Content Delivery Networks (CDNs)

A Content Delivery Network (CDN) is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and performance by distributing the service spatially relative to end users. CDNs serve a large portion of the Internet content today, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social media sites.

CDN Providers

Cloudflare

Global cloud platform that provides a range of network services including CDN, DNS, DDoS protection, and security services.

AWS CloudFront

Amazon's global content delivery network service that securely delivers data, videos, applications, and APIs with low latency.

Akamai

One of the world's largest CDN providers, delivering 15-30% of all web traffic with extensive global infrastructure.

Core Concepts of CDNs

Edge Servers: Physical servers located at various geographical points around the world that cache content closer to users.
Origin Server: The main server where your original content is hosted before being distributed to edge servers.
Cache Hit/Miss: A cache hit occurs when requested content is found on the edge server; a cache miss requires fetching from the origin server.
TTL (Time To Live): Determines how long content stays cached on edge servers before being refreshed from the origin.
Point of Presence (PoP): A physical location where CDN providers have deployed servers to serve content to users in that region.

How CDNs Work

When a user requests content from a website using a CDN:

DNS Resolution: The user's DNS request is routed to the CDN's DNS servers
Edge Server Selection: The CDN selects the optimal edge server based on the user's location, server health, and network conditions
Content Delivery: If the content is cached (cache hit), it's served immediately. If not (cache miss), the edge server fetches it from the origin server
Caching: The content is cached on the edge server for future requests according to caching rules

CDN Benefits

Reduced Latency: Content is served from servers geographically closer to users, reducing round-trip time.
Improved Performance: Faster load times lead to better user experience and higher conversion rates.
Reduced Server Load: Origin servers handle fewer requests as content is served from edge caches.
Scalability: CDNs can handle traffic spikes and distribute load across multiple servers.
DDoS Protection: Many CDN providers offer built-in DDoS mitigation and security features.
Global Reach: Serve users worldwide without deploying infrastructure in every region.

Common CDN Use Cases

Your interviewer will be impressed if you can discuss practical applications of CDNs in system design.

Static Content Delivery: Images, CSS, JavaScript files, and other static assets are perfect candidates for CDN caching.
Video Streaming: CDNs are essential for delivering high-quality video content with minimal buffering, used by platforms like Netflix and YouTube.
API Acceleration: Caching API responses at edge locations to reduce latency for frequently requested data.
Dynamic Content Optimization: Even non-cacheable content benefits from CDN's optimized routing and connection pooling.
Security: CDNs provide Web Application Firewall (WAF) capabilities and DDoS protection at the edge.

CDN Caching Strategies

Different types of content require different caching approaches:

Static Assets: Long TTL (hours to days) for images, stylesheets, and JavaScript files that rarely change.
HTML Pages: Short TTL (minutes to hours) or dynamic generation with edge-side includes for personalized content.
API Responses: Cache based on request parameters and user context, with appropriate cache invalidation strategies.
Personalized Content: Use edge computing to generate personalized responses while still benefiting from CDN infrastructure.

How to Use CDNs in a System Design Interview

When you're in a system design interview, you should be able to articulate why you would choose a CDN and how it fits into your architecture.

Here are some key points to mention:

Global User Base: When designing systems that serve users worldwide, CDNs are essential for providing consistent performance across different geographical regions.
High Traffic Websites: Real-world examples include social media platforms like Instagram serving millions of images, or e-commerce sites like Amazon delivering product images and videos globally.
Video Streaming Platforms: CDNs are crucial for services like Netflix, YouTube, or TikTok to deliver high-quality video content without buffering.
Mobile Applications: CDNs significantly improve mobile app performance by reducing data transfer times and bandwidth usage.
Cost Optimization: CDNs reduce bandwidth costs on origin servers and can significantly lower infrastructure expenses for high-traffic applications.
SEO Benefits: Faster loading times improve search engine rankings, making CDNs valuable for content-heavy websites.
Trade-offs: Discuss the complexity of cache invalidation, consistency challenges, and the additional cost considerations when implementing CDNs.

By discussing these points and providing concrete examples like how social media platforms handle image delivery or how e-commerce sites optimize product catalog performance, you'll demonstrate a solid understanding of CDNs in real-world applications.

Example System Design Problems

Here are examples of system design problems where CDNs play a crucial role:

Design Netflix: CDNs are essential for delivering video content globally with minimal buffering. Netflix uses multiple CDN providers and even deploys their own Open Connect CDN.
Design Instagram: A CDN is crucial for serving billions of photos and videos to users worldwide, with different optimizations for mobile vs. desktop viewing.
Design a News Website: CDNs help deliver articles, images, and breaking news content quickly to readers around the globe while handling traffic spikes during major events.
Design an E-commerce Platform: CDNs serve product images, handle API requests for product catalogs, and ensure fast checkout experiences regardless of user location.