What is idempotency in API design?

Question

Accepted Answer

**Idempotency** — same request produces same result.

**Why important:**
- Safe retries.
- Network reliability.
- Prevent duplicate operations.

**Idempotent Methods:**
```
GET    — always idempotent (read-only)
PUT    — idempotent (replace resource)
DELETE — idempotent (remove resource)
POST   — NOT idempotent (create new)
PATCH  — depends on implementation
```

**Example:**
```
POST /orders (not idempotent):
  Request 1: Create order → Order #1
  Request 2: Create order → Order #2 (duplicate!)

PUT /orders/123 (idempotent):
  Request 1: Update order #123 → Success
  Request 2: Update order #123 → Same result
```

**Implementation:** Use idempotency keys for POST.

Systems Analyst: What is idempotency in API design?