https://vubon.me/tags/idempotency/Recent content in Idempotency on Vubon NotesHugoen-usMon, 04 May 2026 11:00:00 +0000https://vubon.me/posts/system-design-interview-idempotency/Mon, 04 May 2026 11:00:00 +0000https://vubon.me/posts/system-design-interview-idempotency/<p>Almost every candidate faces this question during a system design interview:</p> <p><strong>How do you handle duplicate transactions?</strong></p> <p>Most people answer: <em>Idempotency key + Cache.</em></p> <p>To be honest — that&rsquo;s a good start.</p> <p>But the real lesson comes when you build an actual payment system.</p> <hr> <h2 class="heading" id="duplicates-are-normal-behavior"> Duplicates are normal behavior <a class="anchor" href="#duplicates-are-normal-behavior">#</a> </h2> <p>In distributed systems, duplicates are not edge cases. They are normal behavior.</p> <p>Duplicates can originate from anywhere:</p> <ul> <li>client double-clicks</li> <li>mobile retry on timeout</li> <li>API gateway retries</li> <li>reverse proxies</li> <li>silent network failures</li> <li>third-party timeouts</li> </ul> <p>So the problem is not <em>if</em> duplicates happen.</p>