[RFC] Select which interfaces/integrations get installed

[Jake Nabasny]

A customer would like to be able to choose from the charm configuration which interfaces get installed for storage integration. For example, they would like an option to prevent the installation of "interface:mount" since they do not use NFS [1].

[1] https://github.com/charmed-kubernetes/charm-kubernetes-worker/blob/34ad8e0d4805b2b74f27f8c8d221d2ea5aa73319/layer.yaml#L22

[George Kraft (cynerva)]

This request doesn't entirely make sense to me. The interface layer is included in the charm when it is built. There should be no runtime installation occurring as long as the interface is not related to anything.

Is interface:mount causing problems for the customer? When they say "prevent the installation", what exactly are they asking for?

It's worth noting that we are about to rewrite the kubernetes-worker charm and other charms to use the Operator framework, which does not have layers or a layer.yaml file. We will however be including charm libraries to handle relation interfaces in a similar way.

[Jake Nabasny (slapcat)]

Sorry for the confusing description. Essentially, they just want the "nfs-common" package removed. I tested with removing that line from layer.yaml and the charm still deployed, but I'm not sure if this is entirely safe to do in case anything else depends on this package.

So the original request was to remove this package or offer a charm-level config key to prevent its installation.

[George Kraft (cynerva)]

Ahh okay, thanks for clarifying. Aside from nfs-common I can see that this could apply to other packages like ceph-common as well.

I would guess that nothing else is depending on nfs-common so I think it should be okay to omit it. It's not a scenario we've tested so I can't really say for sure.

It seems like we could probably move these apt packages out of the layer.yaml and into a charm config option that could be tweaked as desired. We'll need to make sure the documentation is quite clear that some of those packages may be necessary.