With the continuous growth of data volume and the increase of data operations, the concurrency issue of database transaction operations has become more and more important. In a relational database system like MySQL, how to ensure the correctness and reliability of concurrent operations is a very complex issue. This article will introduce how to implement concurrent operation control and improve the performance of data query from the aspects of MySQL transaction characteristics, transaction isolation level and concurrency control techniques.
MySQL transaction characteristics
MySQL's transaction mechanism is a mechanism that guarantees ACID characteristics (atomicity, consistency, isolation, and durability). Specifically, when MySQL operates data, either all operations succeed or all fail, ensuring atomicity; the database state remains consistent after the operation, ensuring consistency; each operation does not affect each other, ensuring isolation; the operation results are persistent , ensuring durability.
Transaction isolation level
MySQL supports four transaction isolation levels: uncommitted read (Read Uncommitted), committed read (Read Committed), repeatable read (Repeatable Read), and serialization (Serializable). The following are the characteristics of each isolation level:
Uncommitted read: One transaction can read uncommitted data of another transaction, and dirty data may be read. The problem of non-repeatable read or phantom read is very serious.
Committed read: A transaction can only read data that has been submitted by another transaction, but repeatable reads and phantom reads are not guaranteed.
Repeatable read: The data read by one transaction will not be modified by other transactions during the transaction, but new transactions can insert new data, causing phantom read problems.
Serialization: Concurrent access is limited, and all read or write operations must be queued to avoid all concurrency problems.
Concurrency control techniques
In order to improve the concurrency performance of MySQL, you need to use some concurrency control techniques, as follows:
1. Transaction level selection
In high concurrency scenarios, it is recommended to use repeatable read or committed read, which can effectively avoid dirty read and phantom read problems. For low concurrency scenarios, you can use uncommitted reads, which can improve query efficiency. However, you need to pay attention to data consistency and resolve update conflicts by yourself.
2. Optimize the locking mechanism
The locking mechanism in MySQL is a very important mechanism for controlling concurrency behavior and can be used to ensure data consistency and prevent the occurrence of concurrency problems. There are generally two types of locks: row locks and table locks. Row locks can avoid dirty reads between transactions, and table locks can avoid deadlock problems. When using the locking mechanism, it is necessary to choose the lock granularity reasonably to reduce the occurrence of lock conflicts and deadlock problems.
3. Increase caching
In high concurrency scenarios, for frequent reading of data, caching can be used to reduce the pressure on the database and improve query efficiency. You can use caching solutions such as Redis to cache data in memory to avoid frequent IO operations.
4. Use sub-database and sub-table
When the data scale is very large, you can use the sub-database and sub-table strategy to avoid the concurrency bottleneck of a single database. By dispersing data into different databases or tables, the concurrency capability of the system can be improved.
In short, concurrency control for large systems is a very complex task. Only through the reasonable selection and use of transaction characteristics, isolation levels and concurrency control techniques can the stability and reliability of system operation be ensured.
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