What is a CDN and how does it work

CDN, the full name of which is Content Delivery Network or Content Distribution Network, is a distributed network service that transmits content to users through the Internet. Its core purpose is to cache the website content by deploying server nodes around the world, so as to improve the response speed and availability of users visiting the website and reduce the load of the original server. According to Akamai's statistics, websites using CDN services can reduce the page loading time by nearly half and significantly improve the user experience.

Request routing is the primary link of CDN service, and its efficiency directly affects the user's access experience. According to Verizon's 2019 report, 50% of users expect the page to be loaded within 2 seconds, and the optimization of request routing can increase this ratio to more than 70%. CDN directs users' requests to the best edge nodes through intelligent DNS resolution or HTTP redirection technology. This decision-making process takes into account the geographical location of users, network conditions, load and health status of edge nodes and other factors. For example, Akamai's routing algorithm can complete this decision-making process in milliseconds, ensuring that user requests are routed to the nearest nodes quickly and accurately.

Content caching is one of the core functions of CDN, which reduces the delay and improves the response speed by storing the content copy of the source server at the edge node. According to the report of Limelight Networks in 2018, for every 1% increase in cache hit rate, the average loading time of websites can be reduced by 0.3 seconds. CDN nodes will store the content according to the preset caching policy. When users request resources, if the resources have been cached on the nodes, the nodes will directly provide the content, otherwise, they will request and cache copies from the source server. This mechanism not only reduces the request pressure on the source server, but also speeds up the delivery of content.

Content distribution is the key link for CDN to ensure that content is delivered to users quickly and reliably. When the edge node does not cache the content requested by the user locally, it will request the content from the source server, and at the same time, it will store the copy of the content locally for subsequent requests. According to the statistics of CDNetworks, the content distributed through CDN is more than 50% faster than that obtained directly from the source server. This distribution mechanism effectively makes use of the redundancy of the Internet and transmits content through multiple paths, thus improving the stability and efficiency of content transmission.

Load balancing is an important means for CDN to improve the usability and stability of the website. By distributing users' requests to multiple edge nodes, CDN can effectively reduce the load of the source server and prevent the server from being overloaded due to excessive visits. For example, during the double 11 Shopping Festival in 2019, Alibaba successfully handled hundreds of thousands of requests per second by using CDN services, ensuring the stable operation of the platform.

Performance optimization is another key link for CDN to improve user experience. CDN reduces the size of transmitted data and speeds up loading by compressing pictures, using more efficient coding formats and merging files. For example, by using the HTTP/2 protocol, CDN can compress the header information more effectively and reduce the delay. According to Google's 2016 report, the average page loading time of websites using HTTP/2 is 16% faster than that of websites using HTTP/1.1.

Security protection is an important part of CDN service, which ensures the security of website and user data by providing DDoS attack protection, data encryption and security certificate management. For example, the DDoS protection service provided by Cloudflare can resist the attack traffic up to 1.3 Tbps, ensuring the usability and security of the website. According to Imperva's 2019 report, the frequency of DDoS attacks on websites using CDN services is 70% lower than that of websites not using CDN.

Making full use of the performance optimization features provided by CDN, such as Brotli compression, HTTP/2 and HTTP/3, can further improve the transmission efficiency. For example, according to Google's report, the average page loading time of websites using HTTP/2 is 16% faster than that of websites using HTTP/1.1. In addition, the image optimization and video transcoding functions of CDN can also significantly enhance the user experience