dba-support

ACID in MySQL

ACID is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. These are the key properties that ensure reliable transaction processing in a database system like MySQL. Here’s a brief explanation of each property and how it applies to MySQL:

  1. Atomicity:
    • Definition: Ensures that all operations within a transaction are completed successfully. If any operation within the transaction fails, the entire transaction is rolled back.
    • MySQL Implementation: MySQL uses transaction logs and rollback mechanisms to maintain atomicity. If a transaction fails, MySQL will undo any changes made during that transaction.
  2. Consistency:
    • Definition: Ensures that a transaction brings the database from one valid state to another, maintaining database invariants.
    • MySQL Implementation: MySQL uses constraints, triggers, and cascades to enforce consistency. For example, primary keys, foreign keys, and unique constraints help maintain data integrity.
  3. Isolation:
    • Definition: Ensures that concurrently executing transactions do not affect each other. The changes made by one transaction are not visible to other transactions until the transaction is committed.
    • MySQL Implementation: MySQL supports multiple isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. The default isolation level is REPEATABLE READ. These levels help control the visibility of uncommitted changes to other transactions, minimizing issues like dirty reads, non-repeatable reads, and phantom reads.
  4. Durability:
    • Definition: Ensures that once a transaction is committed, it will remain so, even in the event of a system failure.
    • MySQL Implementation: MySQL uses the InnoDB storage engine for its transaction management, which includes mechanisms like the redo log to ensure durability. Once a transaction is committed, changes are written to the disk, making them permanent.

Example of an ACID Transaction in MySQL

Here is an example of how an ACID-compliant transaction might look in MySQL:

sqlCopy codeSTART TRANSACTION;

-- Atomicity and Consistency: All operations succeed or none do
UPDATE accounts SET balance = balance - 100 WHERE account_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE account_id = 2;

-- Isolation: Changes are not visible to other transactions until commit
SELECT * FROM accounts WHERE account_id IN (1, 2);

-- Commit transaction to ensure Durability
COMMIT;

In this example:

  • The transaction ensures atomicity because if either UPDATE fails, the ROLLBACK command can be issued to undo both updates.
  • Consistency is maintained by ensuring that the total balance remains unchanged.
  • The isolation level can be set to control the visibility of these changes to other transactions.
  • The COMMIT command ensures durability, making sure that the changes are saved to the database permanently.

By adhering to the ACID principles, MySQL ensures reliable, consistent, and isolated transaction processing, which is crucial for maintaining data integrity in a multi-user environment.

amit