With the advent of the DevOps and Infrastructure as Code movements, tools have emerged that allow building a complete HPC solution from scratch based only on open source software. At UCLouvain, one of our clusters, and the services on which it depends, is built on a full FOSS stack. From the operating system, to the deployment tools, monitoring, scheduling, and user software installation, everything is built from open source software that inter-operate gracefully. For instance, for deployment/provisioning, we use a combination of Ansible and Salt which we find work perfectly together even if they are often considered to be mutually exclusive. This talk will share our experience with making FOSS software co-operate smoothly and will offer our point of view on choosing the right tool for the right job. It will also present some of the contributions we have made to the open source community.