Saying what you will not build

Before you start

• Requirements

  Estimates and APIs start from an agreed list of what the system must do.

  Open lesson

Why it matters

A text feed can quietly expand into ranking, ads, stories, recommendations, media, and moderation before its core behavior is settled. Those additions hide the two cases that actually stress the first version: a reader following 500 accounts and an author with millions of followers. Home must remain reverse-chronological, fresh within seconds, and under 200 ms for the 500-account reader.

A user posts a short text update. Followers see it in a reverse-chronological home timeline.

Scope the feed to post, follow, and reverse-chronological home; require seconds-level freshness and a 200 ms home read for 500 followees, preserve the millions-follower celebrity case, and defer ranking, ads, and stories.

Describe the first version with three user verbs: publish a short text post, follow an account, and open home newest first. Attach observable limits: new posts appear within a few seconds, home answers within 200 ms, a reader may follow 500 accounts, and an author may have millions of followers.

Give every non-goal a reason tied to extra work. Ranking needs signals and a model, ads need an auction, and stories need media storage. These reasons preserve the boundary without pretending the features were forgotten.