Reel Notes
Notes for every @masstdev system design reel — diagrams, interview words and the follow-up questions interviewers ask.
Every reel on @masstdev gets a notes page here: the diagram from the reel, the words to use in an interview, and the follow-up questions you will actually be asked. Each page ends with links into the full system design course on this site, so you can go deeper on the same topic.
Case studies
| Reel | What happened | The lesson |
|---|---|---|
| #39 · AWS DNS outage (Oct 2025) | An empty DNS record took down thousands of apps | Race conditions and version checks |
| #38 · CrowdStrike outage (Jul 2024) | One update file crashed 8.5M Windows PCs | Canary rollouts |
How it works
| Reel | Question | Concept |
|---|---|---|
| #50 · Why YouTube views got stuck at 301 | Why did view counts freeze at 301? | Eventual consistency, sharded counters |
| #49 · How Find My Device finds an offline phone | Who sent the location if the phone was offline? | Crowdsourced network, E2E encryption |
| #48 · Why your neighbour's TV shouts SIX first | Why is the stream 40 s behind? | Latency vs buffering, LL-HLS |
| #47 · Does the lift door-close button work? | How does a lift decide where to go? | SCAN, design an elevator |
| #46 · How does Uber find the nearest car? | Your cab in 2 seconds among thousands | Geospatial index, H3 |
| #45 · How does Google Maps know the road is red? | 99 phones in a cart made an empty street red. How? | Stream aggregation |
| #44 · How does your phone keyboard really work? | How does it know your next word, offline? | Typeahead, trie, federated learning |
| #43 · Let's design ChatGPT | Why does the answer arrive word by word? | Tokens, KV cache, streaming |
| #42 · Let's design UPI | Scan → 2 seconds → paid. What happens in between? | UPI stack, idempotency |
| #41 · Swiggy on IPL final night | 6,200 orders at the peak. Why was "order placed" instant? | Message queues |
| #40 · Virat posts, 25 crore feeds | How does one post reach crores of feeds in seconds? | Fan-out (push vs pull) |
| #37 · WhatsApp ticks | Phone was off all night. Where was the message? | Store and forward |
| #36 · Free CDN with Cloudflare | How do you make a site fast worldwide for free? | CDN and edge caching |
New reels every day at 9:30 PM IST on Instagram: @masstdev.