Idempotency
Operation that produces the same result whether run once or many times. Crucial in distributed systems where retries are common — without idempotency, retries can create duplicates.
HTTP: GET, PUT, DELETE are idempotent; POST and PATCH are not. Stripe + other payment APIs support an Idempotency-Key header: send the same key on retry → API deduplicates. Database upserts (INSERT … ON DUPLICATE KEY UPDATE) are idempotent. Note: idempotency ≠ pure function — it can have side effects, but only once.