Defining a charm that supports multiple interfaces for the same relationship

[Bruno Girin]

Hi all,

I had a chat with Marco on Thursday about supporting multiple interfaces
on the same relationship and we couldn't come to a sensible conclusion
(beer may not have helped) so we concluded that we should put the
question to the wisdom of the mailing list. Here goes.

Use case: say you have a web app that needs to connect to a database and
supports either MySQL or PostgreSQL. How should I define that in
metadata.yaml and how do I handle it in the relation-changed hook?

If I specify this:

requires:
  db1:
    interface: mysql
  db2:
    interface: postgresql

it's a bit verbose and does not enforce the fact that I want either db1
or db2 but not both. Can I use the same name for both relationships? As in:

requires:
  db:
    interface: mysql
  db:
    interface: postgresql

Or even specify several interfaces for the same relationship:

requires:
  db:
    interface: mysql
    interface: postgresql

Or even better:

requires:
  db:
    interface: [ mysql, postgresql ]

Then, will I be able to call something like "relation-get interface" in
the db-relation-changed hook in order to install the correct client
libraries depending on the interface supported by the other side of the
relationship?

Cheers,

Bruno