What is idempotency in APIs? | API Interview Questions

What You'll Learn

What idempotency means
Why it matters
Idempotent HTTP methods

What is Idempotency?

An operation is idempotent if making the same request multiple times has the same effect as making it once.

Why It Matters

Safe retries on network failures
Prevents duplicate operations
Critical for payment/transaction APIs

HTTP Methods

Method	Idempotent	Safe
GET	Yes	Yes
HEAD	Yes	Yes
PUT	Yes	No
DELETE	Yes	No
POST	No	No
PATCH	No*	No

*PATCH can be idempotent depending on implementation

Examples

code.jsJavaScript

// Idempotent - same result every time
PUT /users/1 { "name": "John" }
// Always sets name to "John"

DELETE /users/1
// First call deletes, subsequent calls return 404

GET /users/1
// Always returns same user

// NOT Idempotent
POST /orders { "item": "book", "qty": 1 }
// Creates new order each time!

Implementing Idempotency

code.jsJavaScript

// Use idempotency keys for POST
POST /payments
Idempotency-Key: unique-request-id-123
{ "amount": 100 }

// Server checks if key exists
// If yes, return cached response
// If no, process and store response