CDN

System Design Fundamentals: Caching - CDN (Content Delivery Network)

This document outlines the fundamentals of using a Content Delivery Network (CDN) as a caching strategy in system design.

1. Introduction: Why CDN?

Caching is a crucial technique for improving performance, reducing latency, and lowering costs in distributed systems. While various caching layers exist (browser, proxy, server-side), a CDN takes caching to a global scale.

Problem: Serving content to users geographically distant from your origin server results in:

• High Latency: Data travels further, increasing response times.
• Increased Load on Origin Server: Every request hits the origin, potentially overwhelming it.
• Higher Bandwidth Costs: Transferring data over long distances is expensive.

Solution: CDN

A CDN is a geographically distributed network of proxy servers (Points of Presence - PoPs) that cache static and dynamic content. When a user requests content, the CDN attempts to serve it from the PoP closest to the user. If the content isn't cached, the PoP retrieves it from the origin server and caches it for future requests.

2. How CDN Works: The Flow

1. User Request: A user requests content (e.g., an image, video, webpage) from a website.
2. DNS Resolution: The DNS server directs the request to the CDN. CDNs often use Anycast routing, directing the request to the closest PoP.
3. PoP Check: The PoP checks its cache for the requested content.
   • Cache Hit: If the content is cached, the PoP serves it directly to the user. This is fast and reduces load on the origin.
   • Cache Miss: If the content is not cached, the PoP requests it from the origin server.
4. Origin Server Retrieval: The PoP forwards the request to the origin server.
5. Origin Response: The origin server responds with the content.
6. PoP Caching: The PoP caches the content for a specified duration (TTL - Time To Live).
7. User Response: The PoP serves the content to the user.
8. Subsequent Requests: Future requests from nearby users will likely be served directly from the PoP's cache.

3. Key CDN Concepts

• Points of Presence (PoPs): Geographically distributed servers that cache content. More PoPs = lower latency for more users.
• Origin Server: The original server hosting the content. The CDN fetches content from here when it's not in the cache.
• Time To Live (TTL): The duration for which content is cached in the PoP. Determines how often the CDN checks for updates from the origin. Shorter TTLs ensure freshness but increase origin load. Longer TTLs reduce origin load but may serve stale content.
• Cache Invalidation: The process of removing outdated content from the CDN cache. Important when content on the origin server changes. Methods include:
  • TTL Expiration: Content automatically expires after its TTL.
  • Purge: Manually remove content from the cache. Often done via CDN control panel or API.
  • Versioned URLs: Changing the URL when content updates (e.g., image.jpg?v=2).
• Anycast Routing: A network addressing and routing methodology where multiple servers share the same IP address. Requests are routed to the closest server based on network latency.
• Dynamic Content Acceleration (DCA): Techniques to improve the delivery of dynamic content (e.g., personalized webpages). Often involves route optimization, TCP connection optimization, and edge-side includes (ESI).
• Edge Computing: Running code (e.g., serverless functions) at the edge (PoPs) to process requests closer to the user.

4. CDN Use Cases

• Static Content: Images, CSS, JavaScript, videos, downloadable files. This is the most common use case.
• Dynamic Content: Personalized webpages, API responses. Requires more sophisticated caching strategies (DCA, ESI).
• Streaming Media: Video and audio streaming. CDNs are essential for delivering high-quality streaming experiences.
• Software Downloads: Distributing software updates and installers.
• Gaming: Delivering game assets and updates.
• Security: CDNs can provide DDoS protection and web application firewall (WAF) capabilities.

5. CDN Providers

• Akamai: One of the largest and most established CDN providers.
• Cloudflare: Popular for its ease of use, security features, and free tier.
• Amazon CloudFront: Integrated with AWS services.
• Google Cloud CDN: Integrated with Google Cloud Platform.
• Fastly: Known for its performance and control.
• Microsoft Azure CDN: Integrated with Azure services.

6. Considerations when choosing a CDN

• Global Coverage: Does the CDN have PoPs in the regions where your users are located?
• Performance: What is the CDN's average response time?
• Pricing: How is the CDN priced (e.g., bandwidth, requests)?
• Features: Does the CDN offer the features you need (e.g., security, DCA, edge computing)?
• Integration: How easy is it to integrate the CDN with your existing infrastructure?
• Support: What level of support does the CDN provider offer?

7. CDN and Other Caching Layers

CDNs work in conjunction with other caching layers:

• Browser Caching: Browsers cache static assets locally.
• Proxy Caching: Intermediate servers (e.g., corporate proxies) cache content.
• Server-Side Caching: Caching within your application server (e.g., using Redis or Memcached).

The caching hierarchy is typically:

Browser -> Proxy -> CDN -> Server-Side Cache -> Origin Server

8. Conclusion

CDNs are a powerful tool for improving the performance, scalability, and reliability of web applications. By caching content closer to users, CDNs reduce latency, lower bandwidth costs, and offload traffic from the origin server. Choosing the right CDN provider and configuring it properly are crucial for maximizing its benefits. Remember to consider TTLs, cache invalidation strategies, and integration with other caching layers to create a robust and efficient caching solution.