What is idempotent API?

An Idempotent API is an API where making the same request multiple times produces the same result as making it once.

👉 In simple terms:
Repeat the request any number of times — the outcome won’t change.

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Simple Definition

Idempotency = Same request → Same result (no side effects after first call)

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Example

Idempotent Operation

PUT /users/101
{
  "name": "Harish"
}

• First request → updates user name
• Second request → still "Harish" (no extra change)

✔ Result remains the same

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Non-Idempotent Operation

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

• First request → creates user (ID: 101)
• Second request → creates another user (ID: 102)

Different result each time

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Idempotent vs Non-Idempotent

Feature	Idempotent API ✅	Non-Idempotent API ❌
Multiple Requests	Same result	Different results
Side Effects	No additional effect	Creates/changes data
Example Methods	GET, PUT, DELETE	POST

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HTTP Methods and Idempotency

✔ Idempotent Methods:

• GET → Fetch data (no change)
• PUT → Update/replace resource
• DELETE → Remove resource (once removed, same result)

Non-Idempotent:

• POST → Creates new resource each time

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Real-Time Example

Payment API 💳

Imagine a payment request:

POST /pay

If not idempotent:

• Multiple clicks → Multiple charges 😱

If idempotent:

• Only one payment processed ✅

👉 That’s why idempotency is critical in real-world systems

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Idempotency Key

To make APIs idempotent (especially POST), systems use an Idempotency Key.

POST /pay
Idempotency-Key: 12345

• Same key → Same response
• Prevents duplicate processing

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Benefits of Idempotent APIs

• Prevents duplicate operations
• Safe retries (network failures)
• Improves system reliability
• Essential for distributed systems

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Important Note

Even if response status differs (like 200 vs 404),
👉 The system state must remain the same

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Conclusion

An Idempotent API ensures that repeating the same request does not cause unintended side effects. It’s a key concept in building robust, fault-tolerant systems, especially in microservices and payment systems.

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