Problem Framing and the Fan-Out Architecture
~40 min read
Scoping the feed ranking problem, establishing scale assumptions, and solving the fan-out challenge: how posts reach users' feed caches before ranking can even begin.
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Key points
- •Fan-out on write is fast to read but creates massive write amplification for high-follower accounts; fan-out on read avoids write amplification but is slow for accounts with many follows
- •The hybrid approach (write for regular users, read for celebrities, with a follower count threshold) is the production standard for social feed systems
- •Algorithmic expansion (posts from beyond the user's follows) requires a separate candidate generation path, typically two-tower retrieval from a global post index