How do I get the size of the largest connected component of a graph in Spark?

Question

I'm building a graph from an RDD of tuples of source and destination nodes, like this:

Graph.fromEdgeTuples(rawEdges = edgeList, 1)

1. First off, I did not quite understand what the second parameter is. From the documentation,

   defaultValue the vertex attributes with which to create vertices referenced by the edges

   I still don't get it.

2. Second, I cannot find anything to compute the size of the biggest component. There is no foreach implemented, nor map or reduceByKey, or anything else after invoking the connectedComponents method.

Answer 1

1. defaultValue is an attribute assigned to all created edges:

   val graph = Graph.fromEdgeTuples(sc.parallelize(Seq(
     (1, 2), (2, 3), (4, 5))), 1)
   
   graph.edges.map(_.attr).distinct.collect 
   // Array[Int] = Array(1)
   

2. Extract component ids and do a worcount:

   val ids = graph.connectedComponents.vertices map((v: (Long, Long)) => v._2)
   ids.map((_, 1L)).reduceByKey(_ + _)