--- title: "#system-design-interview articles | Hashnode" description: "Explore 44 articles tagged #system-design-interview on Hashnode. Tutorials, guides, and how-tos from the community." url: "https://hashnode.com/tag/system-design-interview" --- Tag # #system-design-interview 44 posts · 7 followers [Parminder singh](https://hashnode.com/@param007) · [2d ago](https://hashnode.com/posts/flash-sale-push-notifications-fcm-topics/6af6762a23037bb26d250a7e) ### [How to notify 50 million users about a flash sale: pull, push and FCM topics](https://systemdesignwithparam.hashnode.dev/flash-sale-push-notifications-fcm-topics?utm_source=hashnode&utm_medium=feed) The sale starts at 12:00. Marketing wants all 50 million users to know, and most of them have the app closed. Your server can't call a phone, and 50 million is a lot of phones to reach in one minute. 7 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/flash-sale-push-notifications-fcm-topics?utm_source=hashnode&utm_medium=feed) [Parminder singh](https://hashnode.com/@param007) · [5d ago](https://hashnode.com/posts/payment-failed-money-deducted-idempotency/6af2d4383c36fe3908d4fd5a) ### [Payment failed but money deducted: how idempotency keys stop double charges](https://systemdesignwithparam.hashnode.dev/payment-failed-money-deducted-idempotency?utm_source=hashnode&utm_medium=feed) The app says "Payment failed". A second later your phone gets an SMS: ₹2,000 debited. You tap Pay again. Now ₹4,000 is gone, and you still have no order. https://www.youtube.com/watch?v=A6hrGCQidgM 7 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/payment-failed-money-deducted-idempotency?utm_source=hashnode&utm_medium=feed) [2](https://hashnode.com/posts/payment-failed-money-deducted-idempotency/6af2d4383c36fe3908d4fd5a) [Parminder singh](https://hashnode.com/@param007) · [Nov 4](https://hashnode.com/posts/bloom-filter-username-check/6aeaf0ffd499967824bc7def) ### [How Instagram checks 1 billion usernames and tells you in milliseconds if yours is free](https://systemdesignwithparam.hashnode.dev/bloom-filter-username-check?utm_source=hashnode&utm_medium=feed) You type rahul\_07 on a signup page. Before you finish, it says "This username isn't available." Now imagine 1 billion users and a check on every keystroke. Sending every check to the database creates 9 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/bloom-filter-username-check?utm_source=hashnode&utm_medium=feed) [Parminder singh](https://hashnode.com/@param007) · [Nov 1](https://hashnode.com/posts/flash-sale-system-design-redis-queue/6ae797891a33172e761e951d) ### [Flash sale system design: why the atomic update dies at 10 lakh buyers](https://systemdesignwithparam.hashnode.dev/flash-sale-system-design-redis-queue?utm_source=hashnode&utm_medium=feed) The atomic update never oversells, but 10 lakh buyers queue on one row. Why it breaks, and the Redis gate and queue that fix it. 9 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/flash-sale-system-design-redis-queue?utm_source=hashnode&utm_medium=feed) [Parminder singh](https://hashnode.com/@param007) · [Oct 31](https://hashnode.com/posts/2gb-file-upload-system-design/6ae6048dd902ed290445b81b) ### [2 GB File Upload System Design: Why Streaming Isn't Enough](https://systemdesignwithparam.hashnode.dev/2gb-file-upload-system-design?utm_source=hashnode&utm_medium=feed) I crashed a server with one upload, then measured what actually fixes it 9 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/2gb-file-upload-system-design?utm_source=hashnode&utm_medium=feed) [Parminder singh](https://hashnode.com/@param007) · [Oct 31](https://hashnode.com/posts/fix-overselling-race-condition/6ae64112c3aa5bca9b6edefc) ### [1 Item Left, 1000 Buyers, Sold 3 Times: How to Fix Overselling?](https://systemdesignwithparam.hashnode.dev/fix-overselling-race-condition?utm_source=hashnode&utm_medium=feed) From the race condition to the atomic update 7 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/fix-overselling-race-condition?utm_source=hashnode&utm_medium=feed) [Devang Pandya](https://hashnode.com/@cloudtechbydevang) · [Oct 30](https://hashnode.com/posts/sticky-sessions-explained-the-story-of-two-waiters-load-balancer-session-affinity/6ae4a07a701013feab652e93) ### [Sticky Sessions Explained: The Story of Two Waiters (Load Balancer Session Affinity)](https://cloudtechbydevang.hashnode.dev/sticky-sessions-explained-the-story-of-two-waiters-load-balancer-session-affinity?utm_source=hashnode&utm_medium=feed) By Devang Pandya · Cloud Tech by Devang · Cloud • DevOps • AI — Made Simple This post is the full breakdown for the Two Waiters Reel. If you came here after commenting STICKY, welcome! The story in 8 min read · [cloudtechbydevang.hashnode.dev](https://cloudtechbydevang.hashnode.dev/sticky-sessions-explained-the-story-of-two-waiters-load-balancer-session-affinity?utm_source=hashnode&utm_medium=feed) [Anonymous](https://hashnode.com/@imsam) · [Oct 23](https://hashnode.com/posts/your-system-design-looks-fine-on-a-whiteboard-until-traffic-hits/6adb15b63bd2d77f11cadcc2) ### [Your System Design Looks Fine on a Whiteboard. Until Traffic Hits.](https://stackmonkey.hashnode.dev/your-system-design-looks-fine-on-a-whiteboard-until-traffic-hits?utm_source=hashnode&utm_medium=feed) Explore how traffic spikes, cache failures and database bottlenecks affect system design, with practical examples and hands-on experiments. 4 min read · [stackmonkey.hashnode.dev](https://stackmonkey.hashnode.dev/your-system-design-looks-fine-on-a-whiteboard-until-traffic-hits?utm_source=hashnode&utm_medium=feed) [Parminder singh](https://hashnode.com/@param007) · [Oct 20](https://hashnode.com/posts/rate-limiter-system-design/6ad7534de9f3db9e640c5ec2) ### [Rate Limiter System Design: 5 Algorithms Explained Simply](https://systemdesignwithparam.hashnode.dev/rate-limiter-system-design?utm_source=hashnode&utm_medium=feed) Fixed window, sliding log, sliding counter, token bucket and leaky bucket, with live simulators 15 min read · [systemdesignwithparam.hashnode.dev](https://systemdesignwithparam.hashnode.dev/rate-limiter-system-design?utm_source=hashnode&utm_medium=feed) [skillcopilotai](https://hashnode.com/@candyindex999) · [Oct 9](https://hashnode.com/posts/how-to-explain-rate-limiter-tradeoffs-in-a-system-design-interview/6ac91207ad2a8b52d916bb10) ### [How to Explain Rate Limiter Tradeoffs in a System Design Interview](https://skillcopilotai-guides.hashnode.dev/how-to-explain-rate-limiter-tradeoffs-in-a-system-design-interview?utm_source=hashnode&utm_medium=feed) Practice explaining token buckets, atomic counters, failure policies, and multi-region rate limiting with a concrete system design interview exercise. 5 min read · [skillcopilotai-guides.hashnode.dev](https://skillcopilotai-guides.hashnode.dev/how-to-explain-rate-limiter-tradeoffs-in-a-system-design-interview?utm_source=hashnode&utm_medium=feed)