sunbeam-charms/ops-sunbeam/doc/howto-config-context.rst

How-To Write a config context

A config context is an additional context that is passed to the template renderer in its own namespace. They are usually useful when some logic needs to be applied to user supplied charm configuration. The context has access to the charm object.

Below is an example which applies logic to the charm config as well as collecting the application name to construct the context.

class CinderCephConfigurationContext(ConfigContext):
    """Cinder Ceph configuration context."""

    def context(self) -> None:
        """Cinder Ceph configuration context."""
        config = self.charm.model.config.get
        data_pool_name = config('rbd-pool-name') or self.charm.app.name
        if config('pool-type') == "erasure-coded":
            pool_name = (
            config('ec-rbd-metadata-pool') or
            f"{data_pool_name}-metadata"
            )
        else:
            pool_name = data_pool_name
        backend_name = config('volume-backend-name') or self.charm.app.name
        return {
            'cluster_name': self.charm.app.name,
            'rbd_pool': pool_name,
            'rbd_user': self.charm.app.name,
            'backend_name': backend_name,
            'backend_availability_zone': config('backend-availability-zone'),
        }

Configuring Charm to use custom config context

The charm can append the new context onto those provided by the base class.

import ops_sunbeam.charm as sunbeam_charm

class MyCharm(sunbeam_charm.OSBaseOperatorAPICharm):
   """Charm the service."""

    @property
    def config_contexts(self) -> List[sunbeam_ctxts.ConfigContext]:
        """Configuration contexts for the operator."""
        contexts = super().config_contexts
        contexts.append(
            sunbeam_ctxts.CinderCephConfigurationContext(self, "cinder_ceph"))
        return contexts