Idempotency

Idempotency means: doing the same operation multiple times has the same effect as doing it once.

IHTTP / APIs (common interview + system design topic)

• Usually idempotent
  • GET /users/123 Read-only. Repeating doesn’t change state.
  • PUT /users/123 with the full user object, replace the resource with the same content each time → same final state.
  • DELETE /users/123 Deleting again still ends up “deleted” (even if the server responds differently).
• Usually not idempotent
  • POST /orders (create a new order)
    • retrying can create multiple orders unless you protect it.

How to make a “create” operation safe: Idempotency Key

For non-idempotent actions like POST /payments, clients send a unique key:

• Header: Idempotency-Key: 7f2c...
• Server stores: key → result (or key → created resource id)
• If the same key is seen again, server returns the same result instead of creating a new one.

This is how payment APIs (e.g., Stripe) avoid double-charging.