Design a News Feed – Design

For a read-heavy feed at moderate scale (e.g., 1M DAU):

High-Level Architecture

1. Post service: Creates and stores posts (DB: PostgreSQL or similar).

2. Graph service: Tracks follow relationships.

3. Feed service: Builds timelines. Options:

   • Pull (fan-out on read): Fetch posts from followed users on each request. Simple, but slow for heavy follow graphs.
   • Push (fan-out on write): Precompute feed per user when new posts arrive. Fast reads, expensive writes.

4. Cache: Redis for hot feeds.

Inline Check 1

When is fan-out on read (pull) acceptable?

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