def peer_store(key, value, relation_name='cluster'):
    """Store the key/value pair on the named peer relation `relation_name`."""
    cluster_rels = relation_ids(relation_name)
    if len(cluster_rels) > 0:
        cluster_rid = cluster_rels[0]
        relation_set(relation_id=cluster_rid,
                     relation_settings={key: value})
    else:
        raise ValueError('Unable to detect '
                         'peer relation {}'.format(relation_name))
Store the key/value pair on the named peer relation `relation_name`.