When the conversation drifts to CDN and Cache servers, many people wrongly assume that CDNs need caching technologies to transfer and distribute information. Not exactly. IT specialists prefer term “delivery node” to the term “cache”, frequently applied in this case. What is the difference between CDN and Cache server then? Let’s clear it up.
| Cache server | CDNs |
| Has web cache, software that tracks and temporarily stores web objects got via HTTP. | Has delivery node within that serves to deliver these objects to users. Can be both hardware or software. |
| Cashing is a part of HTTP that is needed to define such mechanisms as validation. | CDN has some cache functions, but may work without caching, exploiting some other mechanisms. |
| Caching is not a registered part of not-HTTP, RTSP, RTMP and other streaming protocols. | In this case, streaming protocols are not cached (it is not supposed by the technology). Instead, streaming objects may be transferred to delivery node with the help of multicasting or unicasting. |
| Cashes are good for small objects as images, documents and HTML, while videos slow it down. | CDN uses several technologies to pull video files, which greatly improves its effectiveness. |
| As a rule, web caches are placed in the way between the original HTTP source and its end users. Being called transparent caching, this technology serves to catch HTTP objects in the Web. | When CDNs are operating, web cache is not placed between the original HTTP source and end users. It’s CDN that redirects a user to the cache. To be able to distribute content via CDN, the operator must have legal or commercial relationship with publisher of content. |
Thus, in order to function properly, CDN should use not only HTTP delivery, but other technologies, such as pull (caching), push and relaying technologies for streaming protocols.
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