def get_subscribe_broadcast_messages(received_message, subscription_id,
    connection_id):
    """
    Return a BroadcastMessage to be delivered to other connections,
    possibly connected through redis pub/sub

    This message is called whenever an user subscribes to a topic.
    """
    assert received_message is not None, 'get_subscribe_broadcast_message requires a received_message'
    assert subscription_id is not None, 'get_subscribe_broadcast_message requires a subscription_id'
    assert connection_id is not None, 'get_subscribe_broadcast_message requires a connection_id'
    return []
