Practice — Design a News Feed (Twitter / Facebook-style Timeline) (5 questions)

You are asked to design a news feed for a new social app with the following assumptions: 50 million daily active users, each opening the feed 8 times a day; 5% of DAU post once per day; the average user has 150 followers; assume a peak-to-average traffic ratio of 3×.

1. Compute average and peak feed-read QPS.
2. Compute average post-write QPS and the average number of fan-out writes per day if every post is pushed to every follower's feed.
3. Based on these numbers, is pure fan-out-on-write viable for all users in this system? Justify with the arithmetic, not just intuition.