Idempotency of methods in Rest API

Introduction

Idempotency means doing the same action many times has the same effect as doing it once. This helps avoid mistakes when requests are repeated.

When a user clicks a button multiple times but you want only one action to happen.

When a network error causes a request to be sent again and you want to avoid duplicate changes.

When updating or deleting data and you want to make sure the result is stable no matter how many times the request runs.

Syntax

Rest API

HTTP methods like GET, PUT, DELETE are idempotent by design.
POST is usually not idempotent.

Example:
PUT /items/123 {"name": "book"}

This request can be sent many times and the item will be the same after each call.

GET requests only read data and never change it, so they are always idempotent.

PUT replaces or updates a resource fully, so repeating it does not change the result after the first time.

Examples

GET is safe and idempotent because it only reads data.

Rest API

GET /users/1

// This just fetches user data. No matter how many times you do it, the user data stays the same.

PUT is idempotent because the resource ends up the same after each request.

Rest API

PUT /users/1 {"name": "Alice"}

// This updates user 1's name to Alice. Sending this many times keeps the name as Alice.

DELETE is idempotent because deleting something that is already deleted has no extra effect.

Rest API

DELETE /users/1

// This deletes user 1. Doing it once or multiple times results in user 1 being gone.

POST is not idempotent because it creates new resources each time.

Rest API

POST /users {"name": "Bob"}

// This creates a new user. Sending this multiple times creates multiple users, so POST is not idempotent.

Sample Program

This program sends GET and PUT requests twice to the same URL. The GET requests return the same data both times. The PUT requests update the resource with the same data twice, so the result is the same after each call.

Rest API

import requests

url = 'https://jsonplaceholder.typicode.com/posts/1'

# GET request - idempotent
response1 = requests.get(url)
response2 = requests.get(url)

print('GET request repeated:')
print(response1.json())
print(response2.json())

# PUT request - idempotent
put_data = {"title": "foo", "body": "bar", "userId": 1}
response3 = requests.put(url, json=put_data)
response4 = requests.put(url, json=put_data)

print('\nPUT request repeated:')
print(response3.json())
print(response4.json())

Important Notes

Idempotency helps make APIs reliable and safe to retry.

Not all HTTP methods are idempotent; know which ones are to avoid bugs.

Sometimes APIs add special headers or tokens to make POST requests idempotent.

Summary

Idempotent methods produce the same result no matter how many times they run.

GET, PUT, DELETE are idempotent; POST usually is not.

Using idempotent methods helps avoid duplicate changes and errors.

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