Init process
Main, first started process (typically id = 1, daemon)
Started by kernel, if start is unsuccessful, kernel panic occurs.
Multiple init implementations
Upstart
systemd
Maintains userspace applications
Each application is described in unit files. They use more declarative syntax instead of bash scripts.
Typically located in /etc/systemd/system
unit file
Contains information nearly about anything: a service, socket, device, mount point, etc.
File name is used for control (via systemctl), though unit file can declare aliases (Alias=) that can be used too.
service declaration
Typically systemd controls one service but it is possible to spawn and control multiple instances of service.
In order to enable multi instance management:
1. create service file with @.service suffix
2. in unit file use %i placeholder for "some application instance"