Sysunit

Minimal system state management tool

Rationale

Configuration managment tools such as Ansible and Puppet, while fitting for many applications, are generally overly complex for basic environments. Shell scripts, on the other extreme, are generally brittle and difficult to maintain over time.

Sysunit aims to facilitate management of a single system's state by providing an execution engine that composes simple executables, written in any language, to idempotentally apply small, easily tracked changes to a system.

Units

Units are executables which are meant to be very simple, and apply only minor alterations to system state. They are invoked by sysunit to perform some operation.

For example, this unit will idempotently download a cat image to some target file:

#/etc/units/kitty

#!/bin/sh

set -eu

case $1 in
check) [ -f $target ] && echo "ok";;
apply) curl 'https://placekitten.com/500x500' > $target;;
rollback) rm $target;;
deps) echo pkg name=curl;;
esac

When applied with sysunit apply kitty target=~/pics/awww.jpg, sysunit will first invoke the unit with the equivalent of target=~/pics/awww.jpg /etc/units/kitty check, followed by target=~/pics/awww.jpg /etc/units/kitty rollback, if it is not present.

Directory Units

Units may contain data which needs a place in the filesystem, or supporting libraries which a unit author may wish to break out into their own files. Sysunit allows units to be provided as directories, which must contain a ./unit executable which behaves as a stand-alone executable unit would. This will be executed in its definition directory so accompanying files are easily referenced with relative paths.

Care should be given not to alter accompanying files in the unit's directory, as these changes will not be reverted at the end of unit execution and may result in subsequent unit executions having their behavior altered from intended.

Operations

• apply alters the system with the unit's target state if it is not present
• rollback removes the unit's target state from the system
• check determines whether the state which the unit applies is present
• deps provides a list of other units with the parameters which the state this unit affects is dependant upon

Configuration

SYSUNIT_PATH may contain a colon-delimited list of directories which will be searched for unit executables or directories.

Testing

• There's a docker_compose configuration provided with this project that runs some minimal Alpine containers.
• The ./test directory contains some files for provisioning of these containers and running specs (via Ruby's serverspec) against them.
• Use dc run test to run the primary integration specs against an alpine container.