#bytescrolls: Content Delivery Network

For the better performance of a website, the use of Content Delivery network as a backbone provides internet bandwidth efficiently.

What are CDNs ?

A content delivery network (CDN) is a collection of web servers distributed across multiple locations to deliver content more efficiently to users. The server selected for delivering content to a specific user is typically based on a measure of network proximity. For example, the server with the fewest network hops or the server with the quickest response time is chosen.

When we request a page, the browser requests to particular domain only. If any other hypermedia contents are embedded they are loaded by the browser by requesting to the resource from the location domain.These assets are originally located in a server. Most browsers support only a few persistent requests to the same domain. It is possible to direct the clients' requests to a surrogate server or a network is commonly called Request routing or Content routing or Content Redirection. It is used in the Content Delivery Networks (CDNs).Content Networks deal with the routing and forwarding of requests and responses for content. Replication is a well-known technique to improve the accessibility of Web sites. But replicating resources may not be an ideal solution.Which objects are selecting the objects to replicate? Object selection is directly related to the granularity of replication. In practice, whole Web sites are taken as the unit for replication, coz its been observed that grouping Web documents can considerably improve the performance of replication schemes at relatively low costs. These CDNs provide replication for digital content providers. These web replica hosting systems host the documents of a Web site and manage replication automatically.Nowadays the web gives more relevance to content as more and more users seek the web for data. The burst of video streaming and social networks made web to imbibe more data than ever. It grows exponentially as the web provides a potential market for the new gen business. These content networks can be seen as a new virtual overlay to the OSI stack: a "content layer", to enable richer services that rely on underlying elements from all 7 layers of the stack. The CDNs are an upgraded idea of caching proxies. They were used by ISPs for the benefit of customers using dial up or cable modem. Server farms are also used to improve performance of a website which is a form of content network. They worked as if the groups of servers as a single server through a load balancing mechanisms. The requests are rooted to the resource for a failure situation. They have an important role is maintaining state management. By placing static files (css/js/images) to a cdn sub domain, visitors can load content with twice the allowed amount of persistent requests. CDN nodes are deployed in multiple locations, often over multiple backbones. These nodes cooperate with each other to satisfy requests for content by end users, transparently moving content behind the scenes to optimize the delivery process. Optimization can take the form of reducing bandwidth costs, improving end-user performance, or both.The number of nodes and servers making up a CDN varies, depending on the architecture, some reaching thousands of nodes with tens of thousands of servers.Requests for content are intelligently directed to nodes that are optimal in some way. When optimizing for performance, locations that can serve content quickly to the user may be chosen. This may be measured by choosing locations that are the fewest hops or fewest number of network seconds away from the requestor, so as to optimize delivery across local networks. When optimizing for cost, locations that are less expensive to serve from may be chosen instead. Often these two goals tend to align, as servers that are close to the end user sometimes have an advantage in serving costs, perhaps because they are located within the same network as the end user.

Remember that 80-90% of the end-user response time is spent downloading all the components in the page: images, stylesheets, scripts, Flash, etc. This is the Performance Golden Rule. Rather than starting with the difficult task of redesigning your application architecture, it's better to first disperse your static content. This not only achieves a bigger reduction in response times, but it's easier thanks to content delivery networks.

Akamai is one of the largest CDNs currently deployed, with tens of thousands of replica servers placed all over the Internet. To a large extent, Akamai uses well-known technology to replicate content, notably a combination of DNS-based redirection and proxy-caching techniques.Akamai transparently mirrors content (usually media objects such as audio, graphics, animation, video) stored on customer servers. Though the domain name is the same, the IP address points to an Akamai server rather than the customer's server. The Akamai server is automatically picked depending on the type of content and the user's network location.In addition to image caching, Akamai provides services which accelerate dynamic and personalized content, J2EE-compliant applications, and streaming media to the extent that such services frame a localized perspective.

RFC 3568

ACM Article

Akamai