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#system-design-interview

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How to notify 50 million users about a flash sale: pull, push and FCM topics

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 readsystemdesignwithparam.hashnode.dev

Parminder singh5d ago

Payment failed but money deducted: how idempotency keys stop double charges

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 readsystemdesignwithparam.hashnode.dev

2

Parminder singhNov 4

How Instagram checks 1 billion usernames and tells you in milliseconds if yours is free

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 readsystemdesignwithparam.hashnode.dev

Parminder singhNov 1

Flash sale system design: why the atomic update dies at 10 lakh buyers

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 readsystemdesignwithparam.hashnode.dev

Parminder singhOct 31

2 GB File Upload System Design: Why Streaming Isn't Enough

I crashed a server with one upload, then measured what actually fixes it

9 min readsystemdesignwithparam.hashnode.dev

Parminder singhOct 31

1 Item Left, 1000 Buyers, Sold 3 Times: How to Fix Overselling?

From the race condition to the atomic update

7 min readsystemdesignwithparam.hashnode.dev

Devang PandyaOct 30

Sticky Sessions Explained: The Story of Two Waiters (Load Balancer Session Affinity)

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 readcloudtechbydevang.hashnode.dev

AnonymousOct 23

Your System Design Looks Fine on a Whiteboard. Until Traffic Hits.

Explore how traffic spikes, cache failures and database bottlenecks affect system design, with practical examples and hands-on experiments.

4 min readstackmonkey.hashnode.dev

Parminder singhOct 20

Rate Limiter System Design: 5 Algorithms Explained Simply

Fixed window, sliding log, sliding counter, token bucket and leaky bucket, with live simulators

15 min readsystemdesignwithparam.hashnode.dev

skillcopilotaiOct 9

How to Explain Rate Limiter Tradeoffs in a System Design Interview

Practice explaining token buckets, atomic counters, failure policies, and multi-region rate limiting with a concrete system design interview exercise.

5 min readskillcopilotai-guides.hashnode.dev

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Trending tags this week

How to notify 50 million users about a flash sale: pull, push and FCM topics

Payment failed but money deducted: how idempotency keys stop double charges

How Instagram checks 1 billion usernames and tells you in milliseconds if yours is free

Flash sale system design: why the atomic update dies at 10 lakh buyers

2 GB File Upload System Design: Why Streaming Isn't Enough

1 Item Left, 1000 Buyers, Sold 3 Times: How to Fix Overselling?

Sticky Sessions Explained: The Story of Two Waiters (Load Balancer Session Affinity)

Your System Design Looks Fine on a Whiteboard. Until Traffic Hits.

Rate Limiter System Design: 5 Algorithms Explained Simply

How to Explain Rate Limiter Tradeoffs in a System Design Interview