A CDN (Content Delivery Network) is a highly-distributed platform of servers that helps minimize delays in loading web page content by reducing the physical distance between the server and the user. This helps users around the world view the same high-quality content without slow loading times.
Mechanics
Generally speaking, transmitting data over the Internet is faster when the source data and recipient are closer together. One way to bring source data and recipients closer together is by caching copies of the source data in locations that are closer to the recipients. When a user needs the data, retrieving it from the closest location will be faster than retrieving it from the origin, which might potentially be much further away. This practice is commonly known as edge caching and is the role of the CDN.
How it works: CDN sits between users and origin
Edge servers are proxy caches that work in a manner similar to the browser caches. When a request comes into an edge server, it first checks the cache to see if the content is present. If the content is in cache and the cache entry hasn’t expired, then the content is served directly from the edge server.
If, on the other hand, the content is not in the cache or the cache entry has expired, then the edge server makes a request to the origin server to retrieve the information. The origin server is the source of truth for content and is capable of serving all of the content that is available on the CDN. When the edge server receives the response from the origin server, it stores the content in cache based on the HTTP headers of the response.
CDN “pulls” the assets from the origin “Just In Time” the first time they are needed.
