What are ACID properties in SQL transactions?

ACID properties are fundamental principles that ensure reliable transaction processing in SQL databases. ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties work together to guarantee that database transactions are processed safely, even in cases of errors, crashes, or concurrent operations. A transaction is a sequence of operations (like inserting, updating, or deleting data) treated as a single logical unit.

Atomicity ensures that a transaction either fully completes or has no effect at all. If any part of a transaction fails, the entire transaction is rolled back, leaving the database unchanged. For example, consider a bank transfer where money is withdrawn from one account and deposited into another. If the deposit fails after the withdrawal, atomicity ensures the withdrawal is reversed, preventing partial updates. Consistency guarantees that a transaction brings the database from one valid state to another, adhering to predefined rules like constraints, triggers, or unique indexes. For instance, if a transaction attempts to assign a duplicate primary key, consistency ensures the operation fails, preserving data integrity. Isolation manages how concurrent transactions interact. Without isolation, simultaneous transactions might interfere, leading to incorrect results (e.g., two users reading the same data before it’s finalized). Isolation levels (like Read Committed or Serializable) control visibility of changes between transactions. Durability ensures that once a transaction is committed, its effects persist even after system failures. This is typically achieved through write-ahead logging or storing data on non-volatile storage.

Each property addresses specific challenges. Atomicity prevents partial updates, Consistency enforces data rules, Isolation handles concurrency, and Durability safeguards against data loss. For example, an e-commerce order processing system relies on atomicity to ensure inventory deduction and order creation either both succeed or both fail. Consistency ensures stock levels never go negative. Isolation prevents conflicting reads/writes during checkout surges. Durability guarantees orders aren’t lost if the database crashes post-payment.

Developers use ACID-compliant databases for scenarios requiring reliability, such as financial systems or record-keeping. While strict ACID adherence can impact performance, modern databases optimize this through techniques like locking, multiversion concurrency control (MVCC), or configurable isolation levels. Choosing the right isolation level (e.g., Read Committed for balance between consistency and speed) is crucial. Understanding ACID helps design robust systems where data accuracy and reliability are non-negotiable.