In database management systems, locks are an important mechanism to ensure data integrity and concurrency control. Different database engines use different lock policies, among which myisam and innodb are two common storage engines, and they have significant differences in lock handling. This article provides an in-depth analysis of the table locks and row locks of Myisam and InnoDB, as well as their trigger timing and compatibility between locks in different situations, to help you choose the best database lock strategy.
1. Myisam's table lock
MyISAM is one of the default storage engines for MySQL, and it only supports table-level locks. This means that when one thread writes to a myisam table, the entire table is locked, and other threads cannot read or write to the table until the lock is released. This locking mechanism is simple and fast, but it can cause performance issues in high-concurrency environments.
There are two modes of myisam table-level locking: table read lockTable Write Lock.
2. InnoDB's row locks and table locks
Unlike Myisam, InnoDB is another popular MySQL storage engine that primarily uses row-level locking. This means that when a thread writes to a row of data in an InnoDB table, only that row is locked, and other threads can still access the data outside of the locked row. This helps improve concurrency performance, especially in high-concurrency systems.
There are two modes of innodb row-level locks: shared locks and exclusive locks.
There are two types of InnodDB table-level locks: Intention Shared Locks and Intention Exclusive Locks.
3. The timing of the lock
The Myisam storage engine supports table-level locking, where read locks are shared and write locks are exclusive. This means that when a query is performing a write operation (such as insert, update, or delete) on a Myisam table, other queries cannot read or write to the table until the first query completes.
Shared locks: Shared locks, also known as read locks, referred to as S locks, as the name suggests, shared locks are multiple transactions that can share a lock on the same data, and the data can be accessed, but only the data can be read and cannot be modified, and the lock is added through "lock in share mode".
Exclusive locks: Also known as write locks, exclusive locks cannot coexist with other locks, if a transaction obtains an exclusive lock on a data row, other transactions can no longer obtain the lock on the row (shared lock, exclusive lock), only the transaction that obtains the exclusive lock can read and modify the data row, (other transactions to read data can come from snapshots). Lock method: delete update insert adds x lock by default select … for update。
Intent Shared Lock (IS) means that a transaction is ready to add a shared lock to a data row, that is, an IS lock must be obtained before a data row can be added to a shared lock, and an Intent Exclusive Lock (ix) means that a transaction is ready to add an exclusive lock to a data row, that is, the IX lock of the table must be obtained before an exclusive lock is added to a data row, and the intent lock (IS, IX) is automatically added before the InnoDB data operation and does not require user intervention.
It should be noted that the innodb row lock is implemented by locking the index entries on the index. InnoDB uses row-level locks only when data is retrieved using index criteria, otherwise table locks are used. If the primary key index condition is satisfied, you need to add one lock, and when the secondary index condition is satisfied, you need to add two locks. If the index is not met by the condition, then the entire table is locked.
Fourth, the compatibility between locks
It's clear that MySQL's read-lock mode doesn't prevent other readers from reading, but it doesn't allow writes;Write locks, on the other hand, block access for both readers and write users.
When the lock mode requested by a transaction is compatible with the lock mode currently in use, INNODB grants the requested lock to the transactionOtherwise, the transaction waits for the lock to be released. Intent locks are automatically added by innodb;For update, delete, and insert statements, innodb automatically adds an exclusive lock (x) to the design datasetFor common select statements, innodb does not add locks.
5. How to choose the best database lock strategy?
Choosing the best database lock strategy depends on your application needs and database usage. Here are some suggestions:
High concurrent read operations: If your application has a large number of reads and few writes, myisam may be a better choice because its table-level locks can reduce the blocking of read operations.
High concurrent write operations: For applications with a large number of write operations, InnoDB's row-level locks can provide better concurrency performance.
High data integrity requirements: If data integrity is important, you may need to weigh the speed of Myisam against the isolation level of InnoDB.
Requires transaction support: If your application needs to support transactions, then InnoDB is the only option because it supports transactions and row-level locking.
Backup and recovery: Backup and recovery of Myisam tables is generally faster, while InnoDB offers more features and better crash recovery capabilities.
Data volume size: For very large datasets, Myisam may provide better performance in some cases because it does not require maintenance of transaction logs.
6. Summary
Understanding the locking mechanisms and characteristics of different database engines is critical to choosing the best database locking strategy. You need to select the appropriate storage engine based on the requirements of the application. Hope you find these articles helpful!If you have any further questions or need more help, leave a comment!List of high-quality authors