Content Delivery Network (CDN)

Phase 1 — Clarify & Scope

Interviewer: Design a Content Delivery Network — think Cloudflare or AWS CloudFront. What do you want to clarify?

Candidate: A few clarifying questions:

1. What types of content — static assets (images, JS, CSS), video streaming, or dynamic API responses too?
2. What's the geographic footprint — global, or specific regions?
3. What's the traffic volume — requests per second and total bandwidth?
4. Cache invalidation requirements — how quickly must content updates propagate?
5. Do we need edge compute (running logic at the edge) or purely caching?

Interviewer: Primarily static assets and video streaming. Global footprint — 450+ Points of Presence (PoPs). Peak traffic: 10 million requests/sec, 2 Tbps bandwidth. Cache invalidation must propagate globally within 60 seconds. Edge compute is a stretch goal.

Candidate: Let me size the PoP infrastructure:

$\text{Per PoP (450 PoPs)} = \frac{10\text{M req/sec}}{450} \approx 22{,}222 \text{ req/sec average}$

$\text{Per PoP bandwidth} = \frac{2\text{ Tbps}}{450} \approx 4.4 \text{ Gbps average}$

With typical peak-to-average ratio of 3×: each PoP must handle ~66K req/sec and ~14 Gbps at peak. A tier-1 PoP (major city) might handle 10× that — 220K req/sec.

Target cache hit rate: 90%+ (only 10% of requests reach origin):

$\text{Origin requests} = 10\text{M} \times 0.10 = 1\text{M req/sec to origin}$

$\text{Bandwidth saved} = 2\text{ Tbps} \times 0.90 = 1.8 \text{ Tbps}$

Interviewer: Good. What are the core requirements?

Candidate:

• Route user requests to the nearest PoP (<…