master-application-relation-joined

#!/usr/bin/env python3
import os
import sys
sys.path.insert(0, os.path.join(os.environ['CHARM_DIR'], 'lib'))
from hookenv import relation_set, relation_get, log, relation_id
import random

def generateWorkerKey():
    """Return a random key of length 4,
    which will be passed as new workers join."""
    return ''.join(random.choice('0123456789ABCDEF') for i in range(4))

def master_application_relation_joined():
    """
    https://discourse.juju.is/t/charm-hooks/1040

    [name]-relation-joined is run only when that remote unit is first observed by the unit.
    It should be used to relation-set any local unit settings that can be determined using no more than the name of the
    joining unit and the remote private-address setting, which is always available when the relation is created and is
    by convention not deleted.

    You should not depend upon any other relation settings in the -joined hook because they’re not guaranteed to be
    present; if you need more information you should wait for a -changed hook that presents the right information.
    """

    log(" ========= hook: master-application-relation-joined  ========")

    # Generate a worker-key
    workerKey = generateWorkerKey()

    # Get the remote unit name so that we can use that for a composite key.
    remoteUnitName = os.environ.get('JUJU_REMOTE_UNIT', None) # remote_unit()

    # Get the worker remote unit private-address for logging
    workerAddr = relation_get('private-address', unit=remoteUnitName)

    log(f"Joined with WORKER at private-address: {workerAddr}")

    # Assemble the relation data.
    relation_data = { f"{remoteUnitName}-worker-key": workerKey }

    # Set the relation data on the relation.
    relation_set(relation_id(), relation_settings=relation_data )

if __name__ == "__main__":
    master_application_relation_joined()