@Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException
    {
        nodeId = ExternalizableUtil.getInstance().readSafeUTF(in);
        serviceId = ExternalizableUtil.getInstance().readSafeUTF(in);
        uniqueNodeIdentifier = new Node.UniqueIdentifier( serviceId, nodeId );
    }
Finds the pubsub service for the pubsub node that is the subject of this task.

Note that null, instead of a pubsub service instance, might be returned when the pubsub service is not currently
loaded in-memory on the cluster node that the task is executing on (although there is no guarantee that when this
method returns a non-null pubsub service, it was previously not loaded in-memory)! The rationale for this is that
this cluster tasks does not need to operate on data that is not in memory, as such operations are the
responsibility of the cluster node that initiates the cluster task.

@return A pubsub service
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/cluster/NodeTask.java