources.
2. Implementation principle of row lock
The implementation principle of MySQL row lock is based on the index. This is because MySQL has a rule: row-level locks are only used when index conditions are used to retrieve data. This means that row-level locks cannot be used for operations such as full table scans.
When MySQL executes the SELECT statement, it will find the records to be accessed according to the conditions of the WHERE clause. If the WHERE condition has an index, MySQL will use this index.
If the index is a clustered index, locking the record locks the entire clustered index.
If the index is a non-clustered index, then locking the record is locking the index items in the non-clustered index.
Row-level locks are automatically processed, and the concurrency of transactions is determined according to whether the records are locked or the type of the lock. When the transaction that locks the row is committed, the transaction automatically releases the lock, and other transactions can continue to access these records.
row lock type
MySQL's row-level locks are implemented using a variety of lock types. Here we will introduce three common types of row locks and their application scenarios. They are:
1. Shared lock
Shared locks, also known as read locks, are the most commonly used row-level locks and are used to control the concurrency of read and write operations. When reading data, MySQL will automatically add a shared lock, which allows multiple threads to read the same data block at the same time, but cannot write at the same time.
In MySQL, shared locks can be acquired in the following ways:
SELECT ... FROM ... WHERE ... LOCK IN SHARE MODE;
This statement acquires a read lock and waits for other transactions holding shared locks to complete before releasing the lock. By adding this LOCK IN SHARE MODE clause, you can ensure that no data modification operations occur during the SELECT query, thus avoiding concurrency issues.
In addition, MySQL also provides the SELECT ... FOR SHARE statement, which can also acquire shared locks. But its effect is very different from LOCK IN SHARE MODE. SELECT ... FOR SHARE obtains the shared lock at the beginning of the transaction, and does not release the lock until the end of the transaction. LOCK IN SHARE MODE acquires the lock only while executing the query and releases the lock immediately after the query is completed.
If a thread already has a shared lock on a data block, other threads can only apply for a shared lock, but not an exclusive lock, because the data block is being read and cannot be modified. Therefore, a shared lock can be shared by multiple transactions, but it cannot coexist with other exclusive locks. In the case of acquiring a shared lock, other transactions can still read data, but cannot modify it until the lock is released. Therefore, data consistency still needs to be considered in concurrent read and write scenarios.
2. Exclusive lock
Exclusive locks in MySQL are used to control the concurrency of write operations, just the opposite of shared locks. An exclusive lock can only be held by one transaction, and other transactions cannot acquire the lock. When one transaction holds an exclusive lock, other transactions cannot hold shared or exclusive locks.
In MySQL, an exclusive lock can be obtained by:
SELECT ... FROM ... WHERE ... FOR UPDATE;
This statement acquires a write lock and waits for other transactions holding shared or exclusive locks to complete before releasing the lock. By adding the FOR UPDATE clause, you can ensure that no data modification operations occur while the SELECT statement is executing within a transaction, thereby avoiding concurrency problems.
It should be noted that if a piece of data is already held by other transactions, the current transaction cannot obtain an exclusive lock on the data. Concurrent write operations need to consider how to control the execution order of transactions so that the data can avoid race conditions to a certain extent and ensure the correctness and integrity of the data.
3. Intention lock
The Intention Lock in MySQL is mainly used to coordinate the relationship between read locks and write locks. It is a lock level that plays an important role in transaction concurrency control. There are two types of Intention Lock: Intention Share Lock (IS) and Intention Exclusive Lock (IX).
When a transaction wants to read a row of data (using the SELECT statement), it will request an IS lock. If a transaction request requests an IS lock on a table, it means that it wants to read some rows of this table, but it does not need an exclusive lock, and multiple transactions can hold IS locks at the same time.
When a transaction wants to write a row of data (using UPDATE or DELETE statement), it will request an IX lock. If a transaction requests an IX lock on a table, it means that it wants to modify some rows of this table, requiring other transactions not to perform read/write operations, and only itself can modify it. IX locks are more exclusive.
In MySQL, multiple transactions are allowed to hold IS locks at the same time, but it is not allowed to hold IX locks while holding IS locks at the same time. This is because if a transaction holds an IX lock, it means that it wants to modify the row data, and other transactions cannot read/write. Therefore, if a transaction holding an IS lock is allowed to also hold an IX lock, other transactions will not be able to perform read operations, thus violating the sharing of IS locks.
scenes to be used
In actual development, row locks are widely used, which can solve data consistency problems under high concurrent access.
1. Scenarios of concurrent reading and writing
If the system performs read and write operations on the same table, in order to ensure data consistency, it is necessary to consider using row locks. For example, in a fund account, one user deposits money into the account while another user withdraws money from the account. In order to avoid concurrency problems caused by these two operations, row-level locks must be used to ensure data consistency.
2. The uniqueness constraint of data operation
Using row locks can ensure that uniquely identified data is only modified by one transaction, thus ensuring the uniqueness of data operations.
3. For scenarios that need to operate larger tables
Table-level locks are inefficient when operating large tables and will cause resource bottlenecks. Using row-level locks can solve this problem well, because row-level locks only lock the accessed data and avoid locking the entire table.
Lock Optimization
1. Reduce lock competition
When multiple threads want to acquire the same record at the same time, lock competition will occur. At this time, it is necessary to consider how to reduce lock competition.
A commonly used method is to divide databases and tables, and divide data into different tables or different databases, which can effectively reduce lock competition and improve concurrency performance.
Another method is to use caching, which can reduce direct access to the database, thereby reducing lock competition.
2. Reasonable use of lock types
When using row-level locks, you need to choose different lock types according to the actual situation to avoid the situation where the lock cannot be obtained.
For scenarios with more reads and fewer writes, shared locks can be used to ensure data consistency;
For scenarios with more writes and fewer reads, exclusive locks can be used to ensure data consistency;
For scenarios with both read and write operations, intent locks can be used to ensure data consistency, which can effectively avoid lock competition.
3. Simplify the scope of business
Try to control the operation range of the transaction to the minimum range, which can effectively reduce the chance of lock conflicts and improve concurrency performance. Especially when updating a large table, it can be processed for a partition or a page instead of locking the entire table.
Summarize
This article introduces the basic concepts, implementation principles, types and usage scenarios of row-level locks in MySQL. MySQL row-level locks are fine-grained locks, which can ensure the isolation of access to the same data by different threads. There are many types of locks used in row locks. We need to choose different lock types according to the actual situation to avoid the situation that the lock cannot be obtained. By using row-level locks, we can better solve the problem of data consistency in high concurrency situations and improve the concurrency performance of the system.