Idempotency keys for safe retries in Rest API - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an idempotency key header to the API request.

Rest API

headers = {"Content-Type": "application/json", "[1]": "12345-abcde"}

Drag options to blanks, or click blank then click option'

AAccept

BAuthorization

CIdempotency-Key

DUser-Agent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Authorization' instead of 'Idempotency-Key'.

Forgetting to add the idempotency key header.

The Idempotency-Key header is used to safely retry requests without creating duplicates.

2fill in blank

medium

Complete the code to check if the idempotency key already exists in the server's storage.

Rest API

if [1] in stored_keys:
    return stored_keys[[1]]

Drag options to blanks, or click blank then click option'

Arequest.body

Brequest.headers.get('Idempotency-Key')

Crequest.method

Drequest.url

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the request body instead of headers.

Using the URL or method instead of the idempotency key.

We check if the Idempotency-Key from the request headers is already stored to avoid duplicate processing.

3fill in blank

hard

Fix the error in the code that stores the response using the idempotency key.

Rest API

stored_keys[[1]] = response

Drag options to blanks, or click blank then click option'

Arequest.headers.get('Idempotency-Key')

Brequest.body

Cresponse.status_code

Drequest.url

Attempts:

3 left

💡 Hint

Common Mistakes

Using request body or URL as the key instead of the idempotency key.

Storing the status code instead of the full response.

The response must be stored with the Idempotency-Key from the request headers as the key.

4fill in blank

hard

Fill both blanks to safely retry a POST request using idempotency key logic.

Rest API

key = request.headers.get('[1]')
if key and key [2] stored_keys:
    return stored_keys[key]

Drag options to blanks, or click blank then click option'

AIdempotency-Key

Bnot in

Cin

DAuthorization

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'not in' instead of 'in' for the check.

Using wrong header names like 'Authorization'.

The code gets the Idempotency-Key from headers and checks if it is in stored keys to return the saved response.

5fill in blank

hard

Fill all three blanks to implement idempotency key storage and response retrieval.

Rest API

key = request.headers.get('[1]')
if key [2] stored_keys:
    return stored_keys[key]
response = process_request(request)
stored_keys[[3]] = response

Drag options to blanks, or click blank then click option'

AIdempotency-Key

Bin

Ckey

DAuthorization

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Authorization' instead of 'Idempotency-Key'.

Using 'not in' instead of 'in' for the check.

Storing response with wrong dictionary key.

The code gets the Idempotency-Key, checks if it is in stored keys, returns the stored response if found, otherwise processes the request and stores the response with the key.

Practice

1. What is the main purpose of using an idempotency key in REST APIs?

easy

A. To speed up the server response time

B. To prevent duplicate processing of the same request

C. To encrypt the request data

D. To authenticate the user

Solution

1. Step 1: Understand idempotency key concept

   An idempotency key is a unique identifier sent with a request to ensure the server processes it only once.

2. Step 2: Identify the main purpose

   This prevents duplicate actions like double payments or repeated orders if the client retries the request.

3. Final Answer:

   To prevent duplicate processing of the same request -> Option B

4. Quick Check:

   Idempotency key = prevent duplicates [OK]

Hint: Idempotency keys stop repeated actions safely [OK]

Common Mistakes:

• Confusing idempotency key with authentication token
• Thinking it speeds up response time
• Assuming it encrypts data

2. Which HTTP header is commonly used to send an idempotency key in a REST API request?

easy

A. Idempotency-Key

B. Cache-Control

C. Content-Type

D. Authorization

Solution

1. Step 1: Recall standard header for idempotency

   The standard practice is to send the unique idempotency key in the Idempotency-Key HTTP header.

2. Step 2: Differentiate from other headers

   Authorization is for credentials, Content-Type defines data format, and Cache-Control manages caching.

3. Final Answer:

   Idempotency-Key -> Option A

4. Quick Check:

   Idempotency key header = Idempotency-Key [OK]

Hint: Idempotency keys go in Idempotency-Key header [OK]

Common Mistakes:

• Using Authorization header for idempotency key
• Confusing Content-Type with idempotency key
• Sending key in request body instead of header

3. Consider this simplified server logic for handling POST requests with idempotency keys:

stored_keys = {}

def handle_request(idempotency_key, data):
    if idempotency_key in stored_keys:
        return stored_keys[idempotency_key]
    response = process_data(data)
    stored_keys[idempotency_key] = response
    return response

# Client sends:
print(handle_request('abc123', 'order1'))
print(handle_request('abc123', 'order1'))
print(handle_request('xyz789', 'order2'))

What will be the output?

medium

A. None, None, None

B. process_data output for order1, None, process_data output for order2

C. process_data output for order1, process_data output for order2, process_data output for order2

D. process_data output for order1, process_data output for order1, process_data output for order2

Solution

1. Step 1: Analyze first call with key 'abc123'

   Key not in stored_keys, so process_data('order1') runs, result saved and returned.

2. Step 2: Analyze second call with same key 'abc123'

   Key found, stored response returned without reprocessing.

3. Step 3: Analyze third call with new key 'xyz789'

   Key not found, process_data('order2') runs, result saved and returned.

4. Final Answer:

   process_data output for order1, process_data output for order1, process_data output for order2 -> Option D

5. Quick Check:

   Repeated key returns cached response [OK]

Hint: Repeated keys return cached response, new keys process data [OK]

Common Mistakes:

• Assuming second call reprocesses data
• Confusing keys and responses
• Thinking None is returned on repeats

4. A developer wrote this code snippet to handle idempotent POST requests:

cache = {}
def post_request(key, data):
    if key not in cache:
        process(data)
    return cache[key]

What is the bug in this code?

medium

A. It never stores the processed result in cache

B. It processes data twice for the same key

C. It raises an error if key is missing

D. It uses the wrong HTTP method

Solution

1. Step 1: Check caching logic

   The code checks if key not in cache, then calls process(data), but never saves the result in cache.

2. Step 2: Identify consequence

   Since cache[key] is never set, returning cache[key] will cause a KeyError or return None incorrectly.

3. Final Answer:

   It never stores the processed result in cache -> Option A

4. Quick Check:

   Missing cache storage causes errors [OK]

Hint: Always save processed result in cache before returning [OK]

Common Mistakes:

• Assuming process(data) stores result automatically
• Ignoring missing cache assignment
• Thinking HTTP method affects caching here

5. You are designing a payment API that uses idempotency keys to avoid duplicate charges. Which approach best ensures safe retries without charging twice if the client resends the same request due to a timeout?

hard

A. Ignore the idempotency key and process every request independently

B. Reject any request with an idempotency key that was seen before

C. Store the payment result linked to the idempotency key and return it on repeated requests

D. Process the payment again but log a warning for duplicate keys

Solution

1. Step 1: Understand safe retry requirements

   Safe retries mean the client can resend the same request without causing duplicate charges.

2. Step 2: Evaluate options

   Store the payment result linked to the idempotency key and return it on repeated requests stores the payment result linked to the key and returns it on repeats, preventing double charges. Ignore the idempotency key and process every request independently processes every request, risking duplicates. Reject any request with an idempotency key that was seen before rejects repeats, which may cause client errors. Process the payment again but log a warning for duplicate keys processes again, causing duplicates.

3. Final Answer:

   Store the payment result linked to the idempotency key and return it on repeated requests -> Option C

4. Quick Check:

   Store and reuse result for idempotency [OK]

Hint: Save and reuse payment result by key to avoid duplicates [OK]

Common Mistakes:

• Rejecting repeated keys causing client errors
• Processing duplicates causing double charges
• Logging warnings but still charging twice