/ˈeɪ-sɪd/
n. “Guaranteeing your data behaves — no surprises allowed.”
ACID is an acronym describing the foundational properties of reliable database transactions: Atomicity, Consistency, Isolation, and Durability. These principles ensure that when data is read, written, or updated, it behaves predictably — even under crashes, concurrent access, or network issues.
Atomicity guarantees that each transaction is “all or nothing.” If any part of a transaction fails, the entire operation is rolled back, leaving the database in its prior state. Think of transferring money: either the funds leave one account and arrive in the other, or nothing changes.
Consistency ensures that transactions move the database from one valid state to another, obeying all rules, constraints, and integrity checks. For example, foreign key relationships or unique constraints are maintained automatically.
Isolation addresses the problems of concurrent transactions. Each transaction behaves as if it were running alone, even when multiple operations occur simultaneously. This prevents “dirty reads,” lost updates, or other anomalies that could corrupt data.
Durability promises that once a transaction commits, its effects are permanent. Even a sudden system crash or power failure cannot undo committed changes, thanks to mechanisms like write-ahead logs or transaction journals.
ACID principles are critical in relational databases such as SQLServer, PostgreSQL, and MySQL. They form the backbone for applications where data integrity is non-negotiable: banking, e-commerce, enterprise resource planning, and any system where failed or inconsistent writes could be catastrophic.
Beyond traditional relational databases, ACID concepts influence modern distributed systems. Techniques like two-phase commit or consensus protocols extend ACID guarantees across multiple nodes, enabling reliable cloud databases and transaction systems. While NoSQL databases sometimes relax ACID in favor of availability and scalability, understanding ACID remains essential for designing systems that require trust in every operation.
In practice, ACID provides developers and users confidence: you can update, delete, or insert data knowing that the database will never leave you in an inconsistent, partially updated, or corrupted state. It’s the reason that when you check your bank account after transferring money, it always matches expectations.