Idempotency Keys | Double Spending Problem | System Design Interview Question

Ever wondered how payment systems ensure you are not charged twice, even if your network fails or you accidentally tap the Pay button multiple times?

In this video, we dive into one of the most important backend system design concepts - Idempotency. You will learn how modern payment platforms prevent duplicate transactions caused by retries, network failures, and distributed system delays.

Topics covered:

00:00 What is the Double Spending Problem?
00:35 Why duplicate payments happen
04:00 Idempotency
12:38 Safety
13:00 Safety vs Idempotency 
13:24 PUT vs PATCH

You get the opportunity to pause the video, think through the solution yourself, and then compare your approach with the discussion.

If you are preparing for backend or system design interviews at companies like Stripe, PayPal, Uber, Airbnb, DoorDash, Amazon, Microsoft, Google, LinkedIn, Razorpay, PhonePe, and Cashfree, this is an essential concept to master. Idempotency and duplicate payment prevention are common discussion points in payment system and distributed systems interviews.

If you enjoy practical interview preparation, consider subscribing for more mock interviews and videos covering Low Level Design, High Level Design, Distributed Systems, Java, Machine Coding, DSA, and Backend Engineering.

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