Design Instagram: photo sharing with a follower feed

Design a photo and video sharing app like Instagram. Users post photos and videos, follow each other and scroll a feed of posts from people they follow. Cover upload, storage, feed generation and serving media fast worldwide.

Requirements

• Functional: upload photos and videos with captions; follow and unfollow; a home feed of posts from followed accounts, newest or ranked; likes and comments; user profiles with their posts.
• Non-functional: hundreds of millions of users; far more reads than writes; feed loads in well under a second; media served quickly worldwide; accounts with many millions of followers.

Clarifying questions

• How many daily users, posts per day and feed views per day?
• Chronological or ranked feed?
• Do we need stories, reels, direct messages, or only the main feed?
• What video lengths and quality?
• How quickly must a new post appear in followers' feeds?

What a strong answer covers

• Upload: direct upload to object storage, then processing into several sizes and formats, served through a CDN.
• Data model: users, posts, follows (a graph partitioned by user), likes and comments.
• Feed generation: fan-out on write into each follower's feed cache vs fan-out on read; the hybrid where accounts with huge followings are merged at read time.
• Ranking: candidate generation from the feed cache, a ranking model, and caching the result.
• Storage choices and partitioning for posts and the follow graph; counters for likes at scale.
• Global serving: CDN, regional caches and replicas, and what happens when a region fails.

Follow-ups

• A celebrity with 100 million followers posts. What happens?
• A user follows 5,000 accounts.
• Delete a post everywhere, including caches and the CDN.
• Add an Explore page of posts from people the user does not follow.