graphx-ConnectedComponents

1.

ConnectedComponents返回一个与原图结构相同的图，只是顶点的属性变为连通图中最小的顶点ID。

2.算法 grpahx源码

import scala.reflect.ClassTag

import org.apache.spark.graphx._


object ConnectedComponents {
  /**
    * Compute the connected component membership of each vertex and return a graph with the vertex
    * value containing the lowest vertex id in the connected component containing that vertex.
    *
    * @tparam VD the vertex attribute type (discarded in the computation)
    * @tparam ED the edge attribute type (preserved in the computation)
    * @param graph the graph for which to compute the connected components
    * @param maxIterations the maximum number of iterations to run for
    * @return a graph with vertex attributes containing the smallest vertex in each
    *         connected component
    */
  def run[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED],
                                      maxIterations: Int): Graph[VertexId, ED] = {
    require(maxIterations > 0, s"Maximum of iterations must be greater than 0," +
      s" but got ${maxIterations}")

    val ccGraph = graph.mapVertices { case (vid, _) => vid }
    def sendMessage(edge: EdgeTriplet[VertexId, ED]): Iterator[(VertexId, VertexId)] = {
      if (edge.srcAttr < edge.dstAttr) {
        Iterator((edge.dstId, edge.srcAttr))
      } else if (edge.srcAttr > edge.dstAttr) {
        Iterator((edge.srcId, edge.dstAttr))
      } else {
        Iterator.empty
      }
    }
    val initialMessage = Long.MaxValue
    val pregelGraph = Pregel(ccGraph, initialMessage,
      maxIterations, EdgeDirection.Either)(
      vprog = (id, attr, msg) => math.min(attr, msg),
      sendMsg = sendMessage,
      mergeMsg = (a, b) => math.min(a, b))
    ccGraph.unpersist()
    pregelGraph
  } // end of connectedComponents

  /**
    * Compute the connected component membership of each vertex and return a graph with the vertex
    * value containing the lowest vertex id in the connected component containing that vertex.
    *
    * @tparam VD the vertex attribute type (discarded in the computation)
    * @tparam ED the edge attribute type (preserved in the computation)
    * @param graph the graph for which to compute the connected components
    * @return a graph with vertex attributes containing the smallest vertex in each
    *         connected component
    */
  def run[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]): Graph[VertexId, ED] = {
    run(graph, Int.MaxValue)
  }
}