CDN: Content Delivery Network
CDN: Content Delivery Network
Created on Aug 24, 2025, Last Updated on Aug 25, 2025, By a Developer
Content Delivery Network (CDN) is a geographically distributed network of servers that store cached copies of website content closer to users.
When user try access internet resource in other countries or even continents, the internet traffic will go from local network domain all the way up to cross-sea network cable, and come back with all the response. This is necessary for at least the first time, but can be optimized for the following requests.
Content Caching
- CDN provider creates network with servers at edge and a lot of peering agreements with network providers.
- Content provider signs up with CDN provider and specifies Origin Server.
- Client Requests go to CND servers.
- CDN server cannot find contents, and contacts origin server and cache the response.
- CDN serve cached content for subsequent accesses.
Live Video Streaming
Traditionally, live video streaming require huge amount of network bandwidth, since server need to send traffic to all users.
But with help of CDN, servers no longer do unicast to perform many point to point transmission, but perform multicast, only talking to one router, and let router to send traffic to multiple endpoint when necessary.
This approach saves huge amount of bandwidth from the server aspect. From users’ perspective, they are not talking to streaming server any more, but pulling from CDN edge server instead.
Application Delivery
Not all content is cacheable, a lot application data is highly dynamic, changing from moment to moment. CDN cannot be used to cache them, but application can still benefit from CDN.
- Path optimization: CDN can allow traffic flow through their private path, which is faster than public Internet.
- Packet loss reduction - CDN network is somewhat more reliable. On top of that, if packet retransmission is required, this can be done in the edge network level, without requiring a full round trip.
- Transport protocol optimizations (e.g., QUIc, HTTP/3 for reusing connection).
- Application optimization (parse and prefetch content)
DNS Based Load Balancing
One domain can be mapped multiple IP addresses on DNS, CDN can perform load balancing at this level. This can be treated as an alternative to normal Load Balancer.
Edge Function
Apart from caching, CDN can serve a some functionality closer to user as well. The middleware functionality a lot frontend framework provides is actually leveraging this, which is commonly used for request rejection, redirection, request overwrite and etc. These operations are light weight and not really have to happen on the source server.