CDN concept and usage in HLD - System Design Exercise

Design: Content Delivery Network (CDN)

Design the CDN architecture focusing on caching, content distribution, and request routing. Exclude detailed origin server design and content creation.

Functional Requirements

FR1: Deliver static content (images, videos, scripts) to users globally with low latency

FR2: Handle high traffic spikes efficiently

FR3: Reduce load on origin servers

FR4: Provide high availability and fault tolerance

FR5: Support cache invalidation and content updates

FR6: Secure content delivery with HTTPS

Non-Functional Requirements

NFR1: Serve content with p99 latency under 100ms globally

NFR2: Support at least 1 million concurrent users

NFR3: Achieve 99.9% uptime annually

NFR4: Cache consistency delay should be under 5 minutes for updates

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Edge servers (cache nodes) distributed globally

Origin servers hosting original content

DNS-based request routing or Anycast IP

Cache management and invalidation system

Load balancers

SSL/TLS termination

Monitoring and logging infrastructure

Design Patterns

Caching with TTL and cache invalidation

Geo DNS or Anycast for routing user requests to nearest edge

Load balancing across edge servers

Failover and redundancy for high availability

Security patterns like HTTPS and token-based access

Reference Architecture

DNS Resolver

directs to→Nearest Edge Server (Cache Node)

↓

Edge Server Cache

↓

Origin Server

↓

Load Balancer

↓

Monitoring & Logging

Components

Edge Server (Cache Node)

Distributed cache servers (e.g., Nginx, Varnish, or custom)

Serve cached content close to users to reduce latency and origin load

Origin Server

Web servers or storage hosting original content

Provide original content when cache misses occur

DNS Resolver with Geo Routing

Geo DNS or Anycast IP routing

Route user requests to the nearest or best-performing edge server

Load Balancer

Software or hardware load balancer (e.g., HAProxy, AWS ELB)

Distribute requests evenly among origin servers

Cache Invalidation System

API or control plane to purge or update cached content

Ensure updated content is served by removing stale cache

SSL/TLS Termination

Edge server or dedicated SSL terminator

Secure content delivery with HTTPS

Monitoring and Logging

Centralized logging and monitoring tools (e.g., Prometheus, ELK stack)

Track performance, errors, and availability

Request Flow

1. User requests content URL

2. DNS resolver directs user to nearest edge server based on location

3. Edge server checks cache for requested content

4. If content is cached and fresh, edge server returns content immediately

5. If cache miss or stale content, edge server requests content from origin server

6. Origin server responds with content

7. Edge server caches the content and returns it to user

8. Cache invalidation requests update or purge cached content as needed

9. Monitoring system collects metrics and alerts on failures or performance issues

Database Schema

Not applicable as CDN primarily uses distributed caches and metadata stores for cache management rather than traditional databases.

Scaling Discussion

Bottlenecks

Edge server cache capacity limits causing frequent cache misses

DNS resolver or routing service becoming a single point of failure

Origin server overload during cache misses or traffic spikes

Cache invalidation delays causing stale content delivery

Network bandwidth limits at edge locations

Solutions

Add more edge servers and increase cache storage to improve hit ratio

Use multiple redundant DNS servers and Anycast IP for routing resilience

Scale origin servers horizontally and use load balancers

Implement efficient cache invalidation protocols with low latency

Use CDN providers with large global network capacity or partner with ISPs

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing architecture and data flow, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain how CDN reduces latency by caching content near users

Discuss request routing methods like Geo DNS and Anycast

Describe cache management and invalidation strategies

Highlight security considerations like HTTPS termination

Address scalability challenges and solutions

Mention monitoring and fault tolerance for high availability