public static void presenceSubscriptionNotRequired(PubSubService service, Node node, JID user) {
        // Check that no node is requiring to be subscribed to this user
        for (Node hostedNode : service.getNodes()) {
            if (hostedNode.isPresenceBasedDelivery(user)) {
                // Do not unsubscribe since presence subscription is still required
                return;
            }
        }
        // Unscribe from the user presence
        Presence subscription = new Presence(Presence.Type.unsubscribe);
        subscription.setTo(user);
        subscription.setFrom(service.getAddress());
        service.send(subscription);
    }
Requests the pubsub service to subscribe to the presence of the user. If the service
has already subscribed to the user's presence then do nothing.

@param service the PubSub service this action is to be performed for.
@param node the node that originated the subscription request.
@param user the JID of the affiliate to subscribe to his presence.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/PubSubEngine.java