Question 4 · Track 1 · How to run the interview

Find the bottleneck, then trade off

The last ten minutes are the interview.

How to run the interview. 3 clarifying questions to ask first, the answer in 4 moves, the follow-up that catches a memorised answer and 3 supporting topics to watch or read.

What it is really testing

Whether you know what your own design costs. Anyone can add a cache; the signal is knowing what it broke.

Ask these first

Before a single box goes on the board. The answers change the design, which is the point of asking out loud.

At the peak number we estimated, what falls over first?
Which of these components is stateful?
What is allowed to be stale, and for how long?

The answer, in 4 moves

In this order. Each move earns the next one — say them out loud rather than drawing all four and narrating afterwards.

1Name the first component to fall over, and why it is that one.
2Apply ONE lever: cache, shard, queue, or replica.
3Say what the lever costs — staleness, ordering, complexity, money.
4Say what you would measure to know it worked.

The trap

The follow-up that separates a rehearsed answer from a real one.

Every answer ends with "here is what I would give up". A design with no stated cost reads as one you have not run in production.

Watch or read

The pieces of this answer, each covered on its own. Take them whichever way suits you, then give the whole answer without looking.

One person says ship it, another says the system is not stable enough, and neither of them has a number — so the loudest voice wins. Here is the arithmetic that ends that argument: what an SLI actually measures and where to measure it, why an SLO is meaningless without a time window, how an error budget turns a target into an allowance you are meant to spend, and why burn rate is the only one of the four that tells you how much time you have left.

Reel Read

Monitoring vs observability

Your dashboards are green during the outage because monitoring answers questions you asked in advance. Observability keeps the question open. Learn the mechanism, the cardinality trap, and when to use which.