new-sdn-charm.rst

New SDN Charm

Before writing the charm the charm author needs to have a clear idea of what applications the charm is going to need to relate to, what files and services the charm is going to manage and possibly what files or services do other charms manage that need updating.

In the example below we will assume that a new charm, VirtualTokenRing, is needed to install a component on compute nodes and to inject some configuration into nova.conf.

Prerequists

This will change once the OpenStack templates are on pypi

git clone git@github.com:gnuoy/charm_templates_openstack.git
cd charm_templates_openstack
sudo ./setup.py install

Create Charm

Charm tools provides a utility for building an initial charm from a template. During the charm generation charm tools asks a few questions about the charm.

charm-create  -t openstack-neutron-plugin virtual-token-ring
INFO: Generating charm for virtual-token-ring in ./virtual-token-ring
INFO: No virtual-token-ring in apt cache; creating an empty charm instead.
What is the earliest OpenStack release this charm is compatable with? liberty
What packages should this charm install (space seperated list)?

Build Charm

The charm now needs to be built to pull down all the interfaces and layers the charm depends on and rolled into the built charm which can be deployed.

cd virtual-token-ring
charm build -s xenial -o build src

Deploy Charm

cd build
juju deploy cs:xenial/nova-compute
juju deploy local:xenial/virtual-token-ring
juju add-relation nova-compute virtual-token-ring

juju status will now show both charms deployed. The nova-compute status will show some missing relations but thats not an issue for this demonstration.

Updating nova.conf

The charm will have installed any packages that were listed at charm creation time but that is all it has done. The charm has access to its neutron plugin interface. this allows the virtual-token-ring to send configuration to the nova-compute charm for inclusion in the nova.conf.

Return to the virtual-token-ring directory and edit src/reactive/virtual_token_ring_handlers.py adding the following:

@reactive.when('neutron-plugin.connected')
def configure_neutron_plugin(neutron_plugin):
    neutron_plugin.configure_plugin(
        plugin='ovs',
        config={
            "nova-compute": {
                "/etc/nova/nova.conf": {
                    "sections": {
                        'DEFAULT': [
                            ('random_option', 'true'),
                        ],
                    }
                }
            }
        })

This tells the charm to send that configuation to the principle where the neutron-plugin.connected event has been raised. Then repeat the Build Charm steps/

Deploy Update

The freshly built charm which contains the update now needs to be deployed to the environment.

juju upgrade-charm virtual-token-ring

Check Update

juju run --unit nova-compute/0 "grep random_option /etc/nova/nova.conf"
random_option = true