Boost Web Performance with CDN: Basics, How It Works, and Best Practices

From the PC era to mobile internet, users increasingly watch series, read news, stream videos, and play games online, raising expectations for user experience.

The “8‑second rule” once dictated that users would abandon a page if it took longer than 8 seconds to respond; today the tolerance has dropped to about 3 seconds.

To dramatically shorten page load times, Content Delivery Network (CDN) technology is essential.

1. What Is a CDN?

A CDN (Content Delivery Network) is a virtual network built on top of the existing Internet, consisting of distributed cache servers. By storing and delivering content from servers nearest to users, it reduces network congestion, improves response speed, and increases hit rates. In simple terms, a user's request is routed to the closest cache node, which serves the content, e.g., a Beijing user accesses a Beijing CDN node.

CDNs accelerate static resources such as images, HTML/CSS/JS files, videos, and documents, but cannot cache dynamically generated pages or data.

2. How CDN Works

Consider a Beijing user requesting a product image from a store in Zhejiang.

The client resolves the domain name via Local DNS to obtain an IP address.

Local DNS delegates the resolution to a CDN‑specific DNS server.

The CDN DNS returns the IP address of a Global Server Load Balancer (GSLB).

The user sends the image request to the GSLB.

The GSLB selects the cache server closest to the user’s region and returns its IP.

The user contacts that cache server. If the image is not cached, the server fetches it from the origin server, caches it, and then delivers it to the client.

If the cache server must retrieve content from the origin, this “origin pull” (回源) increases load on the source server. The cache‑hit ratio, or inverse “origin‑pull rate,” measures CDN effectiveness; a lower origin‑pull rate indicates higher cache utilization.

CDN cache policies follow HTTP standards, using headers such as Cache-Control: max-age and Expires to control freshness. Short cache lifetimes cause frequent origin pulls, raising source server load.

3. Advantages and Drawbacks of CDN

Benefits include faster static‑resource access, reduced latency, lower bandwidth and storage costs, and alleviated pressure on origin servers. CDNs also improve cross‑operator content delivery.

Drawbacks involve cache consistency challenges, requiring mechanisms like cache‑refresh or versioned resource naming to keep content up‑to‑date. Security and access‑control limitations may make CDN unsuitable for highly confidential media.

4. How to Use a CDN

Most public cloud providers offer CDN services (e.g., Alibaba Cloud CDN, Tencent Cloud CDN, Qiniu CDN). Selecting a CDN that matches your cloud platform simplifies integration and ensures consistent performance.

5. Summary

CDNs enable near‑user delivery of static assets, improving performance and reducing origin load. When adopting CDN, choose a mature cloud provider, configure appropriate cache lifetimes, and consider versioned filenames for frequently changing resources.