Comment 1 for bug 1871441

Confirmed. The etcd charm, when calling network-get for the db relation, does not pass in relation IDs. This means it does not fully support cross-model relations.

This won't be easy to fix. The etcd charm will need to call network-get with relation IDs for the db relation. The charm might observe different ingress addresses for each relation, so it will need to be able to send a different address to each relation. The etcd interface doesn't easily support this - it's still using the old RelationBase with scope=GLOBAL, meaning that the etcd units need to communicate with eachother via the cluster relation and agree on a single connection string before sending that off to db relations.

Most likely, fixing this will involve first updating interface-etcd to use the Endpoint class instead of RelationBase.