Idempotent Methods in REST APIs
💡 Concept Name
Idempotent Methods are HTTP methods that, no matter how many times they are executed with the same parameters, produce the same result without causing additional side effects.
📘 Quick Intro
Idempotency is essential in REST APIs to ensure that repeated requests don't cause unintended changes. This reliability is crucial for applications where network retries or duplicate requests might occur.
🧠 Analogy / Short Story
Imagine pressing a doorbell: whether you press it once or ten times, the bell rings only once. Similarly, an idempotent API method guarantees that making the same call multiple times has no further effect after the first.
🔧 Technical Explanation
- 🔁 Definition: An operation is idempotent if repeating it has the same effect as doing it once.
- ✅ Idempotent HTTP Methods: GET, PUT, DELETE, HEAD, OPTIONS are idempotent.
- ⚠️ Non-idempotent Methods: POST is not idempotent because it usually creates new resources each time.
- 🧪 Benefits: Idempotency enables safe retries and consistency in distributed systems and unreliable networks.
🎯 Purpose & Use Case
- ✅ GET: Retrieve data safely without side effects.
- ✅ PUT: Update or create a resource in a way that repeated calls result in the same state.
- ✅ DELETE: Remove a resource; repeated deletes don’t change the outcome after the resource is gone.
💻 Real Code Example
// Idempotent GET request
fetch('/api/users/1');
// Idempotent PUT request - sets user data to the same state
fetch('/api/users/1', {
method: 'PUT',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({ name: 'Alice' })
});
// Idempotent DELETE request - deleting twice is harmless
fetch('/api/users/1', { method: 'DELETE' });
// Non-idempotent POST request - creates new user every time
fetch('/api/users', {
method: 'POST',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({ name: 'Bob' })
});
❓ Interview Q&A
Q: What does it mean for a method to be idempotent?
A: Calling the method once or multiple times results in the same outcome.
Q: Is POST idempotent?
A: No, POST usually creates a new resource every time.
Q: Why is idempotency important in APIs?
A: It allows safe retries without causing unintended side effects.
Q: Are DELETE requests idempotent?
A: Yes, deleting a resource multiple times results in the same state.
Q: Which HTTP methods are idempotent?
A: GET, PUT, DELETE, HEAD, and OPTIONS.
📝 MCQs
Q1. Which HTTP method is not idempotent?
- GET
- PUT
- DELETE
- POST
Q2. What does idempotent mean?
- Creates new state each time
- Always fails safely
- Multiple identical requests yield the same result
- Always returns 404
Q3. Why is GET idempotent?
- It deletes data
- It creates new resources
- It does not change server state
- It retries a request
Q4. Which method may create a new resource every time?
- PUT
- DELETE
- POST
- OPTIONS
Q5. What happens if you DELETE a resource twice?
- It fails
- It creates data
- The result is the same
- It duplicates the delete
Q6. What is the benefit of idempotency?
- Slower APIs
- More authentication
- Safe retries
- More memory usage
Q7. Is PUT idempotent?
- Yes
- No
- Sometimes
- Only if POST is used
Q8. Which HTTP verb is safe and idempotent?
- POST
- PUT
- GET
- PATCH
Q9. Does HEAD method alter server data?
- Yes
- No
- Maybe
- Only with GET
Q10. What is required for idempotent methods to work reliably?
- Use of POST only
- Client authentication
- No retries allowed
- Server must treat repeated requests identically
💡 Bonus Insight
Idempotency is vital in systems like payments, message queues, and retries, where unintended duplicates can cause serious issues. Designing APIs with idempotency in mind improves reliability and safety.
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