[Draft] Charm hooks

[timClicks (Tim McNamara)]

This document will supersede the original when Juju 2.8 is released.

Charm hooks

An application unit’s direct action is entirely defined by its charm’s hooks. Hooks are executable files in a charm’s hooks directory; hooks with particular names (see below) will be invoked by the juju unit agent at particular times, and thereby cause changes to the world. Whenever a hook-worthy event takes place, the unit agent first checks whether that hook is being debugged, and if so hands over control to the user. Otherwise, it tries to find a hook with precisely the right name. If the ho…