Routing table creation at a node in a Pastry P2P network

Question

This question is about the routing table creation at a node in a p2p network based on Pastry.

I'm trying to simulate this scheme of routing table creation in a single JVM. I can't seem to understand how these routing tables are created from the point of joining of the first node. I have N independent nodes each with a 160 bit nodeId generated as a SHA-1 hash and a function to determine the proximity between these nodes. Lets say the 1st node starts the ring and joins it. The protocol says that this node should have had its routing tables set up at this time. But I do not have any other nodes in the ring at this point, so how does it even begin to create its routing tables?

When the 2nd node wishes to join the ring, it sends a Join message(containing its nodeID) to the 1st node, which it passes around in hops to the closest available neighbor for this 2nd node, already existing in the ring. These hops contribute to the creation of routing table entries for this new 2nd node. Again, in the absence of sufficient number of nodes, how do all these entries get created?

I'm just beginning to take a look at the FreePastry implementation to get these answers, but it doesn't seem very apparent at the moment. If anyone could provide some pointers here, that'd be of great help too.

Answer 1

My understanding of Pastry is not complete, by any stretch of the imagination, but it was enough to build a more-or-less working version of the algorithm. Which is to say, as far as I can tell, my implementation functions properly.

To answer your first question:

The protocol says that this [first] node should have had its routing tables set up at this time. But I do not have any other nodes in the ring at this point, so how does it even begin to create its routing tables?

I solved this problem by first creating the Node and its state/routing tables. The routing tables, when you think about it, are just information about the other nodes in the network. Because this is the only node in the network, the routing tables are empty. I assume you have some way of creating empty routing tables?

To answer your second question:

When the 2nd node wishes to join the ring, it sends a Join message(containing its nodeID) to the 1st node, which it passes around in hops to the closest available neighbor for this 2nd node, already existing in the ring. These hops contribute to the creation of routing table entries for this new 2nd node. Again, in the absence of sufficient number of nodes, how do all these entries get created?

You should take another look at the paper (PDF warning!) that describes Pastry; it does a rather good job of explain the process for nodes joining and exiting the cluster.

If memory serves, the second node sends a message that not only contains its node ID, but actually uses its node ID as the message's key. The message is routed like any other message in the network, which ensures that it quickly winds up at the node whose ID is closest to the ID of the newly joined node. Every node that the message passes through sends their state tables to the newly joined node, which it uses to populate its state tables. The paper explains some in-depth logic that takes the origin of the information into consideration when using it to populate the state tables in a way that, I believe, is intended to reduce the computational cost, but in my implementation, I ignored that, as it would have been more expensive to implement, not less.

To answer your question specifically, however: the second node will send a Join message to the first node. The first node will send its state tables (empty) to the second node. The second node will add the sender of the state tables (the first node) to its state tables, then add the appropriate nodes in the received state tables to its own state tables (no nodes, in this case). The first node would forward the message on to a node whose ID is closer to that of the second node's, but no such node exists, so the message is considered "delivered", and both nodes are considered to be participating in the network at this time.

Should a third node join and route a Join message to the second node, the second node would send the third node its state tables. Then, assuming the third node's ID is closer to the first node's, the second node would forward the message to the first node, who would send the third node its state tables. The third node would build its state tables out of these received state tables, and at that point it is considered to be participating in the network.

Hope that helps.