Instagram

How this question breaks down

1. 1UploadWrite path27 minGet a photo from a phone into durable storage before the user taps Share, turn it into every size the app will ask for, and publish the post exactly once.
2. 2DeliveryRead path24 minServe each photo at the size the screen needs, from a cache near the viewer, with URLs that never change and so never need a purge to stay fast.
3. 3FeedRead path23 minTurn follows and new posts into a ranked first page of photo posts in under 200 ms, with big accounts merged at read time and seen posts pushed below "You're all caught up".

Work through the parts in order and you’ve covered the full design. Each part is one topic.