MYSQL: Transactions and ACID properties (Atomicity, Consistency, Isolation, Durability)

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MySQL, like many other relational database management systems (RDBMS), supports transactions and adheres to the ACID properties. ACID stands for Atomicity, Consistency, Isolation, and Durability, which are a set of properties that guarantee the reliability and integrity of database transactions. Let's discuss each of these properties in the context of MySQL:

Atomicity:

Atomicity ensures that a transaction is treated as a single, indivisible unit of work. It means that either all the changes made by a transaction are committed to the database, or none of them are.

In MySQL, you can start a transaction using the BEGIN, START TRANSACTION, or SET autocommit=0 statement. To commit a transaction, you use the COMMIT statement, and to roll back (undo) a transaction, you use the ROLLBACK statement.

Example:

sql code

START TRANSACTION;

-- SQL statements here

COMMIT;

Consistency:

Consistency ensures that a transaction brings the database from one consistent state to another. In other words, it enforces data integrity rules and constraints, preventing the database from being in an inconsistent state.

MySQL enforces constraints like primary keys, foreign keys, unique constraints, and check constraints to maintain data consistency.

Isolation:

Isolation ensures that concurrent transactions do not interfere with each other. It means that the operations of one transaction are isolated from the operations of other concurrent transactions.

MySQL supports different isolation levels, including READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. You can set the isolation level for a transaction using the SET TRANSACTION or SET SESSION statement.

Example:

sql code

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

START TRANSACTION;

-- SQL statements here

COMMIT;

Durability:

Durability guarantees that once a transaction is committed, its changes are permanent and will survive system failures. Even in the event of a crash, the committed changes should not be lost.

MySQL achieves durability by writing transaction logs to disk. These logs can be used for recovery in case of a crash.

Here's a simple example that demonstrates these ACID properties in MySQL:

sql code

-- Atomicity: The entire transaction is atomic.

START TRANSACTION;

INSERT INTO users (id, name) VALUES (1, 'Alice');

UPDATE accounts SET balance = balance - 100 WHERE user_id = 1;

COMMIT;

-- Consistency: Constraints are enforced.

-- Assuming there is a constraint that ensures the balance in the 'accounts' table is non-negative.

-- Isolation: Concurrent transactions do not interfere.

-- Other transactions can run concurrently with the above transaction, and their changes won't affect each other.

-- Durability: Committed changes are permanent.

-- Even after a system crash, the changes will persist in the database.

MySQL provides a robust framework for managing transactions and adhering to ACID properties, allowing developers to build reliable and consistent database applications. The choice of isolation level and careful transaction management are important factors in designing applications that meet specific consistency and performance requirements.

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