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REST Idempotency Interview Questions

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Concept Made Simple

Which HTTP methods are idempotent and why does it matter?

🧠Mind Map Answer

Remember It Faster

Idempotent = calling it once or many times has the same effect on the server. It matters for safe retries on flaky networks.

GET, PUT, DELETEIdempotent ✅
POSTNot idempotent ❌
WhyClients can safely retry idempotent calls

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Learn by Doing

http
PUT /users/101
{ "name": "John" }

# Call it 5 times → user 101 still just has name "John"
# POST /users called 5 times → 5 new users

🔥What If?

Think Beyond the Expected

How do you make POST safe to retry?

Use an idempotency key — the client sends a unique header (e.g. Idempotency-Key) and the server returns the original result for duplicates instead of creating a new resource.

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Idempotency (REST APIs)
Interview question: "Which HTTP methods are idempotent and why does it matter?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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