provide_data doesn't send data when required_keys not satisfied (juju charm using services framework)

Question

I'm not sure if I'm using this the correct way. I have one class defining the required_keys and the provide_data for the same relationship. The following code excecutes provide_data on a relation join/change, but doesn't relation-set namenode_sshkey. If I remove the required_keys line, the code suddenly works (but then I can't use this class to specify the required-data for the relationship).

[...]
    {
        'service': 'namenode',
        'ports': [9000, 50070],  # ports to open after start
        'provided_data': [
            NamenodeRelation()
        ],
        'required_data': [
            NamenodeRelation(),
            {'role' : 'namenode', 'command' : 'hadoop-daemon.sh'}
        ],
        'data_ready': [
            configure_namenode,
            helpers.render_template(
                 source='upstart.conf',
                 target='/etc/init/namenode.conf')
        ],
    },
[...]


class NamenodeRelation(RelationContext):
    name = 'namenode'
    interface = 'dfs'

    def __init__(self, *args, **kwargs):
        self.required_keys = ['slave_IP', 'private-address']
        RelationContext.__init__(self,name=self.name, *args, **kwargs)

    def provide_data(self):
        return {'namenode_sshkey' : get_ssh_key() }

Is this behaviour intended or is this a bug?

Answer 1

That is indeed a bug. I ran into it this week while rewriting a charm using the Services framework. I'm working on a fix for it, but in the meantime, I do have a workaround.

In the ServiceManager setup, pass the key=value that you want to send over the relation wire, and don't pass anything for the required_data relation.

def manage():
    manager = ServiceManager([
        {
            'service': 'example',
            'ports': [80],  # ports to after start
            'provided_data': [
                RabbitMQRelation(username='example', vhost='/')
            ],
            'required_data': [
               RabbitMQRelation(),
            ],
            'data_ready': [
                configure_rabbitmq,
            ],
            'data_lost': [
            ],
        },
    ])
    manager.manage()

The Relation class basically short-circuits the required_keys by only setting it if there's no provided data to handle.

class RabbitMQRelation(helpers.RelationContext):
    name = 'amqp'
    interface = 'rabbitmq'

    vhost = None
    username = None
    required_keys = []

    def __init__(self, username=None, vhost=None):
        """
        This works around a bug with the RelationContext class that expects
        the required keys to be set before it will call provide_data.
        """
        if username and vhost:
            self.username = username
            self.vhost = vhost
        else:
            self.required_keys = ['private-address', 'hostname', 'password']

        super(RabbitMQRelation, self).__init__(self.name)