Transactions and Isolation Levels

The concurrency problem

Transactions can interleave while each statement remains individually correct. Isolation defines which interleavings the database exposes.

• Non-repeatable reads
• Write skew
• Serialization failure

PostgreSQL choices

Read committed is a statement-level snapshot. Repeatable read holds one snapshot, while serializable detects unsafe dependency cycles.

• Read committed
• Repeatable read
• Serializable

Choose from invariants

Write the business invariant first, then test whether the chosen isolation and locking strategy preserves it under concurrency.