A Content Delivery Network (CDN) is a geographically distributed network of servers that cache and deliver content (images, videos, HTML, CSS, JavaScript) from locations near end users. CDNs reduce latency by serving content from servers geographically close to users rather than from a central origin server. This improves user experience, reduces server load, and enables delivering content globally reliably.
How CDNs Work
CDN operation is straightforward:
Global server network - CDN operators maintain servers (called edge locations or points of presence) in cities worldwide.
Content caching - Popular content is cached at edge locations for fast delivery.
User routing - When users request content, DNS and other mechanisms route requests to the nearest edge location.
Cache hits - If content is cached at the edge location, it is served immediately.
Cache misses - If content is not cached, the edge location fetches it from the origin server, caches it, and serves it.
Cache expiration - Content is cached for a configured duration, then refreshed from the origin.
This reduces latency by serving from geographically close servers and reduces origin server load by distributing requests.
CDN Benefits
CDNs provide substantial benefits:
Reduced latency - Serving from nearby servers dramatically reduces latency, improving user experience and page load times.
Improved throughput - Optimised CDN networks deliver higher throughput than typical internet routing.
Reduced origin load - Content served from caches rather than origin servers, reducing origin infrastructure requirements.
Scalability - CDNs absorb traffic spikes, enabling serving traffic volumes without scaling origin infrastructure.
Improved reliability - Geographic redundancy means failure of some edge locations does not affect service.
Cost reduction - Reducing origin server load and bandwidth requirements lowers infrastructure costs.
Static vs. Dynamic Content
CDNs handle different content types differently:
Static content - Images, CSS, JavaScript, static HTML are easily cached at CDN edge locations. These are ideal for CDN delivery.
Dynamic content - Content generated specifically for each user cannot be cached. However, CDN features like edge computing enable some dynamic functionality.
Mixed content - Most sites have both static and dynamic content. CDNs cache static assets whilst dynamic content goes through origin servers.
Proper cache configuration ensures static content is cached aggressively whilst dynamic content is not cached or cached minimally.
Cache Control
Controlling what is cached and for how long is critical:
Cache headers - HTTP headers (Cache-Control, Expires) control caching behaviour.
Time-to-live (TTL) - How long content is cached before refreshing from origin.
Cache invalidation - Forcing cache refresh when content updates.
Cache keys - Determining what constitutes different content (URL, query parameters, headers).
Proper cache configuration balances cache hit rates (better performance) with content freshness.
CDN Providers
Major CDN providers:
CloudFront (AWS) - AWS's CDN integrated with other AWS services. Strong for AWS users.
Cloudflare - Popular, flexible CDN with additional security and performance features.
Akamai - Large, global CDN serving many enterprises.
Fastly - Developer-friendly CDN with strong configuration options.
Netlify - CDN integrated with modern web deployment.
Each CDN has different strengths - choose based on your needs, existing relationships, and budget.
Edge Computing
Modern CDNs offer edge computing capabilities:
Serverless functions at edge - Running code at edge locations close to users.
Custom request processing - Modifying requests/responses at edge (authentication, compression, etc.).
Real-time decision making - Making decisions about routing, caching, and content based on user characteristics.
Edge computing enables sophisticated functionality whilst maintaining edge proximity benefits.
Security Features
CDNs often include security features:
DDoS protection - Absorbing and mitigating DDoS attacks at the network edge.
Web application firewall (WAF) - Filtering malicious requests before reaching origin.
Rate limiting - Limiting requests from single sources.
Bot management - Identifying and blocking bot traffic.
These features protect origin servers from attacks whilst maintaining legitimate traffic.
Performance Optimisation
CDNs enable performance optimisation:
Image optimisation - Automatically optimising images (format, size) for different devices.
Minification - Automatically compressing CSS and JavaScript.
Compression - Gzip and other compression reducing transfer sizes.
Protocol optimisation - Using HTTP/2 and HTTP/3 for performance.
Smart caching - Using machine learning to predict what content to cache.
Analytics and Monitoring
CDNs provide visibility:
Traffic analytics - Understanding traffic patterns, geographic distribution, popular content.
Performance metrics - Cache hit rates, latency, throughput.
Error tracking - Identifying issues and errors.
User experience metrics - Real user experience data from edge locations.
These insights enable optimising content delivery and identifying problems.
Cost Management
CDN costs require management:
Egress charges - Most CDNs charge for data delivered. Typical costs 0.085-0.12 USD per GB.
Request charges - Some CDNs charge per request.
Reserved capacity - Committing to minimum traffic enables discounts.
Optimising - Improving cache hit rates, compressing content, and preventing misuse reduces costs.
CDN at PixelForce
PixelForce leverages AWS CloudFront for content delivery across platforms. CloudFront integration with AWS services like S3 and Lambda enables sophisticated content delivery strategies. For applications with global users, CDN is essential for performance.
Cache Invalidation
When content updates, caches must be refreshed:
Time-based expiration - Content expires after configured TTL, then refreshes from origin.
Manual invalidation - Explicitly triggering cache refresh when content changes.
Version-based caching - Using content versions in URLs (v1, v2) to enable caching new versions without invalidation.
Partial invalidation - Invalidating only changed content rather than entire caches.
Origin Shield
Some CDNs offer origin shield:
Additional caching layer - Shield sits between edge locations and origin, absorbing repeat requests.
Origin protection - Reduces load on origin servers by absorbing requests at shield.
Improved hit rates - Shields increase overall cache hit rates.
Trade-offs - Additional latency and cost.
Conclusion
Content Delivery Networks dramatically improve global content delivery by serving from geographically distributed servers near users. By reducing latency, distributing load, and improving reliability, CDNs enable delivering fast, reliable experiences globally. For applications with global users or significant static content, CDNs are essential for performance.