ACID
A set of transaction properties—atomicity, consistency, isolation, and durability—used to reason about database correctness under failure and concurrency.
Database Context
ACID is not a single switch and each property is implemented through concrete mechanisms such as logging, locking or MVCC, constraints, and recovery. Isolation level may deliberately allow anomalies even though the database is still described as transactional.
Related Database Concepts
- transaction
- Write-Ahead Log
- MVCC
- isolation level
Transaction Boundary and Isolation
The four ACID properties are not provided by one mechanism. Atomicity is commonly tied to rollback and logging, durability to a persistent redo/WAL boundary, and isolation to locking or MVCC behavior; consistency additionally depends on database constraints and application invariants. "ACID compliant" should therefore not be read as a complete correctness guarantee independent of the selected isolation level and failure model.
Transaction scope matters as much as the label. Splitting one business invariant across two independent transactions can create partial success even when each transaction individually has ACID properties.
Related: Transaction, Isolation Level, Database Management Systems, Engineering Meaning of Indexes in Relational Databases.