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

Practice

1. What is the primary purpose of a Content Delivery Network (CDN) in system design?

easy

A. To store copies of content closer to users for faster delivery

B. To increase the size of the main server's database

C. To replace the main server completely

D. To encrypt all user data end-to-end

Solution

1. Step 1: Understand CDN's role in content delivery

   A CDN caches copies of content at locations near users to reduce latency and speed up access.

2. Step 2: Compare options with CDN purpose

   Only To store copies of content closer to users for faster delivery describes storing content near users for faster delivery, which is the main CDN function.

3. Final Answer:

   To store copies of content closer to users for faster delivery -> Option A

4. Quick Check:

   CDN = content near users for speed [OK]

Hint: CDN = cache content near users to speed delivery [OK]

Common Mistakes:

• Thinking CDN replaces main server
• Confusing CDN with encryption service
• Assuming CDN increases database size

2. Which of the following is the correct way to describe how a CDN handles user requests?

easy

A. User requests always go directly to the origin server

B. CDN stores user credentials for authentication

C. CDN modifies user requests to change content

D. User requests are routed to the nearest CDN edge server if content is cached

Solution

1. Step 1: Understand CDN request routing

   CDNs route user requests to the closest edge server that has cached content to reduce latency.

2. Step 2: Evaluate options for request handling

   User requests are routed to the nearest CDN edge server if content is cached correctly states requests go to nearest CDN edge if cached; others are incorrect or unrelated.

3. Final Answer:

   User requests are routed to the nearest CDN edge server if content is cached -> Option D

4. Quick Check:

   CDN routes requests to nearest cache [OK]

Hint: Requests go to nearest cached CDN server, not origin [OK]

Common Mistakes:

• Assuming all requests hit origin server
• Believing CDN changes content on the fly
• Confusing CDN with authentication service

3. Consider a system using a CDN with edge servers in multiple regions. If a user in Europe requests a static image, which server will most likely serve the image?

medium

A. A CDN edge server located in Europe

B. The origin server located in the US

C. A random CDN edge server in Asia

D. The user's local device cache only

Solution

1. Step 1: Identify CDN edge server selection logic

   CDNs serve content from the closest edge server to the user to reduce latency.

2. Step 2: Match user location with server location

   A user in Europe will get content from a European CDN edge server if available, not from origin or distant servers.

3. Final Answer:

   A CDN edge server located in Europe -> Option A

4. Quick Check:

   User location matches nearest CDN edge server [OK]

Hint: User served by nearest CDN edge server geographically [OK]

Common Mistakes:

• Assuming origin server always serves content
• Choosing random or distant CDN servers
• Ignoring CDN edge server caching

4. A developer notices that users are still experiencing slow load times despite using a CDN. Which of the following is the most likely cause?

medium

A. The CDN is encrypting content which slows delivery

B. The CDN is not caching the content properly, causing requests to go to the origin server

C. The CDN is blocking user requests randomly

D. The CDN is storing user passwords insecurely

Solution

1. Step 1: Analyze slow load cause with CDN

   If CDN cache misses occur, requests go to origin server, increasing latency and slowing load times.

2. Step 2: Evaluate options for likely cause

   The CDN is not caching the content properly, causing requests to go to the origin server explains cache miss causing slow loads; others are unrelated or incorrect.

3. Final Answer:

   The CDN is not caching the content properly, causing requests to go to the origin server -> Option B

4. Quick Check:

   Cache miss = slow load [OK]

Hint: Slow CDN? Check if cache is missing content [OK]

Common Mistakes:

• Blaming encryption for speed issues
• Assuming CDN blocks requests randomly
• Confusing CDN with security storage

5. You are designing a global video streaming service. Which CDN usage strategy best improves performance and reduces origin server load?

hard

A. Disable CDN caching to ensure fresh content always

B. Cache only small static files like logos, serve videos directly from origin

C. Cache all static content including videos on CDN edge servers worldwide

D. Use CDN only for user authentication requests

Solution

1. Step 1: Identify content types for CDN caching

   Videos are large static files that benefit greatly from CDN caching near users to reduce latency and origin load.

2. Step 2: Evaluate CDN strategies for global streaming

   Caching all static content including videos on CDN edges worldwide improves performance and reduces origin server load.

3. Final Answer:

   Cache all static content including videos on CDN edge servers worldwide -> Option C

4. Quick Check:

   Cache large static files globally for best performance [OK]

Hint: Cache all static content globally on CDN edges [OK]

Common Mistakes:

• Caching only small files and ignoring videos
• Disabling CDN caching unnecessarily
• Using CDN for unrelated tasks like authentication