s6-rc is a service manager, or, in other words, a machine state manager: given a database of services, with dependencies between them, it can safely bring the global machine state, or live state, to the desired point, making sure dependencies are never broken.
The live state, by definition, is the tuple of all service states on the machine. Changing the live state means bringing services up, or bringing services down.
Supervision suites manage long-lived processes, a.k.a daemons, and sometimes call them services. With s6-rc, things are a little different: a long-lived process is also called a longrun and is a service, but a service does not have to be a longrun. There is a second kind of service, which is called a oneshot, and which represents a change of system state that is not embodied by the presence of a long-lived process. For instance, "mounting a filesystem" is a system state change, but in most cases there is no process hanging around while the filesystem is mounted.
s6-rc handles both oneshots and longruns.
Services can depend on one another. If service A has been declared as depending on service B, then s6-rc will make sure to never start A until it knows that B is up, and will make sure to never stop B until it knows that A is down. This works whether A and B are both oneshots, both longruns, or a oneshot and a longrun.
Oneshots and longruns are called atomic services. By opposition to atomic services, s6-rc also handles an additional kind of service that it calls a bundle. A bundle is just a collection of atomic services, described under a single name. A bundle definition can even contain other bundles, but ultimately a bundle will always represent a set of one or more atomic services. Bundle names can be used anywhere with the s6-rc user interface, and they will internally be converted to a set of atomic services. An atomic service can depend on a bundle: it will simply depend on all the atomic services represented by the bundle. A bundle, however, cannot have dependencies.
Unlike other service managers such as anopa, s6-rc separates the work of analyzing a set of service definitions, resolving dependencies, and so on, from the work of actually applying the dependency graph to perform live state changes. The former is the compilation phase, and is done offline; the latter is the live phase, and is of course done online - it impacts the actual state of the machine.
When the machine boots up:
The administrator can make changes to the live state of the machine by manually calling s6-rc again with the proper arguments. This is more powerful than the old runlevels: it is possible to change the live state to any set of services, not only predefined ones. The only thing that s6-rc will not allow is a state that would break service dependencies; it will always respect the dependency graph.
The s6-rc command is the engine for machine state changes, and it is also true for shutdown. When shutting a machine down, all the services managed by s6-rc should be brought down in the proper order (via the s6-rc -da change command). Once all those services have been brought down successfully, the final shutdown procedure can take place; for instance, if s6-svscan is running as process 1 with the s6-linux-init defaults, s6-svscanctl -6 /run/service will kill the supervision tree and call /etc/rc.shutdown reboot, which should reboot the machine.
The s6-rc command is a one-stop shop for service management as long as the compiled database doesn't change. If an administrator wishes to make a new compiled database the current live one, without rebooting the machine, a bit more work is needed, and that's the job of the s6-rc-update command. This command has been specifically written with Unix distributions in mind: when new packages ship, they come with new service definitions, or upgraded ones, and it is necessary to compile a new service database and update the live state so it matches; if source definitions for s6-rc are provided in the packages, an invocation of s6-rc-compile followed by an invocation of s6-rc-update should be all it takes to keep the live state up to date.
It is possible to change bundle definitions in a compiled service database, including the live one, without recompiling everything by calling s6-rc-compile. The s6-rc-bundle tool can edit compiled databases to add bundles to them, or delete bundles from them.