Content Delivery Network (CDN)

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Definition

A Content Delivery Network (CDN) is a network of servers distributed across many geographic locations worldwide. The server locations are referred to as "Points of Presence", abbreviated PoP. These servers act as reverse proxy caches for web pages. When a user visits a website, his request is sent to the nearest CDN-PoP. The PoP either returns the requested website component from its cache or retrieves it from the server where the website is hosted (origin server).

CDN servers can cache HTML pages, JavaScript files, stylesheets, images, videos, and other static website content that does not require reloading every time a page is accessed. Because CDN PoPs are geographically distributed, they are often closer to users than the origin server of a website. This means that users can request a website more quickly and receive a response faster. Today, most web traffic is served by CDNs, including those from major web sites such as Facebook, Netflix, and Amazon

How a Content Delivery Network works

Content Delivery Network
Figure: CDN - Author: Seobility - License: CC BY-SA 4.0

A Content Delivery Network generally consists of five components. These include:

  • the origin server on which a website is hosted
  • numerous replica servers, which store copies of the website contents and make them available to users
  • a distribution system that distributes the content from the origin server to the replica servers
  • a request routing system that sends requests from users to suitable replica servers
  • an accounting system that logs data traffic and the status of the content delivery network

The distribution system of the Content Delivery Network has the task of distributing the content of web pages from the origin server to the replica servers. At the same time, the distribution system ensures the consistency and timeliness of the content. The distribution system either proactively distributes new or changed content to the replica servers, or sends a message to the servers in case of changes, informing them that stored content is obsolete and that new content must first be retrieved in case of a request. Both procedures are designed to ensure that content distribution is as efficient as possible.

The request routing system sends user requests to the replica servers in the content delivery network. This happens in two steps. In the first step, the request routing system uses the accounting system to determine which replica server is the most suitable to respond to the request. That request is then sent to the chosen replica server, which delivers the requested resources to the user.

The Benefits of a Content Delivery Network

The benefits of using a CDN may vary depending on the size and requirements of an internet property. In general, a CDN shortens loading times by distributing content across multiple servers. The globally distributed nature of a CDN reduces the distance between users and website resources. Instead of having to connect to the origin server of a web site, users can use a CDN to connect to a geographically closer data center. Because visitors tend to leave a slow page quickly, a CDN can reduce bounce rates and increase the time users spend on your website. Both metrics are important signals for Google and can thus influence your ranking in organic search results.

Through caching and other optimizations, CDNs can reduce the amount of data that an origin server must provide, significantly reducing its load. Smaller file sizes also mean faster loading times and reduce broadband usage costs.

Besides, using a CDN increases the availability and redundancy of content. For example, a CDN is more resistant to website interruptions due to high traffic or hardware failures. Load balancing distributes network traffic evenly across multiple servers, making scaling easier when traffic increases rapidly. Intelligent failover provides uninterrupted service even if one or more CDN servers go offline due to hardware malfunctions. Failover allows traffic to be distributed to the other servers.

If an entire data center has technical problems, anycast routing transfers traffic to another available data center to ensure that no users lose access to the website. Anycast is an addressing and routing method where a single address has multiple routing paths to two or more endpoints. Routers select the desired path based on the number of jumps, distance, lowest cost, latency, or least used route. A positive side effect is that a website is indexed internationally because of the anycast IP.

Last but not least, a content delivery network reduces the risk of DDoS attacks. A DDoS (Distributed Denial of Service) attack is a form of cybercrime aimed at triggering a system failure in the victim. DDoS attacks can not only cause large economic losses but can also have a serious impact on a company's reputation and image. The CDN intercepts these attacks before they can reach the origin server, keeping the site up and running.

CDN and duplicate content

Although there may be an unlimited number of pages associated with particular CDN content, that content is not considered duplicate content. CDNs use canonical headers to refer search engines to the original content on the origin servers.

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