Layer 3 · Go deeper · 9 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.
How many rider location pins arrive each second?
What does short polling cost for 100k tracking screens?
Build-up · 8 steps
Trace live order tracking end to end
The worked example’s high-level design for a food-delivery app, built one path at a time. Every number is an assumption you’d propose.
Step 1 of 8
Riders send pins
The rider app sends a pin every 4 seconds to a location service, about 12.5k a second at the dinner peak.
4 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 4
Interviewer
“Your high-level design works end to end. What do you say to move into the deep dives?”
Your move. Pick an approach:
Deep dive 2 of 4
Interviewer
“It’s minute 25 of 45, and your high-level design still isn’t finished. What do you do?”
Your move. Pick an approach:
Deep dive 3 of 4
Interviewer
“Your status endpoint is POST /orders/:id/transitions, and riders on patchy networks retry. What do you add to the API?”
Your move. Pick an approach:
Deep dive 4 of 4
Interviewer
“Pins and status both travel over Redis pub/sub. A customer’s connection blips, and they never see “delivered”. What do you change?”
Your move. Pick an approach: