Discord: billions of messages, then trillions

What you'll be able to do

1. 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.
2. 2Choose a partition key and sort key that turn each listed access pattern into a single lookup or range read.
3. 3Predict each shard's share of the requests, and what a query without the shard key costs, for a given shard key and traffic.
4. 4Explain why adding shards doesn't cool a hot key, since every request for a key goes to the one shard that owns it.
5. 5Choose a fix for a hot key — cache its reads, split its writes over sub-keys, or isolate it — and name what each one costs.

Inside the lesson

steps

5

questions

6

simulations

5

You guess first, then run the system and see if you were right.

Step 1 · The setup