Design: a news feed

The prompt

"Design the news feed" (Facebook, X, Instagram) is one of the most asked questions because the obvious design fails in two opposite ways: build feeds on write and celebrities break you; build them on read and every app open is a query against hundreds of accounts.

1. Requirements

Functional: post a link (with text and an image); follow and unfollow; read the home feed, newest first, with infinite scroll.

Non-functional: feed loads in under 200 ms at p99; a new post appears in followers' feeds within a few seconds (a minute is acceptable for the largest creators); a deleted post disappears at once.

Out of scope: comments, likes, search, ads.

2. Estimates

3. Entities and API

Post    { id (Snowflake), author_id, text, image_key, created_at, deleted? }
Follow  { follower_id, author_id, created_at }        stored both ways (12.04)
Feed    { user_id → list of post IDs, newest first, capped at 500 }

POST /posts                     { text, image_key }       →  201 { id }
GET  /feed?limit=20&before=ID                             →  200 { posts: [...], next: ID }
PUT  /follows/{author_id}       DELETE /follows/{author_id}

Post IDs are Snowflake IDs (05.05 Distributed ID generation): unique, and ordered by time, so the cursor is just an ID.

4. High-level design

5. Deep dives

export type FeedPost = { id: bigint; author: string }; // Snowflake IDs: bigger is newer

export function feedPage(pushed: FeedPost[], pulled: FeedPost[][], limit = 20, before?: bigint) {
  const seen = new Set<bigint>();
  const merged = [...pushed, ...pulled.flat()]
    .filter((p) => (before === undefined || p.id < before) && !seen.has(p.id) && seen.add(p.id))
    .sort((a, b) => (b.id > a.id ? 1 : b.id < a.id ? -1 : 0))
    .slice(0, limit);
  const next = merged.length === limit ? merged[merged.length - 1].id : undefined;
  return { posts: merged, next }; // the client sends `next` back as `before`
}

Tested with 60 posts, a third of them from a pulled celebrity and one of those also pushed (the author had just crossed the threshold): paging 20 at a time returned all 60 posts across the pages, each exactly once, strictly newest first, even though five new posts arrived after the first page. The new posts weren't shown in that scroll; they appear at the top on the next refresh.

Follows, unfollows and deletes. A new follow backfills the author's last few posts into the follower's feed in the background. An unfollow doesn't rewrite the feed: the read path filters out authors the user no longer follows, and the entries age out. A deleted post is filtered during hydration.

Ranking. If the feed isn't chronological, gather a few hundred candidates the same way, score them with a model, and sort. Fan-out gathers; ranking orders. Cursors then point into a ranked list saved for that session.

6. Wrap-up

What breaks first: fan-out lag when many large-but-not-huge authors post at once, so fan-out workers get their own autoscaling queue and the threshold is tuned by measuring; then the feed cache's memory as active users grow, handled by capping lengths and evicting inactive users' feeds.

Recap

Interview questions