System Design on Vubon Notes

System Design Interview: Idempotency

Mon, 04 May 2026 11:00:00 +0000

Almost every candidate faces this question during a system design interview:

How do you handle duplicate transactions?

Most people answer: Idempotency key + Cache.

To be honest — that’s a good start.

But the real lesson comes when you build an actual payment system.

Duplicates are normal behavior #

In distributed systems, duplicates are not edge cases. They are normal behavior.

Duplicates can originate from anywhere:

client double-clicks
mobile retry on timeout
API gateway retries
reverse proxies
silent network failures
third-party timeouts

So the problem is not if duplicates happen.

System Design Interview Prep — Series Introduction

Mon, 04 May 2026 10:00:00 +0000

System design interviews are tough. Not because the questions are impossible - but because they’re open-ended, and most people don’t know where to start.

After years of building distributed systems in FinTech, I’ve been on both sides of the table. I know what interviewers are actually looking for, and it’s not buzzwords. It’s trade-offs, real-world thinking, and knowing why something works, not just that it works.

So I’m putting together this series — practical write-ups on the topics that come up again and again. No fluff, just the stuff that matters.