phantomfive's Journal: systemd - A descendent of Apple's LaunchD

Systemd is a descendent of Apple's launchd, Lennart suggested the world watch this movie to learn about it, so why? What is good about launchd?

OSX has an excellent inter-processing message system, part of the mach kernel (which they got from CMU). Because of this, many times processes talk to each other through the standard messaging queues. If you use a library to launch a browser window, or query wifi strength, it will communicate through a message queue (although the library takes care of the details).

The interesting thing is that launchd can open a message queue before the recipient is running. So launchd doesn't launch services until they are needed, and if they aren't needed, they never get launched. The benefit is that resources are preserved, things don't get launched until someone sends a message.

The benefit here is dependency resolution. Instead of forcing a deep calculation of what depends on what, or forcing the server script to declare everything it depends on, you can say "manage all these thousand services!" and only the few that are needed will be used, when they are needed.

Another thing I think Lennart copied from launchd is the declarative config files. You don't need to write a shell script, you merely need to indicate what should be launched, and maybe give it configuration options like "relaunch on failure" or "launch completely before receiving message." Of course, some people like config files, others prefer scripts.....that's not an argument I want to get into here, I'm merely pointing out things that seem to have come from launchd. (

What is bad:

Launchd is utterly undiscoverable. It would have been nice of them to put a README in /etc/rc/ or something saying, "hey, all your startup scripts are gone, look elsewhere."

Launchd has a complex hierarchy of directories that it searches for startup scripts. They have justifications for why, and a reason behind each one.......but justifications and reasons are not the same as good design. Every bad design decision ever made had a justification and a reason.

The README

[Lord_Jeremy]

Check out the manpages for launchd, launchd.plist and launchctl. There's your README, it's pretty extensive. Also, OS X in general has a hierarchy of configuration and system data.

/System/Library is the root of the hierarchy, intended for system-wide configurations that are part of the original installation. /Library is also system-wide but is intended for components that are added or updated by the user. ~/Library is the user-specific location. Each of these config locations has a LaunchAgents and a Laun