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System Design

Content Delivery Networks (CDNs)

A guide to how CDNs work, their benefits, and their role in modern web performance.

Table of Contents

What is a CDN?

A Content Delivery Network (CDN) is a globally distributed network of proxy servers that cache and serve content from locations closer to the user. By reducing the physical distance between the user and the server, CDNs significantly decrease latency and improve the performance of web applications.

How Does a CDN Work?

  1. User Request: A user makes a request for a web asset (e.g., an image, CSS file, or video).
  2. DNS Resolution: The user’s request is routed to the nearest CDN edge server.
  3. Cache Check: The edge server checks its cache for the requested content.
  4. Cache Hit: If the content is in the cache (a “cache hit”), the edge server delivers it directly to the user.
  5. Cache Miss: If the content is not in the cache (a “cache miss”), the edge server requests it from the origin server (the main web server). The origin server sends the content to the edge server, which then caches it and delivers it to the user.

Note: Subsequent requests for the same content from users in that geographic region will be served directly from the edge server’s cache, resulting in much faster load times.

Benefits of Using a CDN

  • Improved Performance: By serving content from a location closer to the user, CDNs dramatically reduce latency.
  • Reduced Origin Server Load: By caching content, CDNs reduce the number of requests that hit the origin server, lowering bandwidth costs and improving scalability.
  • Increased Availability and Reliability: The distributed nature of a CDN means that if one edge server goes down, traffic can be automatically rerouted to another, providing resilience against hardware failures and traffic spikes.
  • Enhanced Security: Many CDNs offer protection against Distributed Denial-of-Service (DDoS) attacks.

CDN Caching Strategies: Push vs. Pull

StrategyDescriptionBest For
Push CDNThe origin server proactively “pushes” new or updated content to the CDN. The server is responsible for managing what content is on the CDN.Sites with a small amount of traffic or content that is not updated frequently.
Pull CDNThe CDN “pulls” new content from the origin server only when a user first requests it. The content is then cached for subsequent requests.Sites with heavy traffic, as only the most recently and frequently requested content is stored on the CDN.

When to Use a CDN

A CDN is beneficial for almost any public-facing website, but it is particularly important for:

  • High-Traffic Applications: To handle a large volume of requests and reduce server load.
  • Global Audiences: To provide a fast experience for users around the world.
  • Rich Media Content: For sites with many images, videos, or other large static assets.
  • Amazon CloudFront
  • Cloudflare
  • Akamai
  • Google Cloud CDN
  • Fastly

Summary

CDNs are an essential component of modern web infrastructure. By caching content in a distributed network of servers, they reduce latency, decrease the load on origin servers, and improve the overall performance and reliability of web applications. Understanding the difference between push and pull strategies allows you to choose the right approach for your specific needs.