Celebrity day
Pixel’s writes — uploads, likes and comments — have outgrown its one database: 400 a second, each kind through its own web server (no load balancer here: copies of a database would each have to take every write), and the database falls over.
25 minHardMembers kata
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The brief
Pixel’s writes — uploads, likes and comments — have outgrown its one database: 400 a second, each kind through its own web server (no load balancer here: copies of a database would each have to take every write), and the database falls over. Then, at second 40, a celebrity posts and her fans pile on: 400 likes and comments a second on her photos, all keyed by her user id. They come in through her router — the same shard rule, applied to her one key. A database takes 2 CPU units a write (plus 10 idle): a large server holds about 185 writes a second before it passes 95% busy, an xlarge about 375 — and any server kept above 95% busy for 5 seconds goes down and stays down. Keep every write stored, hers included. So work it out: once she posts, how many writes a second land on your busiest shard?
What it checks
- 1Explain why shards add write and storage capacity where read replicas add only read capacity, and why each shard still needs replicas of its own.