Detailed Explanation of CDN Working Principles and Caching Strategies

Detailed Explanation of CDN Working Principles and Caching Strategies

1. Basic Concepts of CDN
A CDN (Content Delivery Network) is a network system composed of server nodes distributed across different geographical locations. Its core goal is to cache content at edge nodes closer to users, thereby reducing network latency and improving access speed.

2. Core Components of CDN

  1. Origin Server: The server storing the original content, serving as the ultimate source of content.
  2. Edge Node: Cache servers distributed globally, providing services directly to users.
  3. CDN Scheduling System: A DNS system that intelligently selects the optimal edge node based on the user's location.

3. Detailed CDN Workflow

  1. Domain Name Resolution Phase

    • User visits www.example.com, and the local DNS queries the authoritative DNS.
    • The authoritative DNS returns a CNAME record pointing to the CDN provider's domain (e.g., example.cdn.com).
    • The local DNS sends a query request to the CDN's scheduling DNS.
    • The CDN scheduling DNS returns the IP address of the optimal edge node based on factors such as the user's IP address and network conditions.

  2. Content Request Phase

    • The user's browser sends an HTTP request to the edge node.
    • The edge node checks whether the requested content exists in its local cache.
    • Cache Hit: Returns the cached content directly to the user.
    • Cache Miss: The edge node fetches the content from the origin server, caches it, and then returns it to the user.

4. In-depth Analysis of CDN Caching Strategies

  1. Cache Expiration Mechanism

    • Controlled via HTTP headers: Cache-Control (max-age), Expires.
    • Example: Cache-Control: max-age=3600 indicates the cache is valid for 1 hour.
    • The edge node returns cached content directly before the cache expires.

  2. Cache Refresh Mechanism

    • Active Refresh: Forcibly purges the cache on edge nodes via the CDN console or API.
    • Passive Refresh: Automatically fetches the latest content from the origin after the cache expires.
    • Conditional Requests: Validates content changes using If-Modified-Since/If-None-Match.

  3. Cache Key Design

    • Default includes: URL path, query parameters.
    • Configurable options: Whether to distinguish protocol (HTTP/HTTPS), Cookie, User-Agent, etc.
    • Optimization Tip: Normalize cache keys to avoid multiple caches for the same content.

5. Advanced CDN Features

  1. Dynamic Content Acceleration

    • Accelerates non-cacheable content such as APIs by optimizing transmission paths.
    • Employs techniques like TCP optimization and route optimization to reduce network latency.

  2. Security Protection

    • DDoS Protection: Edge nodes disperse attack traffic.
    • WAF Functionality: Implements Web Application Firewall rules at edge nodes.
    • HTTPS Acceleration: Terminates SSL/TLS at edge nodes, reducing the load on the origin server.

6. CDN Performance Optimization Practices

  1. Cache Hit Rate Optimization

    • Set appropriate cache durations: Use longer cache times for static resources (e.g., 1 year).
    • Versioned Filenames: Implement automatic cache updates via file hash values.
    • Segmented Caching Strategy: Apply different strategies for frequently changing and rarely changing content.

  2. Origin Pull Strategy Optimization

    • Set up health check mechanisms to avoid pulling from faulty origin servers.
    • Configure multiple origin server backups to improve system availability.
    • Set reasonable origin pull timeout times to balance user experience and system stability.

By understanding CDN working principles and caching strategies, website performance can be significantly enhanced while reducing the load on the origin server. It is an indispensable infrastructure for modern web applications.