Social News Feed

Functional requirements

• Let a user publish a post with text and optional images
• Let a user follow and unfollow other users
• Show a home feed of recent posts from the people a user follows, newest first
• Page through the feed with a stable cursor, not page numbers
• Show a user's own profile timeline of everything they have posted

Non-functional requirements

• The first page of a feed should load in under 200 ms at p99
• A new post may take a few seconds to appear in followers' feeds
• Feed reads must stay fast even when one author has millions of followers
• Losing a post is unacceptable; briefly missing one from a feed is tolerable

Scale assumptions

Worked estimates for this brief, not measurements of any real system. Disagree with them out loud — that is the exercise.

Posts per day

~1 per 5 users, so 10 million (~115/second average)

Feed cache size

500 most recent post ids × 8 bytes ≈ 4 KB per user; 50 million users ≈ 200 GB

Read:write ratio

~50 feed loads per post, so reads must be the cheap side

Average followers

~200, so naive fan-out is ~2 billion feed inserts/day (~23,000/second)

Daily active users

50 million, as a planning assumption

Feed loads per day

~10 per user, so 500 million (~6,000/second average)

Suggested API surface

• POST/v1/posts

  Publish a post; returns its id

• GET/v1/feed?cursor=

  The signed-in user's home feed, one page at a time

• GET/v1/users/{id}/posts

  One author's own timeline

• PUT/v1/users/{id}/follow

  Follow a user; DELETE on the same path unfollows

Decisions you should be able to defend

• When feeds are assembled
• What the feed cache holds
• Feeds for users who rarely visit
• Pagination