For the first three years of it's existence, the Raspberry Pi Dramble cluster was managed as a set of distinct servers, with a one-to-one relationship between Raspberry Pis and software (e.g. one load balancer, two webservers, two database servers).
In 2018, Jeff Geerling wanted to start learning Kubernetes, so he dug in and converted the Dramble architecture to run on containers managed by Kubernetes.
The work is ongoing—the Raspberry Pi Dramble project codebase's transition to Kubernetes is mostly complete, and you can already host a Drupal site on the cluster with Kubernetes easily, but deployments and cluster management are still being improved.
Much of this work will be documented and highlighted in Jeff's 2nd book, Ansible for Kubernetes (a companion to the best-selling Ansible for DevOps!).