What is ACID (Atomicity, Consistency, Isolation, Durability)?

What is ACID?

ACID is a set of four properties expected from reliable database transactions: Atomicity, Consistency, Isolation, and Durability. They matter when an operation must not be left half-done, such as a money transfer, stock deduction, or accounting entry.

In a bank transfer, the system cannot debit one account and fail to credit the other. With an ACID transaction, those steps are treated as one unit: either all changes are committed, or the database rolls them back as if they never happened.

The Four Properties

• Atomicity: The transaction completes in full or is fully rolled back.
• Consistency: The database remains valid according to its constraints and rules.
• Isolation: Concurrent transactions do not corrupt each other’s intermediate states.
• Durability: Once committed, the result survives crashes and restarts.

Where It Matters

Relational databases such as PostgreSQL, MySQL InnoDB, and SQL Server are known for ACID guarantees. In SQL, commands such as BEGIN, COMMIT, and ROLLBACK expose this behavior to application code.

ACID adds strong safety, but it still requires careful choices around locks, isolation levels, and performance. In finance, e-commerce inventory, and enterprise records, that trade-off is often preferable to accepting eventual consistency.