@Override
public void reduce(IntWritable nid, Iterable<PageRankNode> values, Context context)
throws IOException, InterruptedException {
int massMessages = 0;
// Remember, PageRank mass is stored as a log prob.
float mass = Float.NEGATIVE_INFINITY;
for (PageRankNode n : values) {
if (n.getType() == PageRankNode.Type.Structure) {
// Simply pass along node structure.
context.write(nid, n);
} else {
// Accumulate PageRank mass contributions.
mass = sumLogProbs(mass, n.getPageRank());
massMessages++;
}
}
// Emit aggregated results.
if (massMessages > 0) {
intermediateMass.setNodeId(nid.get());
intermediateMass.setType(PageRankNode.Type.Mass);
intermediateMass.setPageRank(mass);
context.write(nid, intermediateMass);
}
}
@Override
public void reduce(IntWritable nid, Iterable<PageRankNode> iterable, Context context)
throws IOException, InterruptedException {
Iterator<PageRankNode> values = iterable.iterator();
// Create the node structure that we're going to assemble back together from shuffled pieces.
PageRankNode node = new PageRankNode();
node.setType(PageRankNode.Type.Complete);
node.setNodeId(nid.get());
int massMessagesReceived = 0;
int structureReceived = 0;
float mass = Float.NEGATIVE_INFINITY;
while (values.hasNext()) {
PageRankNode n = values.next();
if (n.getType().equals(PageRankNode.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.
mass = sumLogProbs(mass, n.getPageRank());
massMessagesReceived++;
}
}
// Update the final accumulated PageRank mass.
node.setPageRank(mass);
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.
totalMass = sumLogProbs(totalMass, mass);
} 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);
}
}