def peer_retrieve(key, relation_name='cluster'):
    """Retrieve a named key from peer relation `relation_name`."""
    cluster_rels = relation_ids(relation_name)
    if len(cluster_rels) > 0:
        cluster_rid = cluster_rels[0]
        return relation_get(attribute=key, rid=cluster_rid,
                            unit=local_unit())
    else:
        raise ValueError('Unable to detect'
                         'peer relation {}'.format(relation_name))
Retrieve a named key from peer relation `relation_name`.