How does transaction isolation levels work internally in MySQL?

Transaction isolation levels in MySQL define the degree to which the operations in one transaction are isolated from those in other transactions. MySQL uses several isolation levels to control how and when the changes made by one operation become visible to others. These levels balance consistency, concurrency, and performance.

The four main isolation levels in MySQL are:

  • Read Uncommitted: Allows reading data changes that are not yet committed. This level provides the least isolation.
  • Read Committed: Only committed changes are visible, but it can still lead to non-repeatable reads.
  • Repeatable Read: Ensures that if a row is read twice in the same transaction, the results will be identical. This is the default isolation level in MySQL.
  • Serializable: The highest level of isolation, which simulates transaction execution one after the other, preventing any concurrency and ensuring complete isolation.

Internally, MySQL uses a combination of row-level locking, multiversion concurrency control (MVCC), and various algorithms to implement these isolation levels.

Here's an example of setting a transaction isolation level in MySQL:

<?php // Connect to MySQL $mysqli = new mysqli("localhost", "user", "password", "database"); // Set transaction isolation level $mysqli->query("SET TRANSACTION ISOLATION LEVEL REPEATABLE READ"); // Start transaction $mysqli->begin_transaction(); // Execute your queries here... // Commit transaction $mysqli->commit(); ?>

Transaction Isolation Levels MySQL Read Uncommitted Read Committed Repeatable Read Serializable MVCC

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