Idempotency in System Design: Handle Duplicate Requests | Idempotency Interview Question

Duplicate POST requests causing double payments, duplicate cart entries, or retry bugs? This is the idempotency problem — and it's one of the most common REST API interview questions at top companies. In this video, I break down idempotency from first principles and walk you through building a complete idempotency handler — the way interviewers actually expect it. What you'll learn: ✅ What idempotency is and why it matters for REST APIs ✅ Which HTTP methods are idempotent by default (GET, DELETE, PUT) — and which aren't (POST) ✅ Real-world examples: duplicate payments, duplicate cart entries ✅ How to design an Idempotency Key system step by step ✅ Handling both sequential and parallel duplicate requests with locking ✅ Scaling across multiple servers using a shared cache (Redis) ⏱ Timestamps: 00:00 Introduction 00:48 What is idempotency? 01:49 HTTP methods: GET, PUT, DELETE, POST 02:30 PUT vs POST — the key difference 03:37 The duplicate payment problem 04:43 How duplicates occur (sequential + parallel) 06:54 Solution: the idempotency key 09:00 Request flow with idempotency key 11:11 Idempotency key lifecycle 12:17 Handling retries and duplicate requests 15:20 Parallel requests and critical sections 17:05 Scaling across multiple servers 🔔 Subscribe for system design breakdowns and career roadmaps.