What is CDN?
Also known as: Content Delivery Network
A CDN (Content Delivery Network) is a geographically distributed network of proxy servers and data centers that deliver web content to users from the nearest edge location, reducing latency and offloading origin servers.
A Content Delivery Network (CDN) is a system of interconnected servers placed at multiple geographic locations, called Points of Presence (PoPs), that cache and serve static and dynamic content to end users. The core idea is to shorten the physical distance between the user and the content, thereby reducing round-trip time and improving page load speed. When a user requests a resource, the CDN routes the request to the nearest edge server that holds a cached copy, rather than sending it all the way back to the origin server.
CDNs work through a combination of DNS-based request routing, anycast or geographic load balancing, and distributed caching strategies. The origin server pushes or pulls content to the edge nodes, which then serve subsequent requests directly. Modern CDNs also handle dynamic content acceleration, TLS termination, DDoS mitigation, and edge computing tasks via serverless functions. The most common caching protocols are based on HTTP cache headers such as Cache-Control, Expires, and ETags, as defined in RFC 7234.
CDNs are a foundational component of the modern internet, used by nearly all major websites and streaming services. They sit between the user and the origin infrastructure, acting as a reverse proxy layer. Without CDNs, global latency would be significantly higher, and origin servers would face unsustainable traffic loads. Major commercial CDN providers include Akamai, Cloudflare, Fastly, and Amazon CloudFront, while open-source alternatives like Varnish and Apache Traffic Server are also widely deployed.
Key facts
- CDNs reduce latency by serving content from edge servers closest to the user's geographic location.
- They offload origin servers by handling a large percentage of total traffic, often over 90%.
- CDNs use HTTP caching headers (RFC 7234) to control how long content is stored at the edge.
- Many CDNs provide DDoS protection by absorbing attack traffic across their distributed network.
- Edge computing capabilities allow CDNs to run serverless functions close to users for low-latency processing.
How it works in practice
Related terms
References
More in CDN & Performance
Apdex Score
The Apdex Score is a standardised metric that measures user satisfaction with application performance by comparing response times against predefined target and tolerable thresholds.
Bandwidth Throttling
Bandwidth throttling is the intentional reduction of data transfer speed by a network operator or service provider to manage congestion, enforce usage policies, or control costs.
Brotli Compression
Brotli is a lossless compression algorithm developed by Google, offering higher text compression ratios than gzip, used by CDNs to reduce page load times.
Cache Hit
A cache hit occurs when a requested resource is found in a CDN edge cache and served directly to the client, bypassing the origin server entirely.
Cache Invalidation
Cache invalidation is the explicit removal of stored web objects from a cache so that new requests must revalidate or refetch them from the origin server.
Cache Miss
A cache miss occurs when a requested resource is not found in a CDN or proxy cache, forcing the request to be forwarded to the origin server and then storing the response for future requests.
Core Web Vitals
Core Web Vitals are a set of three real-world user experience metrics (LCP, INP, CLS) defined by Google to quantify loading, interactivity, and visual stability on web pages.
Cumulative Layout Shift
Cumulative Layout Shift (CLS) is a Core Web Vital metric that measures the sum of all unexpected layout shift scores during a page's lifespan, quantifying visual stability.
Edge Computing
Edge computing is a distributed computing model that processes data and runs application logic at Points of Presence (PoPs) close to end users, minimizing round-trip latency and bandwidth usage compared to centralized cloud regions.
Image Optimization
Image optimization reduces image file size by selecting modern formats (WebP, AVIF), resizing to display dimensions, and tuning quality, improving page load speed and bandwidth usage.