I have a charm that is blocked on startup by a resource that is created by another charm:
https://github.com/juju-solutions/bundle-istio/blob/master/charms/istio-ingressgateway/reactive/charm.py#L46-L73
By default, Juju will run an update-status hook every 5 minutes, which means that charm can only check for the created resources once every 5 minutes, which means users of the charm have to wait a long time for it to completely start up. I can hack around this with
juju model-config update-status-hook-interval=30s
But this comes at the expense of CPU, as each other charm in the bundle also starts running their update status hooks more frequently. Particularly after all charms are started up, there's no reason to keep this small interval.
I would like a way as a charm author to say "I'm waiting on a resource that will be available soon, please poll me again soon".
This feature - allowing charms to register to received cluster events for things like resource lifecycle - is being implemented as a feature this cycle.