@Override
public void map(IntWritable nid, PersonalizedPageRankNode node, Context context)
throws IOException, InterruptedException {
// Pass along node structure.
intermediateStructure.setNodeId(node.getNodeId());
intermediateStructure.setType(PersonalizedPageRankNode.Type.Structure);
intermediateStructure.setAdjacencyList(node.getAdjacenyList());
context.write(nid, intermediateStructure);
int massMessages = 0;
// Distribute PageRank mass to neighbors (along outgoing edges).
if (node.getAdjacenyList().size() > 0) {
// Each neighbor gets an equal share of PageRank mass.
ArrayListOfIntsWritable list = node.getAdjacenyList();
float mass[] = new float[sources.size()];
for (int i = 0; i < sources.size(); i++) {
mass[i] = node.getPageRank(i) - (float) StrictMath.log(list.size());
}
context.getCounter(PageRank.edges).increment(list.size());
// Iterate over neighbors.
for (int i = 0; i < list.size(); i++) {
neighbor.set(list.get(i));
intermediateMass.setNodeId(list.get(i));
intermediateMass.setType(PersonalizedPageRankNode.Type.Mass);
for (int j = 0; j < sources.size(); j++) {
intermediateMass.setPageRank(j, mass[j]);
}
// Emit messages with PageRank mass to neighbors.
context.write(neighbor, intermediateMass);
massMessages++;
}
}
// Bookkeeping.
context.getCounter(PageRank.nodes).increment(1);
context.getCounter(PageRank.massMessages).increment(massMessages);
}
@Override
public void reduce(
IntWritable nid, Iterable<PersonalizedPageRankNode> iterable, Context context)
throws IOException, InterruptedException {
Iterator<PersonalizedPageRankNode> values = iterable.iterator();
// Create the node structure that we're going to assemble back together from shuffled pieces.
PersonalizedPageRankNode node = new PersonalizedPageRankNode();
node.setType(PersonalizedPageRankNode.Type.Complete);
node.setNodeId(nid.get());
int massMessagesReceived = 0;
int structureReceived = 0;
float[] mass = new float[sources.size()];
for (int i = 0; i < sources.size(); i++) {
mass[i] = Float.NEGATIVE_INFINITY;
}
while (values.hasNext()) {
PersonalizedPageRankNode n = values.next();
if (n.getType().equals(PersonalizedPageRankNode.Type.Structure)) {
// This is the structure; update accordingly.
ArrayListOfIntsWritable list = n.getAdjacenyList();
structureReceived++;
node.setAdjacencyList(list);
} else {
// This is a message that contains PageRank mass; accumulate.
for (int i = 0; i < sources.size(); i++) {
mass[i] = sumLogProbs(mass[i], n.getPageRank(i));
}
massMessagesReceived++;
}
}
// Update the final accumulated PageRank mass.
for (int i = 0; i < sources.size(); i++) {
node.setPageRank(i, mass[i]);
}
context.getCounter(PageRank.massMessagesReceived).increment(massMessagesReceived);
// Error checking.
if (structureReceived == 1) {
// Everything checks out, emit final node structure with updated PageRank value.
context.write(nid, node);
// Keep track of total PageRank mass.
for (int i = 0; i < sources.size(); i++) {
totalMass[i] = sumLogProbs(totalMass[i], mass[i]);
}
} else if (structureReceived == 0) {
// We get into this situation if there exists an edge pointing to a node which has no
// corresponding node structure (i.e., PageRank mass was passed to a non-existent node)...
// log and count but move on.
context.getCounter(PageRank.missingStructure).increment(1);
LOG.warn(
"No structure received for nodeid: " + nid.get() + " mass: " + massMessagesReceived);
// It's important to note that we don't add the PageRank mass to total... if PageRank mass
// was sent to a non-existent node, it should simply vanish.
} else {
// This shouldn't happen!
throw new RuntimeException(
"Multiple structure received for nodeid: "
+ nid.get()
+ " mass: "
+ massMessagesReceived
+ " struct: "
+ structureReceived);
}
}
