How does CDN actually work?

Content Delivery Network (CDN)

A Content Delivery Network (CDN) is a technology architecture designed to optimize the transmission of internet content, aiming to provide faster and more reliable content delivery experiences. CDNs distribute content to servers globally, known as edge servers, to reduce the distance between users and the origin server, thereby accelerating content delivery.

CDN Principles

1. Caching and Distribution: CDN providers deploy multiple server nodes globally, referred to as edge servers. When users request specific content (such as images, videos, web pages, etc.), the CDN caches this content on the nearest edge server. Subsequent requests for the same content are served directly from the edge server, eliminating the need to fetch content from the origin server.

2. Proximity Access: With edge servers distributed globally, users can access content from servers closer to them, reducing latency and bandwidth congestion for faster loading speeds and improved user experience.

3. Load Balancing: CDNs use load balancing technology to distribute user requests across different edge servers. This helps prevent server overload, ensuring relative balance in server loads for enhanced overall performance.

4. Dynamic Content Acceleration: CDNs can accelerate dynamic content, such as personalized web pages and dynamically generated content, by utilizing caching techniques and intelligent routing.

5. Cache Strategy: CDN providers create cache strategies based on content characteristics and user behavior. While some static content may be cached for extended periods on edge servers, dynamic content may only be cached briefly to ensure users receive the latest data.

6. Security and Protection: Some CDN providers offer security features like DDoS attack protection and WAF (Web Application Firewall) to safeguard origin servers against malicious attacks.

Distribution Service System

The fundamental unit of operation within the CDN is the Cache servers. They respond directly to user requests, swiftly distributing content to users while also managing content updates to stay synchronized with the origin server.

Depending on content types and service categories, the distribution service system is subdivided into various subsystems like Web Acceleration Service, Media Streaming Service, Application Acceleration Service, etc. Each subsystem is a distributed service cluster composed of functionally similar Cache clusters deployed in close proximity geographically.

In addition to content synchronization, updates, and responding to user requests, the distribution service system must relay information about each Cache device's health, response status, and content cache status to upper-level management and scheduling systems. This facilitates strategic decisions on which Cache device should handle user requests based on pre-set policies.

Load Balancing System

The core of the entire CDN system is the Load Balancing System, responsible for orchestrating all user requests, determining the final access address provided to users.

It employs a hierarchical approach to manage requests, including Global Load Balancing (GSLB) and Local Load Balancing (SLB). GSLB determines the node serving a user primarily based on proximity principles using methods like DNS resolution or application layer redirection (HTTP 3XX redirection). SLB handles internal load balancing within nodes. When a user request is directed from GSLB to SLB, SLB redirects the request based on the working conditions and content distribution of each Cache device within the node. SLB implementations include Layer 4 scheduling (LVS), Layer 7 scheduling (Nginx), and link load balancing scheduling.

Management System

The Management System consists of Operational Management and Network Management subsystems.

Network Management oversees device management, topology management, link monitoring, and fault management within the CDN system, providing centralized visual management of network resources to administrators typically via web interfaces.

Operational Management handles business-related tasks within the CDN system, managing interactions with external systems necessary for data collection, organization, and delivery. This includes user management, product management, billing management, statistical analysis, and more.

Conclusion

By deploying edge servers globally, CDN distribution service systems optimize content delivery, providing enhanced user experiences, reduced latency, lower bandwidth costs, and improved website and application performance and reliability. Such systems are vital for speeding up content delivery, enhancing network security, and elevating user experiences, particularly in large-scale global network applications.