Raph’s Blog

Demystifying CDN and the Fastly Outage

Similar to my previous post about Demystifying Cyber Security Jargon this post will attempt to demystify CDN jargon to non-technical people. In the hope that an understanding of technical concepts become more accessible to more people.

Recently, the media has been reporting on the fastly outage that had caused a raft of high traffic websites to go down. These websites include: Reddit, Amazon, CNN, and the New York Times. The scale of this list meant that a large portion of the internet was partially inaccessible for a small period of time. A post-mortem of the incident has been published by fastly on their blog.

Fastly, is one of the world’s fastest, largest, and growing providers of CDN services. For a service to be down for such a small period of time, and bringing down with it such a large portion of the internet, is in someway impressive. As it suggests that a large portion of the internet was using those services.

Context (How a Web Page is Loaded)

Before any explanation of CDN is attempted, an explanation of how a page is loaded on a browser is needed in order to provide context.

When a human goes to a website such as https://blog.raph.ws a series of other requests is sent to a collection of computers (called servers). These requests then return responses in the form of content. This content is then put together on the browser and the page is then rendered to what a human can consume.

Below is a list of ‘requests’ that are made when loading https://blog.raph.ws:


What is a CDN?

CDN stands for “Content Delivery Network” and it provides content to humans who browse websites. On a rudimentary level, content on the internet can be split into two distinct categories: static content and dynamic content. Static content may include things like: images, style-definitions, videos, and instructions on how elements on a page interact with a human (JavaScript code).

For example: if an email inbox is browsed, the logo of the provider on the top left can be thought of static content, and the list of emails in the center is dynamic content. The image will not change no matter how many times the page is refreshed, however, the email list will change depending on when emails are received.

Why is CDN needed?

The primary reason why CDNs are relied upon is optimising load time of a web page on the internet. They are primarily used for static content, and can increase performance (that is decrease the loading time of a web page) of this content through a variety of techniques. Many of these are explained below:

Less Overhead

When dynamic content is requested from a server (for example: a list of emails) that request will need to go through several other servers in order to receive the list of emails, even if that list of emails has not changed. Each additional server that is needed, creates an overhead in the form of time to load the entire web page. This is because when new content is available is unknown to the human sending that request, so each and every dependant server is needed to return accurate content.

In addition to this overhead, dynamic content needs to execute more code than static content. For example: determining which content to return depending on the route requested (a route is anything that comes after a domain name; on the address bar of this browser, anything that comes after https://blog.raph.ws/ is considered a ‘route’). Routes are a lot more complicated in dynamic content types because they are often constructed with a lot more logic associated with them, whereas static content routes are often simple and have a level of determinism.

Each additional dependant server and each additional code execution adds time to a web page load, which may result in a degraded user experience.

Static content is different than dynamic content, it does not need additional code execution or dependant servers, and often, having the same servers deal with both static content and dynamic content adds an overhead associated when dealing with static content. This is why having a CDN that’s optimised for static content becomes important to performance. CDNs do not have all the additional overheads associated with servers optimised for dynamic content, so they are able to deal with static content in a performant way.

Global Servers

Dynamic content is costly to replicate. For example: a list of emails often exists in a small(er) collection of servers. This is because the list is constantly changing and the compute and storage costs associated with data replication is high. Static content on the other hand, is not costly to replicate because by its very nature, it does not change regularly.

CDNs can replicate ‘where’ this content is stored and store it on a large number of servers dispersed around the world. When a human requests content from a CDN, the CDN has at its disposal multiple servers around the world it can choose. Generally, the closer the server is to the requesting human, the quicker the content will be retrieved. For example: if a human in Sydney, Australia is requesting the same content from servers in Melbourne, Australia and New York, USA (and all other things being equal apart from this one variable); then that content will be delivered to by the server in Melbourne, Australia more quickly.

Browser Caching

In addition to replicating static content on servers around the world, a replication mechanism happens on the user’s computer in the form of ‘caching’. If the same content is requested more than once, more often than not, it is stored on the human’s browser so that they don’t need to request it again. Thus decreasing the overall load time of a website.

For example: websites are generally built using a series of ‘code libraries’ and these code libraries are used across different websites. For example: https://blog.raph.ws may use the same code library as https://www.australia.gov.au. If both these websites used the same CDN to request the same library, then the website that the human goes to second, will not need to download the content again as it is ‘cached’. This increases the overall page load time.

Below is the same list of requests as above. The requests where the ‘Transfer Size’ is (memory) indicate content that is ‘cached’:



There are several more ways that CDNs can optimise content loading times apart from the ones outlined above (such as multimedia optimisation), however, it is not within the remit of this post to explain them all.

Whilst CDNs are great for performance, when they do go down, the websites that use them will go down with them. In the case where a particular CDN is used by a large cross-section of websites on the internet, it may degrade the availability of that cross-section of websites that use it. Like the case of the fastly outage.

I hope this post helped you understand CDNs in a bit more detail in the context of the fastly outage and thank you for reading this post!