16. Transactions and Concurrency

PostgreSQL uses MVCC (multi-version concurrency control). Understanding transactions and locks is essential for correctness under concurrent load.

Decision Guide

Goal	Prefer	Notes
Default OLTP safety	READ COMMITTED	PostgreSQL default
Consistent snapshot reads	REPEATABLE READ	Same snapshot within transaction
Strict anomaly prevention	SERIALIZABLE	May require retries
Work queues	FOR UPDATE SKIP LOCKED	Avoids workers blocking each other

Lessons

• ACID and Transaction Control
• Isolation Levels and Locking
• Deadlocks and Retry Patterns

What's Next

• Continue to 17. User Management and Security