ACID

ACID is the four properties of relational transactions: atomicity (all or nothing), consistency (constraints preserved), isolation (concurrency without interference), and durability (committed data is never lost).

Purpose

To guarantee transactions remain correct and reliable under failures and concurrency; the core promise of relational databases.

Mechanism

Atomicity: a transaction fully commits or fully rolls back (undo log). Consistency: constraints hold before and after the transaction. Isolation: concurrent transactions are equivalent to a serial order (locks/MVCC). Durability: commits are written to persistent storage (redo log/WAL). Isolation levels from weak to strong: read uncommitted → read committed → repeatable read → serializable.

Use cases

Financial transfer transactions
Orders and inventory deduction
Multi-table relational updates
Businesses requiring strong consistency

Examples

Input

BEGIN; UPDATE...; INSERT...; COMMIT

Output

Atomic commit or rollback

All or nothing

Common errors

Long transactions holding locks cause blocking: Shorten transactions; use optimistic locking

Related terms

BASE is the weak-consistency NoSQL model: basically available, soft state, eventually consistent — trading strong consistency for availability and scalability.

The CAP theorem states that under network partition a distributed system can satisfy only one of consistency (C) or availability (A); all three cannot hold at once.

Isolation Level

Isolation levels define visibility between concurrent transactions, from read uncommitted to serializable; higher levels give stronger consistency but lower concurrency.

A transaction is a group of operations executed as a single logical unit satisfying ACID properties — all or nothing.

References