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BRUCE_FE Interview Notes - Page Loading - What is CDN? Why do we need CDN?
In-depth analysis of Content Delivery Network (CDN) operation principles, main advantages and practical application scenarios, including key knowledge of improving page load speed, reducing server load, and complete answers to common interview questions.
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Basic Concepts
CDN (Content Delivery Network) is a distributed network architecture composed of multiple nodes worldwide that can deliver website content to users faster and more reliably. CDN reduces latency and improves loading speed by adding an intermediate layer between users and the origin server, copying content to nodes closest to users.
User Request
│
↓
CDN Edge Node ◄────► CDN Core System
│ │
│ ↓
│ Origin Server
│ │
↓ │
User Browser ◄─────────┘How CDN Works
- 1. Content Distribution: Copy static resources (images, CSS, JS, etc.) to multiple node servers globally
- 2. Proximity Access: User requests are automatically routed to the nearest CDN node, reducing network latency
- 3. Cache Update: When origin server content updates, it's pushed to CDN nodes through:
- 1. Active Push: Server automatically notifies CDN to update cache after changes
- 2. Passive Pull: Set TTL (Time To Live), CDN periodically checks origin server for updates
- 3. Cache Invalidation (Purge): Administrator manually triggers clearing of specific content cache
- 4. Version Control: Use timestamps or version numbers in URLs to force client requests for new resources
CDN Node Distribution Diagram:
┌─────────────────────────────────────────────────────────┐
│ │
│ ★ North America ★ Europe │
│ Node Group Node Group │
│ │
│ │
│ │
│ Origin Server │
│ ★ │
│ │
│ │
│ ★ Asia ★ Australia │
│ Node Group Node Group │
│ │
└─────────────────────────────────────────────────────────┘
When a Taiwan user accesses the website:
┌─────────────────────────────────────────────────────────┐
│ │
│ User ──► Taipei CDN Node ◄──┐ │
│ │ │
│ │ │
│ Origin Server │
│ │
└─────────────────────────────────────────────────────────┘<!-- Example of using CDN to import JavaScript library -->
<script src="https://cdn.example.com/jquery/3.6.0/jquery.min.js"></script>
<!-- Using version control to avoid cache issues -->
<script src="https://cdn.example.com/main.js?v=1.2.3"></script>Main Advantages of CDN
Improve Page Load Speed
- 1. Reduce latency, shorten TTFB (Time To First Byte)
- 2. Provide faster resource loading, improve user experience
- 3. Support HTTP/2 and HTTP/3 protocols, allowing parallel downloads
Enhanced Reliability and Stability
- 1. Distribute traffic, avoid single point of failure
- 2. Provide high redundancy and availability
- 3. Automatic failover mechanism
Reduce Origin Server Load
- 1. Share server pressure, save bandwidth costs
- 2. Reduce load issues during high traffic
- 3. Provide better scalability
Enhanced Security
- 1. Provide DDoS attack protection
- 2. Support HTTPS encrypted transmission
- 3. Can integrate WAF (Web Application Firewall) functionality
CDN Practical Application Scenarios
- 1. Image and Media Optimization: Large media files such as images and videos
- 1. Example: Netflix uses CDN to distribute video content, reducing buffer time
- 2. Static Resource Delivery: Frontend resources like CSS, JavaScript
- 1. Example: Facebook uses CDN to accelerate page load speed for global users
- 3. Dynamic Content Acceleration: Caching API responses and dynamically generated content
- 1. Example: Twitter uses CDN to cache popular tweets and timelines
- 4. Game and Software Downloads: Large file distribution
- 1. Example: Steam platform uses CDN to distribute game updates and downloads
- 5. Live Streaming and Video Streaming: Low-latency content delivery
- 1. Example: YouTube uses CDN to ensure smooth video playback for global users
┌─────────────────────────────────────────────────────────────┐
│ │
│ Real Case of E-commerce Website Using CDN │
│ │
│ Without CDN: │
│ US User ──(200ms)──> Taiwan Server ──(200ms)──> US User │
│ Total Latency: ~400ms │
│ │
│ With CDN: │
│ US User ──(20ms)──> US CDN Node ──(20ms)──> US User │
│ Total Latency: ~40ms (90% improvement) │
│ │
│ High Traffic Events (e.g., Singles' Day Shopping): │
│ - Origin Server: May crash due to excessive traffic │
│ - Using CDN: Distribute traffic globally, maintain stability│
│ │
└─────────────────────────────────────────────────────────────┘Potential Drawbacks of CDN
- 1. Additional Cost: For small websites, CDN services may add unnecessary expenses
- 2. Configuration Complexity: Proper CDN setup requires expertise, improper configuration may cause issues
- 3. Cache Invalidation Challenges: When content updates, ensuring all CDN nodes sync may have delays
- 4. Third-party Dependency: If CDN provider fails, it may affect entire website availability
- 5. Geographic Restrictions: Some regions may not be able to use specific CDN services due to policy restrictions
🔥 Common Interview Questions
(1) How to determine if CDN is needed?
- 1. Wide User Distribution: CDN can significantly improve speed when users come from different countries or regions
- 2. Many Static Resources: Website has lots of images, CSS, JS files
- 3. Unstable Traffic: Frequent traffic peaks, such as promotional events
- 4. Speed is Critical: Like e-commerce sites where speed directly affects sales
- 5. Cost-effectiveness: Small websites need to evaluate if the investment is worthwhile