@when('config.changed')

[Marco Ceppi]

This is awesome, glad to see this wrapped in the reactive framework. Will
make a lot of my layers much simpler!

Marco

On Wed, Feb 17, 2016 at 1:44 PM Cory Johns <cory.johns at canonical.com> wrote:

> Just wanted to give a heads-up about a feature that landed in the reactive
> base layer yesterday.
>
> If a charm config option has changed, the state "config.changed" will be
> set for the duration of the hook.  Additionally, specific states will be
> set for each config option that changed; that is, if option "foo" has
> changed, the state "config.changed.foo" will be set.  An example of code
> using this would be:
>
>     @when('myservice.started', 'config.changed')
>     def update_config():
>         update_config_files()
>         restart_myservice()
>
> This provides a much cleaner way of detecting changes to config, and it is
> recommended that this be used in favor of @hook('config-changed') going
> forward, as the latter can actually run in to some situations, albeit
> rather rarely, where the charm sees new config option values before the
> config-changed hook has fired.  Using the reactive states avoids that
> completely as well as working more naturally with existing @when decorators.
>
> Please note that, while we are not aware of any charms currently using
> "config.changed" as a state, there is some risk of the state set by the
> base layer conflicting with it if set by the charm layer.  The
> recommendation is to always prefix your states by the name of the layer
> setting them,  or the relation name for interface layers.
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