A content delivery network (CDN) is a system of servers spread through­out different data centers located around world. This proxy server network is ideal for extending your website’s reach or expanding the customer base of your web ap­pli­ca­tion. Using such a network means that your webserver connects to many other servers. And with the help of a dis­tri­b­u­tion system, your webserver’s static content can be trans­ferred to replica servers. A request routing system de­ter­mines which replica server in the CDN is best suited for de­liv­er­ing data when users access certain content on your website. Depending on the provider, CDNs vary in several ways: the dis­tri­b­u­tion and number of servers as well as the degree to which high-per­for­mance backbone networks are in­te­grat­ed are a few examples of how they can differ. 

How data is delivered in CDNs

When accessing in­for­ma­tion through a CDN, users are unaware of whether they are accessing data directly from a given website’s exit server or from a replica server. Most users never really catch wind of this process and will only notice the shorter loading times, if anything at all.

The most important aspects regarding the two com­po­nents involved in de­liv­er­ing data within CDNs are the dis­tri­b­u­tion system and the request routing system. The dis­tri­b­u­tion system transfers content to the replica server and also ensures that the right data has been sent and is up to date. There are two ways of going about this process: one method occurs when the dis­tri­b­u­tion system im­me­di­ate­ly sends new and modified data to all other servers within the Content Delivery Network. Another option involves sending the replica server a message telling the CDN that the updated content is available and can be requested when needed. In order to ensure that user requests find their way to the right servers, a request routing system is used. In just two steps, this mechanism makes sure that requests are rerouted in the best possible way.

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Wanted: the right replica server

For the first part of this process, the most suitable server for answering the user’s request needs to be located. The request routing system uses key in­di­ca­tors that are cal­cu­lat­ed with the help of a host of different mea­sure­ment tech­niques. These various in­di­ca­tors identify, for example, whether a client-side or server­side con­nec­tion is at hand. By using server­side readings (e.g. general CPU uti­liza­tion or the number of active server hits), the replica server with the smallest usage rate is iden­ti­fied. By taking into account the prop­er­ties between the client and the server, the request routing system iden­ti­fies the server with the best con­nec­tion to the client. Latency period, the average transfer rate, packet loss rate, and ge­o­graph­ic proximity are all factors of a CDN that are con­sid­ered during this step. Client side key in­di­ca­tors reveal the identity of users re­quest­ing access. This in­for­ma­tion is then used by the CDN should content be delivered in varying degrees of quality (i.e. standard vs. premium customers).

The weighting of various key in­di­ca­tors always depends on the data that’s being requested: dy­nam­i­cal­ly generated content generally requires servers with good key in­di­ca­tors, while a stable con­nec­tion between the client and server is the most important re­quire­ment when using streaming media.

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For­ward­ing requests

Once the target server has been de­ter­mined in the CDN, the second step in this processes consists of for­ward­ing the requests through the request routing system. There are many different methods for this ‘request redi­rec­tion’ process. Most of them vary from one another in that they are directed from different locations: from the client, in the network, or on the replica server.

Directing the request from the client is a rel­a­tive­ly simple method to carry out, and the client also has a large selection of replica servers at their disposal. The dis­ad­van­tage here is that the content delivery network has very little influence in al­lo­cat­ing the client to the right server. The server­side HTTP-302 redi­rec­tion status code is also easy to implement and is equipped with useful in­for­ma­tion on the CDN. In this case, the exit server replies to the request with the status code mentioned above, and through this informs the client about the best suited replica server, which is then au­to­mat­i­cal­ly contacted through a further request. Doubling the required con­nec­tions is the main dis­ad­van­tage of this process.

The most popular for­ward­ing method is known as DNS-based request routing. Requests placed by clients on a local DNS server are forwarded to a special DNS server within the content delivery network, which in turn returns the IP address of a replica server. The biggest dis­ad­van­tage of this process is that con­stant­ly looking up in­for­ma­tion in the domain system leads to access delays.   

The perks of a CDN

Thanks to a global network of servers scattered across the globe, customers benefit from quicker loading times and smaller burdens on band­widths. But operators also have some con­sid­er­able ad­van­tages with CDNs: by for­ward­ing queries, not only are server occupancy rates minimized, cached data is also more secure. This is because security breaches, like DDoS attacks, don’t reach the exit servers at first. Re­quire­ments for pro­tect­ing cached data include keeping user names and passwords out of the CDN, as this increases the chances of a suc­cess­ful attack. Assigning a custom domain for the out­sourced CDN content is well worth the work, since this prevents attacks from spreading onto other parts of the website. Here’s a summary of all the ad­van­tages of a CDN:

•    Static elements like HTML pages, images, stylesheets, documents, or client-side scripts are buffered in the replica server’s cache and needn’t be requested from your webserver any longer. This spares your server from overuse and also shortens the loading time for users. All of this leads to lower bounce rates and good search engine rankings.
•    The limited usage of the server as well as all involved ap­pli­ca­tions protects your server from overuse and other potential threats, such as outages. This has a positive effect on the hardware needed for hosting.
•    Server operators can decide which static elements clients in the content delivery network are supposed to download. For example, should you only wish to store images from your blog, they can then be loaded alongside the rest of the page, ensuring a faster overall loading time.
•    When it comes to streaming videos and music, websites benefit from delivery with CDNs, as large sums of data need to be processed over a short period of time. This also counts for live-streams.
•    Spikes in traffic and in­creas­ing visitor numbers are easy to overcome with a CDN. The request routing system’s search for replica servers with free capacity is es­pe­cial­ly worth­while when working with files that are available for download. It’s also ben­e­fi­cial for those operating online stores.
•    CDNs don’t only cut down on loading times, they also limit the bandwidth usage that is connected with the in­for­ma­tion request of your content.
•    The security of your server, data, and used ap­pli­ca­tions increases by using a CDN. This is because attacks can be detected and dealt with early in the game.

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