> If someone could come up with a systemd replacement which manages to keep the systemd features while using a design philosophy more in line with that of Daemontools, that would be ...
... in the indicative rather than in the subjunctive, and in fact already mentioned here once. http://homepage.ntlworld.com./jonathan.deboynepollard/Softwa...
> The process manager gets killed. How do you recover?
In nosh terminology, this is the service manager. If it gets killed, the thing that spawned it starts another copy. This could be systemd, if one were running the service manager under systemd. It could be the nosh system manager. Of course, recovery is imperfect. If one designs a system like the nosh package, one makes an engineering tradeoff in the design; the same as one does when one designs a package like systemd. The system manager and the service manager are separate, but the underlying operating system kernel will re-parent orphaned service daemon processes if the service manager dies. One trades the risk of that for the greater separation of the twain, and greater simplicity of the twain. The program that one runs as process #1 is a lot simpler, being concerned only with system state, but there's no recovery in a very rare failure mode. Indeed, the simplicity makes that rarity even greater, if anything. systemd makes the tradeoff differently: there's recovery in a very rare failure mode (which I've yet to see occur in either system outwith me, with superuser privileges, sending signals by hand) at the expense of all of the logic for tracking service states, and for trying to recover them (in circumstances where one knows that the process has failed somehow and might possess corrupted service tracking data), all in that one program that runs as process #1.
> If you have respawn logic for it in PID 1, how do you log information about a failure to respawn the process manager?
In the log that is there for the system manager. See the manual page for system-manager, which explains the details of the (comparatively) small log directory and the (one) logging daemon that is directly controlled by the system-manager, both intended to be dedicated to logging only the stuff that is directly from the system manager and service manager.
> Perhaps you build in some basic logic for logging. Where do you store the data?
In a tmpfs, just like systemd-journald does in the same situation. /run/system-manager/log/ in this particular case. Strictly speaking, this "basic logging" isn't built-in. In theory, it is replaceable with whatever logging program one likes, as the system-manager just spawns a child process running cyclog and that name could be fairly simply made configurable. In practice, difficulties with the C++ runtime library on BSDs being placed on the /usr volume rather than the / volume, and indeed the cyclog program itself living on the /usr volume when it has to be under /usr/local/, have made it necessary to couple more tightly than wanted here, so far. But those problems could go away in the future; if the BSD people were persuaded to put the C++ runtime library in the same place as the C runtime library, for example.
> Most proposed alternative stacks to systemd falls down on the very first question above.
In many ways, that's because it's a poor question that focusses on a very rare circumstance. As I said, I've yet to see either system exhibit this failure mode in real-world use absent my deliberately triggering it. (Nor indeed have I ever seen it occur with upstart or launchd.) Much better questions are ones like "Where are inter-service dependencies and start/stop orderings recorded?", "Is there an XML parser in the program for process #1?", "What makes up a service bundle?", "How do system startup and shutdown operate?", "How does the system cope with service bundles that are on the /var volume when /var hasn't been mounted yet?", "How does the system handle service bundles in /etc when the / volume is read-only?", and "What does the system manager do?". Those are all answered in the package's manual pages and Guide, of course.