CDN — Part 3: Staff/Principal-Level Interview Q&A

What is this?

You know how CDNs work now (Parts 1 and 2). This part is about talking about them under pressure, across a table, while someone decides whether to hire you at a senior level. That's a different skill. The interviewer rarely wants the textbook definition — they want to see how you reason: do you reach for the right trade-off, do you know where the bodies are buried, can you put numbers on your claims?

So this is a mock interview. For each question we'll do three things: decode what they're really asking, walk a model answer that shows your reasoning, and end on the senior line — the one sentence that signals you've operated this stuff, not just read about it.

The one-line idea: at Staff/Principal level, nobody's testing whether you can define a CDN — they're testing whether you reason about trade-offs, failure modes, and cost with the confidence of someone who's been paged at 3am.

We'll lean on the whole series: the cache hierarchy and speed-of-light floor from Part 1, the eviction/invalidation/edge-compute machinery from Part 2.