Idempotency of methods in Rest API - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does idempotency mean in the context of REST API methods?

Idempotency means that making the same API request multiple times results in the same effect as making it once. The server state does not change after the first request.

beginner

Which HTTP methods are idempotent by definition?

GET, PUT, DELETE, HEAD, OPTIONS, and TRACE are idempotent methods. POST is not idempotent by default.

beginner

Why is the POST method generally not idempotent?

POST usually creates new resources or triggers operations that change server state each time it is called, so repeating it can cause multiple changes.

intermediate

How can you make a POST request behave in an idempotent way?

By using unique client-generated IDs or tokens with the POST request, the server can recognize repeated requests and avoid creating duplicates.

beginner

What is a real-life example of idempotency in REST APIs?

If you send a DELETE request to remove a user account, sending it multiple times will have the same effect as sending it once: the account is deleted and further requests do nothing.

Which HTTP method is NOT idempotent by default?

A POST

B GET

C PUT

D DELETE

POST is not idempotent because it usually creates new resources or triggers changes each time it is called.

What does it mean if an API method is idempotent?

A Multiple identical requests have the same effect as one

B It returns different data on each call

C It changes server state every time it is called

D It only works once and then fails

Idempotent methods produce the same result no matter how many times the same request is made.

Which HTTP method is typically used to update a resource and is idempotent?

A CONNECT

B POST

C PATCH

D PUT

PUT replaces or updates a resource and is idempotent because repeating it does not change the result.

How can you ensure a POST request is idempotent?

A By sending it multiple times quickly

B By including a unique client-generated ID

C By using GET instead

D By ignoring server responses

Using a unique ID lets the server detect repeated requests and avoid duplicate actions.

Which of these HTTP methods is idempotent?

A POST

B PATCH

C DELETE

D CONNECT

DELETE is idempotent because deleting the same resource multiple times has the same effect as deleting it once.

Explain idempotency in REST API methods and why it matters.

Think about what happens if you send the same request many times.

Describe how you can design a POST method to behave idempotently.

Consider how the server can recognize repeated requests.

Practice

1. Which HTTP method is idempotent, meaning it can be called multiple times without changing the result beyond the initial application?

easy

A. POST

B. CONNECT

C. PUT

D. PATCH

Solution

1. Step 1: Understand idempotency in HTTP methods

   Idempotent methods produce the same result no matter how many times they are called with the same input.

2. Step 2: Identify which methods are idempotent

   GET, PUT, and DELETE are idempotent. POST and PATCH are not because they can create or modify resources differently each time.

3. Final Answer:

   PUT -> Option C

4. Quick Check:

   PUT is idempotent = B [OK]

Hint: Remember: PUT replaces, POST creates new [OK]

Common Mistakes:

• Confusing POST as idempotent
• Thinking PATCH is idempotent
• Assuming CONNECT is idempotent

2. Which of the following is the correct syntax to perform an idempotent DELETE request using curl?

easy

A. curl -X DELETE https://api.example.com/resource/123

B. curl -X POST https://api.example.com/resource/123

C. curl -X PUT https://api.example.com/resource/123

D. curl -X GET https://api.example.com/resource/123

Solution

1. Step 1: Identify the HTTP method for deletion

   DELETE is the HTTP method used to remove a resource and is idempotent.

2. Step 2: Check curl syntax for DELETE

   The correct curl command uses -X DELETE followed by the resource URL.

3. Final Answer:

   curl -X DELETE https://api.example.com/resource/123 -> Option A

4. Quick Check:

   DELETE method syntax = C [OK]

Hint: DELETE uses -X DELETE in curl command [OK]

Common Mistakes:

• Using POST instead of DELETE
• Omitting -X flag for method
• Confusing GET with DELETE

3. Consider this sequence of HTTP requests to update a user's email:

PUT /users/42 {"email": "[e-mail]"}
PUT /users/42 {"email": "[e-mail]"}

What is the expected result after these two requests?

medium

A. User's email remains unchanged after the first update

B. User's email is updated once and remains the same after second request

C. User's email is updated twice, causing duplicate entries

D. An error occurs on the second request

Solution

1. Step 1: Understand PUT idempotency

   PUT replaces the resource with the given data. Multiple identical PUT requests do not change the resource after the first update.

2. Step 2: Analyze the two identical PUT requests

   The first request updates the email. The second request sends the same data, so no further change occurs.

3. Final Answer:

   User's email is updated once and remains the same after second request -> Option B

4. Quick Check:

   PUT idempotent means same result = A [OK]

Hint: PUT with same data does not change resource again [OK]

Common Mistakes:

• Assuming PUT creates duplicates
• Thinking second PUT causes error
• Confusing PUT with POST behavior

4. A developer wrote this code snippet to delete a resource:

fetch('https://api.example.com/items/10', { method: 'POST' })
  .then(response => console.log('Deleted'));

Why is this code incorrect for an idempotent delete operation?

medium

A. The response is not handled properly

B. The URL is missing a query parameter

C. The fetch method is missing headers

D. POST is not idempotent and should not be used for deletion

Solution

1. Step 1: Identify the HTTP method used

   The code uses POST to delete a resource, which is incorrect because POST is not idempotent.

2. Step 2: Understand correct method for deletion

   DELETE method should be used for idempotent deletion, ensuring repeated calls do not cause unintended effects.

3. Final Answer:

   POST is not idempotent and should not be used for deletion -> Option D

4. Quick Check:

   DELETE is idempotent, POST is not = A [OK]

Hint: Use DELETE, not POST, for idempotent deletes [OK]

Common Mistakes:

• Using POST for delete operations
• Ignoring HTTP method semantics
• Assuming POST is idempotent

5. You want to design an API endpoint to update user profile data safely. Which approach ensures idempotency and avoids duplicate updates if the client retries the request multiple times?

hard

A. Use PUT method to update the entire user profile with the same data each time

B. Use GET method with query parameters to update user data

C. Use PATCH method to partially update user data without tracking changes

D. Use POST method to update user data with a unique request ID

Solution

1. Step 1: Understand idempotent update methods

   PUT replaces the entire resource and is idempotent, so repeated calls with the same data do not cause duplicate changes.

2. Step 2: Evaluate options for safe updates

   POST is not idempotent, PATCH is not guaranteed idempotent, GET should not modify data. PUT is the best choice for safe, repeatable updates.

3. Final Answer:

   Use PUT method to update the entire user profile with the same data each time -> Option A

4. Quick Check:

   PUT ensures idempotent updates = D [OK]

Hint: PUT replaces resource fully, safe for retries [OK]

Common Mistakes:

• Using POST for updates expecting idempotency
• Assuming PATCH is always idempotent
• Trying to update data with GET