# ACID Transactions
Confidence: high
Last verified: 2026-05-22
Generation: human_only


## TL;DR

ACID (Atomicity, Consistency, Isolation, Durability) are the four properties guaranteeing reliable database transactions. Atomicity: all-or-nothing — either all operations complete or none. Consistency: transaction brings database from one valid state to another. Isolation: concurrent transactions appear sequential. Durability: committed data survives crashes.

## Core Explanation

Isolation levels (weaker to stronger): Read Uncommitted, Read Committed, Repeatable Read, Serializable. Phenomena prevented: dirty reads, non-repeatable reads, phantom reads. PostgreSQL default: Read Committed. Serializable via: 2PL (Two-Phase Locking) or SSI (Serializable Snapshot Isolation, PostgreSQL). CAP theorem: in a distributed system, you can only have two of: Consistency, Availability, Partition tolerance.

## Further Reading

- [Principles of Transaction Processing (Bernstein & Newcomer)](undefined)