What Are ACID Properties? What Does Each Letter Mean?

ACID Properties

A — Atomicity

All operations in a transaction either all succeed or all roll back

• Example: bank transfer—debit and credit must both succeed or both roll back

C — Consistency

After a transaction, the database must move from one valid state to another

• All constraints (uniqueness, foreign keys, check constraints) must be satisfied

I — Isolation

Concurrent transactions don't interfere with each other; each runs as if alone

• Different isolation levels offer different performance vs consistency trade-offs

D — Durability

Once committed, data is permanently saved (even after a crash)

• Implemented via: WAL (Write-Ahead Logging), periodic checkpoints

ACID vs BASE (NoSQL)

Property	ACID (RDBMS)	BASE (NoSQL)
Consistency	Strong	Eventual
Availability	May be reduced	High
Character	Conservative, precise	Optimistic, flexible