Layer 3 · Go deeper · 6 min
Go deeper
Size it, build it, then make the calls an interviewer will push you on.
Back of the envelope
Size it up
Tap through each step of the arithmetic.
A cricket-final channel takes 5,000 messages a second. How many DynamoDB keys?
Build-up · 5 steps
Follow a write through DynamoDB
One partition, three replicas in three availability zones. Watch who takes the write, who may answer a read, and what happens when the partition grows.
Step 1 of 5
A router finds the partition
Every call lands on a request router, which authenticates it and looks up the key range’s replication group in a metadata service. Placement lives in that metadata, not in your app.
3 interview moments
You’re the candidate
The interviewer pushes on the hard parts. Pick an approach and see how it holds up.
Deep dive 1 of 3
Interviewer
“Your chat app keeps message history in Cassandra. Busy channels take 20 messages a second; family groups, 200 a day. How do you key it?”
Each message is 500 bytes.
Your move. Pick an approach:
Deep dive 2 of 3
Interviewer
“A three-person team on AWS needs a store for spiky, write-heavy event data, read by key. Cassandra or DynamoDB?”
Your move. Pick an approach:
Deep dive 3 of 3
Interviewer
“Queries still change weekly, the data fits one machine, and orders must match payments. The team wants DynamoDB. What do you say?”
Your move. Pick an approach: