Let's start off by looking at what an init system is, how they used to work and what systemd does different before we go into more systemd-specific details.
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system processes that are started include for example FS mounts, network settings, powertop...
system services are long-running processes such as daemons, e.g. SSH, database or web servers, session managers, udev ...
orphans: Process whose parent has finished somehow, gets adopted by init system
-> when a process terminates its parent must call wait() to get its exit() code, if there is no init system adopting orphans the process would become a zombie
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Before systemd there were simple init systems that just did the tasks listed on the previous slide.
Init scripts -> increased greatly in complexity over time, look at incomprehensible skeleton for Debian service init scripts
Runlevels -> things such as single-user mode, full multiuser mode, reboot, halt
Init will run all the scripts, but it will not do much more than print information on success/failure of started scripts
Init scripts run strictly sequential
Init is unaware of inter-service dependencies, expressed through prefixing scripts with numbers etc.
Init will not watch processes after system is booted -> crashing daemons will not automatically restart
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How systemd came to be
Considering the lack of process monitoring, problematic things about init scripts -> legacy init systems have drawbacks
Apple had already built launchd, a more featured init system that monitored running processes, could automatically restart them and allowed for certain advanced features -> however it is awful to use and wrap your head around
Lennart Poettering of Pulseaudio fame and Kay Sievers decided to implement a new init system to address these problems, while taking certain clues from Apple's design
