Idempotency -- design operations that are safe to retry

Network failures and retries can cause operations to run multiple times. Without idempotency: duplicate charges, double-sends, corrupted data.

Techniques: (1) Idempotency keys (client generates unique key, server deduplicates). (2) PUT over POST. (3) UPSERT over INSERT. (4) Track processed message IDs. Stripe uses Idempotency-Key header.

Exactly-once delivery is impossible in distributed systems. At-least-once with idempotent handlers achieves effectively-once.