Note: This section was written by Wolfgang Jährling back in 2003 and documents the original design of what was then known as GNU dmd. The main ideas remain valid but some implementation details and goals have changed.
The general idea of a service manager that uses dependencies, similar to those of a Makefile, came from the developers of the GNU Hurd, but as few people are satisfied with System V Init, many other people had the same idea independently. Nevertheless, the Shepherd was written with the goal of becoming a replacement for System V Init on GNU/Hurd, which was one of the reasons for choosing the extension language of the GNU project, Guile, for implementation (another reason being that it makes it just so much easier).
The runlevel concept (i.e. thinking in groups of services) is
sometimes useful, but often one also wants to operate on single
services. System V Init makes this hard: While you can start and stop
init will not know about it, and use the runlevel
configuration as its source of information, opening the door for
inconsistencies (which fortunatly are not a practical problem
usually). In the Shepherd, this was avoided by having a central entity that is
responsible for starting and stopping the services, which therefore
knows which services are actually started (if not completely
inproperly used, but that is a requirement which is impossible to
avoid anyway). While runlevels are not implemented yet, it is clear
that they will sit on top of the service concept, i.e. runlevels will
merely be an optional extension that the service concept does not rely
on. This also makes changes in the runlevel design easier when it may
The consequence of having a daemon running that controls the services is that we need another program as user interface which communicates with the daemon. Fortunatly, this makes the commands necessary for controlling services pretty short and intuitive, and gives the additional bonus of adding some more flexibility. For example, it is easiely possible to grant password-protected control over certain services to unprivileged users, if desired.
An essential aspect of the design of the Shepherd (which was already mentioned above) is that it should always know exactly what is happening, i.e. which services are started and stopped. The alternative would have been to not use a daemon, but to save the state on the file system, again opening the door for inconsistencies of all sorts. Also, we would have to use a seperate program for respawning a service (which just starts the services, waits until it terminates and then starts it again). Killing the program that does the respawning (but not the service that is supposed to be respawned) would cause horrible confusion. My understanding of “The Right Thing” is that this conceptionally limited strategy is exactly what we do not want.
The way dependencies work in the Shepherd took a while to mature, as it was not easy to figure out what is appropriate. I decided to not make it too sophisticated by trying to guess what the user might want just to theoretically fulfill the request we are processing. If something goes wrong, it is usually better to tell the user about the problem and let her fix it, taking care to make finding solutions or workarounds for problems (like a misconfigured service) easy. This way, the user is in control of what happens and we can keep the implementation simple. To make a long story short, we don’t try to be too clever, which is usually a good idea in developing software.
If you wonder why I was giving a “misconfigured service” as an example above, consider the following situation, which actually is a wonderful example for what was said in the previous paragraph: Service X depends on symbol S, which is provided by both A and B. A depends on AA, B depends on BB. AA and BB conflict with each other. The configuration of A contains an error, which will prevent it from starting; no service is running, but we want to start X now. In resolving its dependencies, we first try to start A, which will cause AA to be started. After this is done, the attempt of starting A fails, so we go on to B, but its dependency BB will fail to start because it conflicts with the running service AA. So we fail to provide S, thus X cannot be started. There are several possibilities to deal with this:
I hope you can agree that the latter solution after all is the best one, because we can be sure to not do something that the user does not want us to do. Software should not run amok. This explanation was very long, but I think it was necessary to justify why the Shepherd uses a very primitive algorithm to resolve dependencies, despite the fact that it could theoretically be a bit more clever in certain situations.
One might argue that it is possible to ask the user if the planned actions are ok with her, and if the plan changes ask again, but especially given that services are supposed to usually work, I see few reasons to make the source code of the Shepherd more complicated than necessary. If you volunteer to write and maintain a more clever strategy (and volunteer to explain it to everyone who wants to understand it), you are welcome to do so, of course…