sysdesign-newsfeed-fanout

A hybrid fan-out design combines write-time propagation for small follow graphs with read-time merging for high-follower accounts. This approach balances latency and resource usage by pushing posts to normal followers while aggregating celebrity posts on read.

To prevent latency spikes, you flag accounts above a specific follower threshold to bypass aggressive write fan-out. Instead, their posts rely on merge-on-read aggregation, protecting system resources and meeting p99 latency budgets during viral events.

You apply inline moderation by enforcing policy checks before materializing feeds. This ensures content is vetted prior to being pushed or merged into the user's materialized feed, maintaining traceability and policy compliance.

The best way to manage bounded storage is to cap per-user feed length within your data model. This limits the maximum size of materialized feeds, preventing unbounded storage growth while preserving recent post history.

You should switch from fan-out on write to merge-on-read when an account crosses a predefined follower cutover threshold. Monitoring p99 latency by follower-graph shape determines this exact transition point for celebrity accounts.