ACID Properties — Atomicity, Consistency, Isolation, Durability

Q: ACID Properties — Atomicity, Consistency, Isolation, Durability

**ACID** is a set of four guarantees that database transactions must satisfy for reliable processing. Atomicity means a **transaction** fully completes or fully rolls back. Consistency ensures the database moves from one valid state to another, honoring all constraints. Isolation controls how concurrent transactions see each other's changes. Durability guarantees that committed data survives crashes.

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ACID is a set of four guarantees that database transactions must satisfy for reliable processing. Atomicity means a transaction fully completes or fully rolls back. Consistency ensures the database moves from one valid state to another, honoring all constraints. Isolation controls how concurrent transactions see each other's changes. Durability guarantees that committed data survives crashes.

Interview Answer3 min

ACID is the set of four guarantees that make database transactions reliable. The bank transfer explains why all four matter — debit account A, credit account B — what happens if the system crashes between those two operations?

Atomicity — all or nothing

Atomicity means the entire transaction commits or the entire transaction rolls back — so if the system crashes after debiting A but before crediting B, the debit is undone on recovery, which means no money disappears even during a crash
The database achieves this through a write-ahead log: changes are recorded in the WAL before being applied; crash recovery replays committed transactions and rolls back incomplete ones

Consistency — constraints are always satisfied

Consistency means a transaction can only bring the database from one valid state to another — schema constraints, foreign keys, and CHECK constraints must all hold after every commit
This is not the same as the C in CAP theorem — CAP consistency means every read returns the latest write across distributed nodes; ACID consistency means schema rules are not violated within one database

Isolation — concurrent transactions don't interfere

Isolation means transactions behave as if they ran sequentially, even when running concurrently — which makes it the most expensive property to enforce
Full isolation (Serializable) prevents all anomalies but adds significant overhead; most databases default to Read Committed, which allows non-repeatable reads; isolation is a dial controlled by the isolation level setting, not a binary switch

Durability — committed data survives crashes

Once a transaction commits, the data must survive even a power failure — the database flushes the WAL to disk before acknowledging the commit
fsync = off in PostgreSQL disables this guarantee for speed — a commit succeeds in memory but a crash loses it; never do this in production on data you cannot reload

If an interviewer asks which property is most expensive: it's Isolation. The others are mostly about logging; Isolation requires lock management or MVCC overhead that directly impacts throughput.

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Interview answer

Atomicity — all or nothing

Atomicity means the entire transaction commits or the entire transaction rolls back — so if the system crashes after debiting A but before crediting B, the debit is undone on recovery, which means no money disappears even during a crash
The database achieves this through a write-ahead log: changes are recorded in the WAL before being applied; crash recovery replays committed transactions and rolls back incomplete ones

Consistency — constraints are always satisfied

Consistency means a transaction can only bring the database from one valid state to another — schema constraints, foreign keys, and CHECK constraints must all hold after every commit
This is not the same as the C in CAP theorem — CAP consistency means every read returns the latest write across distributed nodes; ACID consistency means schema rules are not violated within one database

Isolation — concurrent transactions don't interfere

Isolation means transactions behave as if they ran sequentially, even when running concurrently — which makes it the most expensive property to enforce
Full isolation (Serializable) prevents all anomalies but adds significant overhead; most databases default to Read Committed, which allows non-repeatable reads; isolation is a dial controlled by the isolation level setting, not a binary switch

Durability — committed data survives crashes

Once a transaction commits, the data must survive even a power failure — the database flushes the WAL to disk before acknowledging the commit
fsync = off in PostgreSQL disables this guarantee for speed — a commit succeeds in memory but a crash loses it; never do this in production on data you cannot reload

If an interviewer asks which property is most expensive: it's Isolation. The others are mostly about logging; Isolation requires lock management or MVCC overhead that directly impacts throughput.

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