Mock round

The six steps

The minutes are a suggested split: the lessons give the order, not the time.

System Design, from Design the Whole Thing

1. Clarifywhat it does, how fast it must be, how available it must be. Ask, don't assume.about 5 min
2. Estimaterequests per second, read-to-write ratio, storage. The numbers decide the rest.about 8 min
3. APIthe two or three requests the outside world can make, and what each returns.about 5 min
4. Datawhat you store, and the one lookup that must be fast.about 5 min
5. Designthe simplest thing that works: client → load balancer → app → cache → database.about 10 min
6. Scalefind each bottleneck, add the one component that removes it.about 12 min

Low-level design, from Who Owes Whom

1. Clarifyask until every answer lands on a class: sizes, rules, what is refused.about 5 min
2. Entitiesone sentence per class, with no "and". Cross out the nouns that are values.about 5 min
3. Class sketchwho holds whom and how many, each fact in one place.about 8 min
4. The tricky part, in codewrite the one method the design hinges on, and run a case through it.about 15 min
5. Follow-upguess the next requirement and count what changes: a field, a class, a line.about 7 min
6. Concurrencytwo callers at once: what can break, and what keeps it whole.about 5 min