Content delivery with CDN in HLD - System Design Exercise

Design: Content Delivery Network (CDN) System

Design covers CDN architecture for static content delivery including caching, cache invalidation, and global distribution. Does not cover dynamic content generation or user authentication.

Functional Requirements

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

FR2: Support at least 1 million concurrent users

FR3: Ensure content is cached close to users to reduce origin server load

FR4: Provide cache invalidation to update content when needed

FR5: Handle sudden traffic spikes without downtime

FR6: Support HTTPS for secure content delivery

Non-Functional Requirements

NFR1: 99.9% availability (less than 8.77 hours downtime per year)

NFR2: API response latency p99 under 100ms for cached content

NFR3: Origin server should not be overwhelmed by requests

NFR4: Content freshness must be maintained with cache TTL and invalidation

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Edge servers located globally for caching content

Origin servers hosting the original content

Load balancers to distribute user requests

Cache invalidation mechanism

DNS routing to direct users to nearest edge server

HTTPS termination at edge

Design Patterns

Cache-aside pattern for cache population

Geo-DNS or Anycast for routing users to nearest edge

TTL-based cache expiration

Push vs pull CDN content update strategies

Reference Architecture

DNS Resolver

directs to nearest→Edge CDN Server Cluster

↓

Cache Storage

↓

Origin Server

Components

DNS Resolver

Geo-DNS or Anycast DNS

Direct users to the nearest edge server based on location

Edge CDN Server Cluster

Distributed edge servers with caching software (e.g., Nginx, Varnish)

Cache and serve content close to users to reduce latency

Cache Storage

In-memory or disk cache on edge servers

Store cached content for fast retrieval

Origin Server

Web servers hosting original static content

Serve content when not available in cache

Cache Invalidation Service

API or messaging system

Update or remove cached content when origin content changes

Load Balancer

Global and local load balancers

Distribute incoming requests evenly across edge servers

Request Flow

1. User requests content via browser.

2. DNS resolver directs user to nearest edge CDN server.

3. Edge server checks cache storage for requested content.

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

5. If content is missing or stale, 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 service updates or removes cached content when origin content changes.

9. Subsequent user requests get fresh content from edge cache.

Database Schema

Not applicable as CDN primarily uses cache storage and origin servers; metadata about cached content includes content key, TTL, last updated timestamp, and invalidation status.

Scaling Discussion

Bottlenecks

Origin server overload during cache misses or traffic spikes

Edge server cache storage limits causing frequent cache evictions

DNS resolver latency or failure affecting user routing

Cache invalidation delays causing stale content delivery

Solutions

Use origin server autoscaling and rate limiting to handle load

Increase edge cache capacity and use efficient eviction policies (LRU)

Deploy multiple DNS resolvers with failover and low latency

Implement near real-time cache invalidation with push updates or short TTLs

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 close to users

Discuss cache miss handling and origin server protection

Describe cache invalidation strategies to maintain content freshness

Highlight global distribution and DNS routing for scalability

Mention security considerations like HTTPS termination at edge