Comment 3 for bug 1940595

The particular use case is for updating machines in Landscape, to allow a list of applications hosted on each machine to be used to do things like "update all the packages on the Keystone units in AZ1", or operations like that.

For that, I'd want the landscape-client subordinate charm to be able to 'look' at the machine, and obtain a list of all the 'other' units on the machine (including units that are principal units or subordinates unrelated to the charm itself).

One example:

Machine 15 has two principal units:
nova-compute
ceph-osd

each of those has a variety of subordinate units for things like monitoring etc.

I'd like landscape-client, which is (e.g.) a subordinate of nova-compute, to know about the subordinates of ceph-osd, even though there's no relation at all. That's easy right now with a simple 'ls /var/lib/juju/agents' (for linux, at least), this is more a request to be able to get that list using a tool within the charm that is more operating system independent. In this example, that list (along with other environmental info like availability zone) gets sent to Landscape for adding tags.