What's a "system service"? How do we model that in GuixSD? In most people’s mind, system services are a bunch of daemons that simply needto be started at boot time, or whenever they are actually needed. Possiblyservices form a dependency graph, and possibly they are actions other thanspawning a daemon, such as mounting a file system. As always, the devil is in the detail, and reality is that “system services”on a modern GNU/Linux system—with udev, dbus, polkit, along with moretraditional Unix services—include lots of different “activities”, with lots ofinteractions among them. That naive picture of a graph of services no longerworks. This talk is going to tell the story of system services in GuixSD. GuixSDstarted from the naive visions of a “dependency graph of actions” to evolveinto a generic model of service composition. I will describe what makesGuixSD’s service composition model unique, and how it helps users andsysadmins reason about the whole system.