Fastly is a CDN provider that uses Varnish. By changing the CNAME of a sub domain, it is possible to serve a response from Fastly. If a Varnish cache record exists, this is served to the user, if not, Fastly makes a request to the web server (Rackspace) and saves the result for future requests. This allows us to serve all current Varnish requests through a CDN. The advantage of this is that the pages will be delivered much quickly in different locations around the global. Another advantage is that you are not reliant on a single Varnish server so there is less chance of suffering a complete loss of caching.
There is an option to soft purge an item, https://docs.fastly.com/guides/purging/soft-purges. This allows you to serve old content while the varnish record is updated in the background.