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.
How often does the refund path actually run?
Build-up · 6 steps
Record one sale, ACID then BASE
Pritchett’s marketplace example, food-delivery style. Record a sale and add it to the restaurant’s running earnings total, first in one transaction, then in local steps.
Step 1 of 6
ACID: one transaction
One transaction inserts the sale and adds 640 to the restaurant’s total. Simple and correct, but it pins both tables to one server forever, and a busy restaurant’s sales queue on one row lock.
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
“Sales and restaurant earnings now live in separate services. Each delivered order must add to the restaurant’s total. How do you write both?”
Your move. Pick an approach:
Deep dive 2 of 3
Interviewer
“Two services, each with its own database up 99.9% of the time, must change together. Do you use two-phase commit?”
Your move. Pick an approach:
Deep dive 3 of 3
Interviewer
“A flash sale has 100 units, and orders flow through events. How do you make sure the 101st buyer isn’t confirmed?”
Your move. Pick an approach: