Is there a universal interface I can use?

Cory Johns cory.johns at
Wed Nov 22 17:51:56 UTC 2017

You can use the "juju-info" interface type for subordinates, but I'm not
sure that this is the correct thing for your case.

You're trying to attach the non-subordinate "database" endpoint (of
interface protocol "juju-info") on your charm to the "database" endpoint
(of interface protocol "cassandra") on the cassandra charm.  Because the
interface protocols don't match, Juju will not establish the relation.
Additionally, because you're not connecting to the subordinate endpoint
(scope: container) and instead to the standard endpoint, even if the
relation was established, you would never see an instance of your charm
created because it has no host application.

All charms have an implicit relation endpoint called "juju-info" of
interface protocol "juju-info" that you can connect to, and as long as the
interface protocol match and are unambiguous, you can omit them.  To
connect your charm using the "juju-info" interface protocol, you would need
to only define the "host-system" endpoint in your charm (as you currently
have it), leave off the "database" endpoint, and omit the endpoint portion
when adding the relation (or, if you want to be explicit about that, you
can use "juju add-relation cassandra-backup:host-system
cassandra:juju-info").  Then, for the reactive portion, you would use
"@when('host-system.available')" as the decorator.

However, I see that you also want to retrieve relation data that the
cassandra charm provides using the "cassandra" interface protocol.  Here it
becomes important to note that whether a relation is subordinate or not is
independent of the interface protocol the relation uses.  Thus, you could
mark the endpoint as "scope: container" and still use the "cassandra"
interface protocol.  You would then want to drop the "host-system" endpoint
from your charm, mark the "database" endpoint as "interface: cassandra",
and connect as you were trying to do.  However, I seem to recall that
Stuart (the maintainer of the cassandra charm) raised a possible issue with
subordinates using relations that the other end doesn't expect to be
subordinate, so there might be some concerns there.  I can't recall exactly
what the issue was, and I can't seem to find the bug report now.  Perhaps
Stuart can chime in.

A final concern is that I don't see an interface layer for the "cassandra"
interface protocol in the layer index (, meaning it
will be more difficult to use in a reactive charm.  Without the interface
layer, you wouldn't get the flags set to react to and would need to
manually inspect the relation data using charmhelpers.core.hookenv.  It
looks like your use case is fairly simple, though, so we can probably get a
very basic interface layer for it put together pretty quickly that Stuart
could then expand upon.  I'm happy to help with that, but with the holidays
in the US, I might be a few days before I can do so.

On Wed, Nov 22, 2017 at 10:02 AM, Tilman Baumann <
tilman.baumann at> wrote:

> I'm writing a reactive subordinate charm for cassandra.
> I can not find a interface for cassandra. But that's ok, since I don't
> really need a full blown database connection client.
> Easy I thought and just re-used the juju-info interface for fun and profit.
> requires:
>   host-system:
>     interface: juju-info
>     scope: container
>   database:
>     interface: juju-info
> And in the code
> @when('database.available')
> def db_changed(cassandra):
>     for conv in cassandra.conversations():
>         username = conv.get_remote('username')
>         password = conv.get_remote('password')
> ...
> However, that doesn't seem to work. Juju complains the relation doesn't
> exist.
> $ juju add-relation cassandra-backup:database cassandra:database
> ERROR no relations found
> So, is there a interface that I can (ab-)use in a similar way?
> I don't  want to build a full blown cassandra interface and at it to the
> list.
> Cheers and thanks
>  Tilman
> --
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