A Content Delivery Network (CDN) is a cache that sits between your website and the user.
This is useful when your site becomes popular and you don't want to have to keep scaling up your webserver to handle the load. This avoids the 'breaks the internet' moment when a website fails due to excessive load. My niece received a christmas present of an invisibility cloak that needs an app to make it work. The website could not handle the load of the majority of the sold products being activated on Christmas morning. Careful use of a CDN (amoung other techniques) can be used to avoid embarrasing mistakes.
CDNs are a great way to massively increase the peformance of your website.
Your website is the origin in this diagram.
By default this will start caching every request. Once you start adding cache headers to your pages then you can control how the CDN will cache the page. There are going to be parts of the site that you will not want to cache. Keep these seperated by a defined path structure to make your life easier (an /api prefix works well here).
The advantage of this is that the CDN will be able to provide content to your users far faster than you can as they will be closer to them. This can take 100ms away from a call to the USA from Europe. This will make a significant difference.
In addition the majority of the load on your site will be removed (the only visitors are the CDN nodes with an expired cache). If the site is static and the cache has been warmed up (each page visited) then it is possible to briefly turn off the origin site and still have the site up. This makes deploying a site that is under heavy load possible.
Another benefit is that you can set the CDN to serve stale content. This means that should a cached page subsequently start to fail then the server will return the previously cached version. This will greatly increase the reliability of your site.
One site that I worked on was able to handle heavy loads (tens of thousands of concurrent sessions on google analytics) despite the website being served from a single heroku node. This site was able to publish updates in around a second. We explicitly managed what we needed to cache, applied the update to our site and then told the CDN to invalidate the changed pages.
Typically a CDN will allow you to have brief bursts of heavy load and will only charge a small amount for the additional usage - which is much cheaper than having to keep scaling up servers.
There are several CDN's available to use, the one that I am most familar with is Fastly. Fastly is a distributed version of Varnish cache. Varnish cache is available to run as a docker image. This means that it is possible to test drive your CDN!
I am going to demonstrate how to configure the express web server to be cached via Varnish in a set of docker images. This will allow tests to be written to demonstrate that the caching is working as expected before deploying to a real environment.
I have written some sample code can be found here:
This demonstrates how to test a cdn in a docker container. This will become more useful if you want to have greater control over the configuration. Varnish is configured using Varnish Configuaration Language: Varnish Documentation - which allows complete control over how a website behaves.
For example you can check for a cookie and return a different page depending upon the value (logged in users get one, unauthenticated get another).
It's possible to conditionally add headers (so that the origin, while public will only respond if sent the correct header). This allows developers to check that the origin is healthy - which will makes isolating problems much easier.
You can also change the response so that details about the application are masked. It is amazing how many sites advertise the exact version of a webserver that they are using. This can be seen simply using curl: curl -v bbc.co.uk
You can use the CDN to alter the behaviour of a hosted website without changing the hosted site itself. This can allow you to apply a quick fix for a problem while a real solution is being prepared. For example you can put a holding page up for a specific url.
It's advisable not to tell a CDN to cache forever as any mistakes may reside in a browser indefinately - this can be a problem if you are caching a javacript library and accidently cache a broken version.
I would also recommend configuring your CDN programatically so that it is testable and repeatable. Be very careful of having the CDN do too much work as you will encounter odd edge conditions such as having an error page cached.
Make sure that you know how to purge a specific page. The site that I worked on added this to a chatbot in slack. It was able to purge a page by sending it a slack message - this was ideal when you had to support a problem and were away from your machine.
It is possible to use AWS S3 as a cache but it's advisable to add a CDN in front of it so that you can control the returned headers.
Learnings while preparing the sample.
Alpine docker images are reasonably fast to start and download compared to the full fat versions. These are the very small docker images
The tests need to wait on the varnish server being started. Normally I would have used the wait-for-it.sh script that allows waiting for a given service to be completed. However since I am using an alpine docker container we don't have bash available. This means that I have used the golang waitforit utility.
Also note the various network options used in the docker-compose. You can use an alias of another docker container name as a local dns entry if it is listed in the depends_on section. This is even accessable to the tests run inside the container.
If you configure your CDN to serve this Heroku app as the origin then you can completely test the behaviour of the CDN. Wiremock gives you a programmable web server that gives you an API that allows the response to change, so it's possible to have a page return a fix value once then start returning errors. This provides the ability for you to test the entirety of the HTTP spec should you wish to do so, but this is beyond the scope of this article.
Typically the CDN is controlled by an infrastructure team within your organisation. Allowing the configuration to be test driven will allow you to use far more of the advanced features of the CDN.