How to manage charm resources

See first: Juju | Resource (charm)

Contents:

• Add a resource to a charm
• Upload a resource to Charmhub
• List all the resources for a Charmhub charm
• List all the revisions for a resource associated with a Charmhub charm
• Set the architectures for a revision of a resource associated with a Charmhub charm

Add a resource to a charm

In the charm’s charmcraft.yaml file, add a resources block.

See more: File charmcraft.yaml > resources

In your charm’s src/charm.py file, use Ops to fetch the path to the resource and then manipulate it as needed.

For example, consider this simple resource definition:

resources:
  my-resource:
    type: file
    filename: somefile.txt
    description: test resource

In the charm code, we can now use Model.resources.fetch(<resource_name>) to get the path to the resource, then manipulate it as we need:

# ...
import logging
import ops
# ...
logger = logging.getLogger(__name__)

def _on_config_changed(self, event):
    # Get the path to the file resource named 'my-resource'
    try:
        resource_path = self.model.resources.fetch("my-resource")
    except ops.ModelError as e:
        self.unit.status = ops.BlockedStatus(
            "Something went wrong when claiming resource 'my-resource; "
            "run `juju debug-log` for more info'"
        ) 
       # might actually be worth it to just reraise this exception and let the charm error out;
       # depends on whether we can recover from this.
        logger.error(e)
        return
    except NameError as e:
        self.unit.status = ops.BlockedStatus(
            "Resource 'my-resource' not found; "
            "did you forget to declare it in metadata.yaml?"
        )
        logger.error(e)
        return

    # Open the file and read it
    with open(resource_path, "r") as f:
        content = f.read()
    # do something

The fetch() method will raise a ModelError if the resource does not exist, and returns a Python Path object to the resource if it does.

Note: During development, it may be useful to specify the resource at deploy time to facilitate faster testing without the need to publish a new charm/resource in between minor fixes. In the below snippet, we create a simple file with some text content, and pass it to the Juju controller to use in place of any published my-resource resource:

echo "TEST" > /tmp/somefile.txt
charmcraft pack
juju deploy ./my-charm.charm --resource my-resource=/tmp/somefile.txt

Upload a resource to Charmhub

Once you’ve uploaded a charm to Charmhub, to upload the resource defined in its charmcraft.yaml > resources to Charmhub as well, run the charmcraft upload-resource command followed by the name of the charm, the name of the resource (cf. charmcraft.yaml), and --filepath=<path to file resource> / --image=<OCI image>. For example:

$ charmcraft upload-resource my-super-charm someresource --filepath=/tmp/superdb.bin
Revision 1 created of resource 'someresource' for charm 'my-super-charm'

$ charmcraft upload-resource my-super-charm redis-image --image=sha256:64aa8983ec5cea7bc143af18829836914fa405184d56dcbdfd9df672ade85249
Revision 1 created of resource 'redis-image' for charm 'my-super-charm'

Charmcraft will first check if that specific image is available in Canonical’s Registry, and just use it if that’s the case. If not, it will try to get it from the developer’s local OCI repository (needs dockerd to be installed and running), push it to the Canonical’s Registry, and then use it.

To update a pre-uploaded resource, run the upload-resource command again. The result will be a new revision.

See more: charmcraft upload-resource

List all the resources for a Charmhub charm

To view all the resources associated with a charm you’ve uploaded to Charmhub, run charmcraft resources followed by the charm name:

charmcraft resources mycharm

See more: charmcraft resources

List all the revisions for a resource associated with a Charmhub charm

To view all the revisions for a resource associated with a charm you’ve uploaded to Charmhub, run charmcraft resource-revisions followed by the charm name and the resource name. For example:

charmcraft resource-revisions mycharm myresource

See more: charmcraft resource-revisions

Set the architectures for a revision of a resource associated with a Charmhub charm

To set the architectures for a revision of a resource associated with a charm you’ve uploaded to Charmhub, run charmcraft set-resource-architectures followed by the name of the charm, the name of the resource, and the architecture(s), using the --resources flag to specify the target resource revision. For example:

charmcraft set-resource-architectures mycharm myresource --revision=1 arm64,armhf

See more: charmcraft set-resource-architectures