How does Postgres implement ACID principles by default?

PostgreSQL implements the ACID principles as follows:

• Atomicity: A transaction is either fully completed or fully rolled back. This is achieved through WAL (Write-Ahead Logging), where all changes are first recorded in a log, allowing recovery in case of failures.

• Consistency: The database transitions from one valid state to another, thanks to integrity constraints, triggers, and checks.

• Isolation: PostgreSQL uses MVCC (Multi-Version Concurrency Control), allowing transactions to work with consistent snapshots of data without locking, providing isolation levels (Read Committed by default, Serializable when needed).

• Durability: After a transaction is committed, data is saved to disk thanks to WAL and fsync mechanisms, ensuring data safety even in power failures.

Thus, PostgreSQL ensures ACID at the core database engine level, using proven logging and multi-versioning mechanisms.