What is the difference between idempotent and non-idempotent operations?

Question

Accepted Answer

**Idempotent** — same result on repeat. **Non-idempotent** — different results.

**Idempotent Operations:**
- Same request, same result.
- Safe to retry.
- PUT, DELETE, GET.

```
Idempotent:
PUT /users/1 {name: "John"}
Request 1: Update → Success
Request 2: Update → Same result
(Safe to retry)
```

**Non-Idempotent Operations:**
- Same request, different results.
- Not safe to retry.
- POST, some PATCH.

```
Non-Idempotent:
POST /orders {item: "book"}
Request 1: Create Order #1
Request 2: Create Order #2 (duplicate!)
(Not safe to retry)
```

**Key differences:**
- **Idempotent** — safe to retry.
- **Non-idempotent** — not safe.
- **Idempotent** — deterministic.
- **Non-idempotent** — side effects.

Systems Analyst: What is the difference between idempotent and non-idempotent operations?