Design a Content Delivery Network (CDN)

Intermediate Infrastructure

The problem

Design a content delivery network that serves images, scripts and video to users around the world with low latency, so most requests never reach the customer's own servers.

You are in a 45-minute interview. The interviewer gave you only the sentence above.

The customers are websites and streaming apps; their servers are the origin.

Think of Cloudflare, Akamai or CloudFront.

You are given

About 50 million daily users, each downloading roughly 2 MB through the CDN.
Peak traffic is several times the average.
Each customer's origin can serve about 1 Gbit/s.
Content is mostly static: images, scripts and video segments.

What we expect from you

Drive the conversation. Ask questions before drawing anything. State your assumptions out loud so they can be corrected early.
Use numbers. Turn the givens into requests per second, storage and bandwidth. Every later choice should point back to one of them.
Start simple, then evolve. Get a working version on the board first. Add caches, sharding and queues only when a number or a requirement asks for them.
Go deep on the hard parts. For this problem the hard parts are routing users to the nearest edge, getting a very high hit rate, and keeping content fresh without hammering the origin.
Name the trade-offs. Say what you chose, what you gave up, and what would make you change your mind.
Think about failure. Point out single points of failure and say what happens when a node, a cache or a whole zone goes down.

Scope

In scope

Routing users to an edge
Caching, fill and eviction
Expiry and purge
Failure handling and security basics

Not required

Video transcoding
Dynamic page personalisation at the edge
Billing and analytics dashboards

Suggested time

Clarify requirements	4 min
Estimate scale	4 min
API and data model	5 min
High-level design	7 min
Deep dive	15 min
Reliability	5 min
Trade-offs and wrap-up	5 min

What makes an answer great

Explains DNS or anycast routing to the nearest healthy edge.
Calculates the hit rate needed to protect the origin.
Uses a shield cache and request coalescing for cold starts.
Prefers versioned file names to purging.

Key concepts

Edge server

A cache server in a city or network close to users.

Origin

The customer's own server that holds the real content.

Anycast

The same IP is announced from many places; routing sends each user to the nearest.

TTL

How long a cached copy may be used before it is checked or replaced.

Cache hit rate

The share of requests answered from cache without contacting the origin.

Signed URL

A link with an expiry and signature the edge can verify without asking anyone.

Numbers worth remembering

50M users x 2 MB = 100 TB a day = about 9 Gbit/s average
Origin share = origin capacity / peak traffic, so you need roughly 97%+ hit rate
Light travels about 1 ms per 100 km in fibre, which is why distance matters

Locked in so far

Decisions you confirm will collect here as you go.

Your design so far