System design interview question

Design Facebook status/post search

Hard · System design (HLD) · Company tags: Meta

The prompt

"Design search over Facebook posts. A user types a query and gets back posts — but only posts they are actually allowed to see. That includes their friends' private posts, public posts, and group posts from groups they belong to. Trillions of posts, 2 billion users."

What this round really tests

The intersection of inverted-index search and per-viewer access control. A normal search engine has one global answer per query. Here the answer is different for every one of 2 billion viewers, and you cannot precompute 2 billion index views. Candidates who have only memorized "build an inverted index" will stall the moment you push on privacy.

Clarifying questions worth asking

Are results ranked or chronological?
How fresh must new posts be?
Do we support phrase queries, operators?
Is privacy per-post or per-user?
Can I return a public-only index?
Scale of the index?

How the round usually runs

0–5 min: Scoping
5–10 min: Estimates
10–18 min: Index design
18–30 min: Deep dive
30–40 min: Deep dive
40–45 min: Wrap

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