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mediumSystem Design

What is idempotency and why does it matter?

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01

Understand the problem

Explain idempotency in APIs.

idempotency
02

Attempt it yourself

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03

Study the solution

What Is Idempotency, and Why Does It Matter?

Target Audience: Junior & Senior Software Engineers preparing for System Design Interviews — no prior system design knowledge assumed. Difficulty: Easy to Medium

How to read this doc: Every concept is explained in plain language first. Right after, you'll s

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04

Read the code

Idempotency key: safe payment retry
Run Playground
class PaymentProcessor {
  constructor() {
    this.seenKeys = new Map();
    this.nextId = 1;
    this.chargesProcessed = 0;
  }

  charge(idempotencyKey, amount) {
    if (this.seenKeys.has(idempotencyKey)) {
      return this.seenKeys.get(idempotencyKey);
    }
    const txnId = `TXN${this.nextId}`;
    this.nextId += 1;
    this.chargesProcessed += 1;
    this.seenKeys.set(idempotencyKey, txnId);
    return txnId;
  }
}

const p = new PaymentProcessor();
console.log('key-1 first call:', p.charge('key-1', 100));
console.log('key-1 retry:', p.charge('key-1', 100));
console.log('key-2 first call:', p.charge('key-2', 50));
console.log('total charges actually processed:', p.chargesProcessed);
05

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