Create content_delivery_network.md

Author: Joseph Luce

system_design/content_delivery_network.md

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+# Content Delivery Network
+CDNs are separate servers which host a portion of your website or content.
+For example, a folder of html and javascript files for your website.
+Generally the user will require these files to access your website, so this content is the first and most important thing to deliver to the user first.
+Reducing the time between delivery is the reason why CDNs exist.
+
+By hosting CDNs across many geographical locations, it allows the content of a website/service to be deliveried quickly globally.
+Greatly reducing the latency in retrieving the content if your servers were only hosted in one part of the world.
+
+## Pros
+- Increases speed. Since location of the content is closer to the user, loading the website is quicker.
+- Reduces load. Since other servers are giving content to the user, less processing is required on the main server.
+- Increases uptime for the user. If one of the CDNs go down, another CDN can delivery that content that maybe closer.
+- Better security through obscurity. Since the main server is no longer on the receiving end of the user, your data is abstracted away from potential harm.
+
+## Pull Based CDNs
+Mainly used when a user or users from a remote geographic location requests for the first time.
+The closest CDN to that request will check if that content is avaliable, if its not, it will then ask the main server for that content.
+Then the main server will deliver the content to the CDN, the CDN then caches that request and finally returns the content to the user.
+
+So this is basically a process a request if asked approach.
+
+## Push Based CDNs
+
+## When to use CDNs?
+A great scenario to use CDNs is if most of the website content is static, say images, videos, documents that do no require input from your main server.
+You can think of this as having the client side of the content independent to the server side content.
+CDNs could also be used to host the static files while a different architecture or set of services handles the dynamic webpages if they were requested.