Design a news feed

Requirements. Users publish posts, follow other users, and view a home feed of posts from people they follow, newest first. Non-functional: loading the feed must be fast (it is the most frequent request), availability matters more than perfect freshness, and a few seconds of delay before a post appears is acceptable. That tolerance is a hint: the feed can be eventually consistent.

• Fan-out on write (push): when someone posts, write the post id into a precomputed feed for each follower (for example a Redis sorted set per user). Reading a feed is one fast lookup, but posting costs one write per follower.
• Fan-out on read (pull): at read time, fetch recent posts from everyone the user follows and merge them. Posting is cheap; reading is slow and expensive.
• Hybrid: push for normal accounts; for celebrities with millions of followers, skip fan-out and merge their recent posts in at read time.

The scale of the problem: a post from someone with 2 followers costs 2 feed writes. A post from an account with 1 million followers costs 1,000,000.

The feed cache stores post ids, not whole posts; the feed service then hydrates ids from a post cache, so an edited post is fixed in one place. Fan-out runs asynchronously through a queue. Paginate with a cursor (“posts older than id X”) instead of an offset, so new posts do not shift pages and cause duplicates. Images and videos go to object storage served through a CDN, and posts store only their URLs. Ranking by relevance is a common extension.