# Design a content delivery network: system design interview question

> Global media delivery with uneven demand, limited cache space, origin limits, and delivery cost trade-offs.

Source: https://sysround.com/questions/design-content-delivery-network
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## How the interviewer opens it

"Design a global network that caches and delivers media near viewers. It must handle popular releases while it limits origin traffic and delivery costs."

## What to clarify first

- Media format
- Viewer scale
- Catalogue and cache capacity
- Read and write mix
- Latency
- Consistency and availability
- Origin capacity and cost
- Durability
- Access and exclusions

## Scope of a strong answer

Core requirements:

- Serve media from nearby, healthy edge sites.
- Handle popular releases without excess origin load.
- Control cache use and delivery costs.
- Serve complete segments and valid byte ranges.

Usually out of scope:

- Live video
- Media encoding and upload workflows
- Digital rights management
- A custom Internet backbone
- Billing

## What interviewers probe

### Popular releases and origin load

Viewers can request the same new segments before any edge has them. Independent misses can exceed origin capacity despite a high average hit rate.

### Site choice and failures

The nearest site can lack capacity or cached media. Viewers can continue attempts to reach a failed site after the network selects an alternative.

### Media requests and cache space

The catalogue exceeds cache capacity, and demand varies by region. Byte ranges and object sizes can make request hit rates hide high origin traffic.

## Common mistakes

- The design treats the network as a metadata service instead of a system that delivers media bytes.
- The capacity estimate counts requests but ignores bandwidth and cache space.
- Each cold request reaches the origin during a popular release.
- The design assumes every edge can store the full catalogue.
- The design ignores byte ranges, incomplete fills, or manifest freshness.
- The cost estimate treats a cache hit as free delivery and ignores paid transit.

## The bar by level

- **Senior:** Designs global site selection and shared miss control. Protects origin capacity during releases and failures. Connects byte hit rates to cache policy and cost.
- **Staff+:** Sets explicit overload behavior and regional failure budgets. Quantifies spare capacity, cold recovery, and transit trade-offs. Separates control failures from delivery failures.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=media-cdn


---
SysRound (https://sysround.com) is an AI practice tool for system design interviews. It has a voice or typed AI interviewer and a shared whiteboard. At the end, you get feedback on each skill and a model answer. The AI feedback can be wrong.