NEW REEL#50 · Why YouTube views got stuck at 301
System design and AI engineering, explained in diagrams.
Free, from the basics to real designs like Netflix, UPI and ChatGPT. A new reel every day at 9:30 PM IST, with notes here.
240+ PAGES · 900+ DIAGRAMS · 47 PRACTICE QUESTIONS
How a request finds a serverLoad Balancer
Health check every 10 sServer 3 removed after 2 failed checks
Three tracks
Learn how systems work, study real designs, then practise for the interview.
Latest reels
Every reel has a notes page: the diagram, the interview words and the follow-up questions.
SYSTEM DESIGN · #50Why YouTube views got stuck at 301@masstdevEvery view is an event, and bots can fake them. YouTube froze public counts at 301 while it verified views, until 2015. Eventual consistency and sharded counters.SYSTEM DESIGN · #49How Find My Device finds an offline phone@masstdevYour offline phone whispers a Bluetooth ID; any nearby Android phone relays its own location, end-to-end encrypted. A crowdsourced network with privacy built in.SYSTEM DESIGN · #48Why your neighbour's TV shouts SIX before your phone@masstdevLive streams are cut into segments; players start a few segments behind live. 6 s × 3 = 18 s of buffer alone. Latency vs buffering, and how Low-Latency HLS fixes it.SYSTEM DESIGN · #47Does the lift door-close button work?@masstdevMost US door-close buttons stopped working after the ADA. Pressing "3" five times is still one request. The SCAN (elevator) algorithm and the LLD "design an elevator" answer.
Design of the week
How Netflix streams to every screen
Encoding, the Open Connect CDN, recommendations and staying up when parts fail. 28 diagrams, then a practice question to try it yourself.