Idempotency of methods in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Idempotency in HTTP Methods

Which HTTP method is idempotent by definition, meaning that making the same request multiple times will have the same effect as making it once?

A CONNECT

B GET

C POST

D TRACE

💡 Hint

Think about methods that retrieve data without changing server state.

GET is idempotent because it only retrieves data and does not change the server state. Calling GET multiple times returns the same result without side effects.

❓ Predict Output

intermediate

Idempotency of PUT vs POST

Consider a REST API where you update a user profile with PUT and create a new user with POST. What is the output of the following sequence of requests?

1. PUT /users/123 with data {"name": "Alice"}
2. PUT /users/123 with data {"name": "Alice"}
3. POST /users with data {"name": "Alice"}

Which statement is true about the server state after these requests?

A POST is idempotent and does not create new users on repeated calls.

B Each PUT creates a new user; POST updates the user with ID 123.

C The server state changes after each PUT and POST; none are idempotent.

D The user with ID 123 is updated twice; a new user is created once. The state after step 2 and step 1 is the same.

💡 Hint

Remember that PUT replaces or creates a resource at a known URL, while POST creates new resources.

PUT is idempotent: calling it multiple times with the same data results in the same server state. POST is not idempotent: each call creates a new resource.

🔧 Debug

advanced

Identifying Non-Idempotent Behavior

Given this pseudo-code for a REST API endpoint:

def delete_user(user_id):
    if user_exists(user_id):
        remove_user(user_id)
        return "Deleted"
    else:
        return "User not found"

What happens if a client sends the DELETE request twice for the same user_id?

A First call deletes the user; second call returns "User not found". DELETE is idempotent.

B Both calls delete the user again, causing an error.

C Second call deletes the user twice, causing data corruption.

D Both calls return "Deleted" even if the user does not exist.

💡 Hint

Idempotent methods can be called multiple times without changing the result beyond the initial call.

DELETE is idempotent because deleting a resource once or multiple times results in the resource being gone. The second call returns "User not found" but does not change server state.

📝 Syntax

advanced

Correct Idempotent HTTP Method Usage

Which of the following HTTP request examples correctly uses an idempotent method to update a resource?

A PUT /items/42 with body {"id": 42, "price": 10.99}

B POST /items with body {"id": 42, "price": 10.99}

C DELETE /items/42

D PATCH /items/42 with body {"price": 10.99}

💡 Hint

PUT replaces the entire resource and is idempotent; PATCH applies partial updates and is not guaranteed idempotent.

PUT is idempotent because sending the same full resource representation multiple times results in the same state. PATCH is not necessarily idempotent because partial updates may accumulate.

🚀 Application

expert

Designing an Idempotent API Endpoint

You are designing a REST API endpoint to process payment transactions. Which approach ensures the endpoint is idempotent?

A Use GET /payments to create payments to ensure idempotency.

B Use POST /payments without any transaction ID and process every request as new.

C Use POST /payments with a unique client-generated transaction ID in the request body and reject duplicates.

D Use DELETE /payments/{payment_id} to create payments and ensure idempotency.

💡 Hint

Idempotency in payment processing often requires client-generated unique IDs to avoid duplicate charges.

Using POST with a unique transaction ID allows the server to recognize repeated requests and avoid processing duplicates, making the endpoint idempotent.