Develop Cinder storage driver charm¶
The example below will walk through the creation of a basic Cinder storage subordinate charm for the Openstack Cinder NFS backend. The charm will use existing operator framework helpers. Once the charm is written it will be composed using charmcraft.
Before writing a new charm the charm author needs to have a clear idea of what files and services the charm is going to manage and what dependencies will be needed for it to perform its designated task.
The Cinder NFS charm created in this example requires an NFS host and export.
Create the skeleton charm¶
The charmcraft tool is needed to build and publish the new charm, and a virtualenv is used for charm creation.
sudo snap install charmcraft sudo apt install python3.9-venv tox venv/bin/pip3 install cookiecutter
Create the charm¶
The OpenStack Charms team has prepared a cookiecutter template to help create a new Cinder driver charm. Running the cookiecutter will prompt the user to answer some questions about the charm and does include defaults.
venv/bin/cookiecutter https://github.com/openstack-charmers/cinder-storage-backend-template driver_name [MyDriver]: NFS driver_name_lc [nfs]: release [queens]: xena additional_package_name : Select driver_install_method: 1 - None 2 - PPA 3 - Juju Resource DEB Choose from 1, 2, 3 : 1 Initialized empty Git repository in /home/chris/code/canonical/cinder-nfs/.git/ [main (root-commit) 88a54ad] Initial Cookiecutter Commit. 18 files changed, 573 insertions(+) create mode 100644 .gitignore create mode 100644 .stestr.conf create mode 100644 README.md create mode 100644 build-requirements.txt create mode 100644 config.yaml create mode 100644 metadata.yaml create mode 100755 pip.sh create mode 100644 requirements.txt create mode 100755 src/charm.py create mode 100644 src/test-requirements.txt create mode 100644 src/tox.ini create mode 100644 test-requirements.txt create mode 100644 tests/bundles/bionic-xena.yaml create mode 100644 tests/tests.yaml create mode 100644 tests/tests_cinder_nfs.py create mode 100644 tox.ini create mode 100644 unit_tests/__init__.py create mode 100644 unit_tests/test_cinder_nfs_charm.py
It’s important to verify that the versions match what you expect. For example,
in the above output we can see that
was generated but we’d expect
tests/bundles/focal-xena.yaml so we’ll need
to update that file name as well as its content.
mv tests/bundles/bionic-xena.yaml tests/bundles/focal-xena.yaml sed -i 's/bionic/focal/g' tests/bundles/focal-xena.yaml
Build the charm¶
The charm now needs to be built to pull down all the dependencies and rolled into the built charm which can then be deployed.
cd ~/cinder-nfs tox -e build
Deploy the charm¶
juju deploy --channel yoga/edge ch:cinder juju deploy ./cinder-nfs.charm juju add-relation cinder:storage-backend cinder-nfs:storage-backend
juju status will now show both charms deployed. The
status will show some missing relations but that’s not an issue for this
Update Cinder configuration¶
During the initial deploy of this charm, the standard openstack.core
default installer will install the packages specified in the class
CharmName.PACKAGES, but it will not do any other configuration.
In order to update
cinder.conf in the cinder principal charm, this
NFS subordinate charm will need to access the
which will allow it to send configuration information to the
cinder principal charm for inclusion in
cinder.conf on the co-located
In the generated charm, we can see that there’s already the minimal code
to send data over the storage-backend relation in
def cinder_configuration(self, config): # Return the configuration to be set by the principal. volume_driver = '' options = [ ('volume_driver', volume_driver) ] return options
What this is doing is sending structured data over the relation that the
cinder charm knows how to convert into native Cinder configuration. To
support the NFS backend, we’ll need to modify the
config.yaml file and
options: nfs_host: default: description: IP or Hostname for NFS server type: string nfs_path: default: description: Path to the NFS export type: string
def cinder_configuration(self, config): # Return the configuration to be set by the principal. volume_driver = 'cinder.volume.drivers.nfs.NfsDriver' options = [ ('volume_driver', volume_driver), ('nas_host', config['nfs_host']), ('nas_share_path', config['nfs_path']), ] return options
This tells the charm to send that configuration to the principal charm where the storage-backend relation changes. Then repeat the steps in the Build the charm section.
Upgrade to new charm¶
To test the updates, upgrade the running charm to the one that was just built.
juju upgrade-charm --path ./cinder-nfs.charm
Verify the update¶
We can confirm that the update has been applied by checking for the driver in the cinder.conf file:
juju run --unit cinder/0 "grep NfsDriver /etc/cinder/cinder.conf" volume_driver = cinder.volume.drivers.nfs.NfsDriver