Medium · 12 steps · ~70 min

Design a ranked feed

Votes pouring in, and a ranking that stays fresh and fair.

Posts are ranked by votes and age. Reads are constant and votes arrive in a flood.

Same problem as RedditHacker NewsProduct Hunt

How it works

• Read: show a precomputed ranking.
• Vote: record the vote, recompute scores in the background.

What you'll build

1. The API and data for posts and votes
2. A system that answers 800 requests a second
3. A cached front page within a budget
4. A queue and workers that tally votes
5. A design for a viral post and a rate limiter against vote bots
6. A system that survives a cold cache, a lost server and a lost primary

Steps

1. 1Design the API
2. 2Design the data model
3. 3Handle 800 requests/sec
4. 4Cache the front page for $900 a month
5. 5Queue the votes at 30,000 requests/sec
6. 6A viral post at 100,000 requests/sec for $2,000
7. 7How fresh should the ranking be?
8. 8Survive vote brigading on $2,200
9. 9Survive a cache restart
10. 10Lose an API server
11. 11Lose the primary database
12. 12Deep dive: a retried vote counts twice