Idempotency

Idempotency is a fundamental concept in Distributed Systems, APIs, and fault-tolerant computing. Here’s what it means and why it matters:

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🔁 What is Idempotency?

An operation is idempotent if performing it multiple times has the same effect as performing it once.

Example:

POST /charge-customer

Without idempotency: charging twice = double charge 💸 With idempotency: charging twice = charged once ✅

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🔧 Why Idempotency is Important in Distributed Systems

In distributed systems (like Temporal, REST APIs, payment systems, etc.), network failures, retries, and crashes can cause the same message or request to be processed more than once. Without idempotency, this can result in:

• Duplicate transactions
• Corrupt data
• Unexpected side effects

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🧠 Real-World Examples

Operation	Idempotent?	Notes
GET /users/123	✅	Reading doesn’t change state
DELETE /users/123	✅	Deleting twice has same effect
POST /create-user	❌/✅	Only idempotent if client provides a unique ID
POST /process-payment	❌	Usually not unless designed carefully

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✅ How to Ensure Idempotency

1. Use unique request IDs:

   • Clients send an Idempotency-Key with each request.
   • Server tracks and ignores duplicates.

2. Store operation status in durable storage:

   • Persist result the first time.
   • On retry, return the stored result.

3. Avoid non-deterministic effects:

   • E.g., sending emails, making payments—wrap with idempotent logic.

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