原创：Spark中GraphX图运算pregel详解

由于本人文字表达能力不足，还是多多以代码形式表述，首先展示测试代码，然后解释：

package com.txq.spark.test

import org.apache.spark.graphx.util.GraphGenerators
import org.apache.spark.graphx._
import org.apache.spark.rdd.RDD
import org.apache.spark.{SparkConf, SparkContext, SparkException, graphx}

import scala.reflect.ClassTag

/**
  * spark GraphX 测试
  * @authorTongXueQiang
  */
object test {

    System.setProperty("hadoop.home.dir","D://hadoop-2.6.2");
    val conf = new SparkConf().setMaster("local").setAppName("testRDDMethod");
    val sc = new SparkContext(conf);

    def main(args: Array[String]): Unit = {
      /*
      val rdd = sc.textFile("hdfs://spark:9000/user/spark/data/SogouQ.sample");//搜狗搜索日志解析
      val rdd1 = rdd.map(_.split("\t")).map(line=>line(3)).map(_.split(" "));
      println("共有"+rdd1.count+"行");
      val rdd2 = rdd1.filter(_(0).toInt == 1).filter(_(1).toInt == 1);
      println("搜索结果和点击率均排第一的共有"+rdd2.count+"行");
      val users:RDD[(VertexId,(String,String))] = sc.parallelize(Array((3L,("rxin","student")),(7L,("jgonzal","postdoc")),(5L,("franklin","prof")),(2L,("istoica","prof"))));
      val relationships:RDD[Edge[String]] = sc.parallelize(Array(Edge(3L,7L,"collab"),Edge(5L,3L,"advisor"),Edge(2L,5L,"colleague"),Edge(5L,7L,"pi")));
      val defaultUser = ("jone","Missing");
      val graph = Graph(users,relationships,defaultUser);
      val result = graph.vertices.filter{case(id,(name,pos)) => pos == "prof"}.count();
      println("职位名称为prof的个数有：" + result + "个");
      println(graph.edges.filter(e => e.srcId > e.dstId).count());
      graph.triplets.collect().foreach(println)
      graph.edges.collect().foreach(println)*/
      /*
      val graph:Graph[Double,Int] = GraphGenerators.logNormalGraph(sc,numVertices = 100).mapVertices((id,_) => id.toDouble)
      println(graph);
      println(graph.vertices)*/

      /*
      val oderFollowers:VertexRDD[(Int,Double)] = graph.mapReduceTriplets[(Int,Double)](
        triplet =>{
          if(triplet.srcAttr > triplet.dstAttr){
            Iterator((triplet.dstId,(1,triplet.srcAttr)));
          } else {
            Iterator.empty
          }
        },
        (a,b) =>(a._1 + b._1,a._2 + b._2)
      )
      val avgAgeOfolderFollower:VertexRDD[Double] = oderFollowers.mapValues((id,value) => {
        value match{
          case (count,totalAge) => totalAge / count
        }
      })

      avgAgeOfolderFollower.collect().foreach(println)*/
　　　 //收集邻居节点，后面有自定义方法
      //collectNeighborIds(EdgeDirection.In,graph).foreach(line => {print(line._1+":"); for (elem <- line._2) {print(elem + " ")};println;});
　　　//以Google的网页链接文件(后面由下载地址)为例，演示pregel方法，找出从v0网站出发，得到经过的步数最少的链接网站，类似于附近地图最短路径算法
      val graph:Graph[Double,Double] = GraphLoader.edgeListFile(sc,"hdfs://spark/user/spark/data/web-Google.txt",numEdgePartitions = 4).mapVertices((id,_) => id.toDouble).mapEdges(edge => edge.attr.toDouble);
      val sourceId:VertexId = 0;//定义源网页Id
      val g:Graph[Double,Double] = graph.mapVertices((id,attr) => if(id == 0) 0.0 else Double.PositiveInfinity)
      //pregel底层调用GraphOps的mapReduceTriplets方法，一会儿解释源代码
      val result = pregel[Double,Double,Double](g,Double.PositiveInfinity)(
        (id,vd,newVd) => math.min(vd,newVd),//这个方法的作用是更新节点VertexId的属性值为新值，以利于innerJoin操作
        triplets => {//map函数
          if(triplets.srcAttr + triplets.attr < triplets.dstAttr){
            Iterator((triplets.dstId,triplets.srcAttr + triplets.attr))
          } else {
            Iterator.empty
          }
        },
        (a,b) => math.min(a,b)//reduce函数
      )
　　　//输出结果，注意pregel返回的是更新VertexId属性的graph，而不是VertexRDD[(VertexId,VD)]
      print("最短节点："+result.vertices.filter(_._1 != 0).reduce(min));//注意过滤掉源节点
    }
　　 //找出路径最短的点
    def min(a:(VertexId,Double),b:(VertexId,Double)):(VertexId,Double) = {
    if(a._2 < b._2) a else b
   }
  /**
    * 自定义收集VertexId的neighborIds
    * @author TongXueQiang
    */
  def collectNeighborIds[T,U](edgeDirection:EdgeDirection,graph:Graph[T,U])(implicit m:scala.reflect.ClassTag[T],n:scala.reflect.ClassTag[U]):VertexRDD[Array[VertexId]] = {
    val nbrs = graph.mapReduceTriplets[Array[VertexId]](
      //map函数
      edgeTriplets => {
        val msgTosrc = (edgeTriplets.srcId,Array(edgeTriplets.dstId));
        val msgTodst = (edgeTriplets.dstId,Array(edgeTriplets.srcId));
        edgeDirection match {
          case EdgeDirection.Either =>Iterator(msgTosrc,msgTodst)
          case EdgeDirection.Out => Iterator(msgTosrc)
          case EdgeDirection.In => Iterator(msgTodst)
          case EdgeDirection.Both =>  throw new SparkException("It doesn't make sense to collect neighbors without a " + "direction.(EdgeDirection.Both is not supported.use EdgeDirection.Either instead.)")
        }
      },_ ++ _)//reduce函数
    nbrs
  }

  /**
    * 自定义pregel函数
    * @param graph 图
    * @param initialMsg 返回的vertexId属性
    * @param maxInterations 迭代次数
    * @param activeDirection 边的方向
    * @param vprog 更新节点属性的函数，以利于innerJoin操作
    * @param sendMsg map函数，返回Iterator[A],一般A为Tuple2,其中id为接受消息方
    * @param mergeMsg reduce函数，一般为合并，或者取最小、最大值……操作
    * @tparam A 想要得到的VertexId属性
    * @tparam VD graph中vertices的属性
    * @tparam ED graph中的edge属性
    * @return 返回更新后的graph
    */
  def pregel[A:ClassTag,VD:ClassTag,ED:ClassTag](graph:Graph[VD,ED],initialMsg:A,maxInterations:Int = Int.MaxValue,activeDirection:EdgeDirection =  EdgeDirection.Either)(
      vprog:(VertexId,VD,A) => VD,
      sendMsg:EdgeTriplet[VD,ED] =>Iterator[(VertexId,A)],
      mergeMsg:(A,A) => A)
  : Graph[VD,ED] = {
    Pregel0(graph,initialMsg,maxInterations,activeDirection)(vprog,sendMsg,mergeMsg)//调用apply方法
  }
　

　//此为节点内连接函数,返回VertexRDD
  def innerJoin[U:ClassTag,VD:ClassTag](table:RDD[(VertexId,U)])(mapFunc:(VertexId,VD,U) => VertexRDD[(VertexId,VD)]) = {
    val uf = (id: VertexId, data: VD, o: Option[U]) => {
      o match {
        case Some(u) => mapFunc(id, data, u)
        case None => data
      }
    }
  }
　//测试Option[T]  def test():Unit = {
    val map = Map("a" -> "1","b" -> "2","c" -> "3");
    def show(value:Option[String]):String = {
      value match{
        case Some(x) => x
        case None => "no value found!"
      }
    }
    println(show(map.get("a")) == "1");
  }
}

下面重点研究Pregel,为了方便，自己重新定义了一个Pregel0

package com.txq.spark.test

import org.apache.spark.Logging
import org.apache.spark.graphx.{EdgeDirection, EdgeTriplet, Graph, VertexId}
import scala.reflect.ClassTag

/**
  * 自定义Pregel object，处理思路：
  */
object Pregel0 extends Logging {
  def apply[VD:ClassTag,ED:ClassTag,A:ClassTag]
  (graph:Graph[VD,ED],
   initialMsg:A,
   maxIterations:Int = Int.MaxValue,
   activeDirection:EdgeDirection = EdgeDirection.Either)
  (vprog:(VertexId,VD,A) => VD,
   sendMsg:EdgeTriplet[VD,ED] => Iterator[(VertexId,A)],
   mergeMsg:(A,A) => A)
  : Graph[VD,ED] =
  {
　　//①对vertices进行更新操作
    var g = graph.mapVertices((vid,vdata) => vprog(vid,vdata,initialMsg)).cache();
    //②compute the messages，注意调用的是mapReduceTriplets方法，源代码：  

def mapReduceTriplets[A](

map: EdgeTriplet[VD, ED] => Iterator[(VertexId, A)],

reduce: (A, A) => A),

activeSetOpt: Option[(VertexRDD[_], EdgeDirection)] = None )

: VertexRDD[A]

    var messages = g.mapReduceTriplets(sendMsg,mergeMsg);
    print("messages:"+messages.take(10).mkString("\n"))
    var activeMessages = messages.count();
    //LOAD
    var prevG:Graph[VD,ED] = null
    var i = 0;
    while(activeMessages > 0 && i < maxIterations){
        //③Receive the messages.Vertices that didn't get any message do not appear in newVerts.
        //内联操作，返回的结果是VertexRDD，可以参看后面的调试信息
        val newVerts = g.vertices.innerJoin(messages)(vprog).cache();
        print("newVerts:"+newVerts.take(10).mkString("\n"))
        //④update the graph with the new vertices.
        prevG = g;//先把旧的graph备份，以利于后面的graph更新和unpersist掉旧的graph
　　　　　//④外联操作，返回整个更新的graph
        g = g.outerJoinVertices(newVerts){(vid,old,newOpt) => newOpt.getOrElse(old)}//getOrElse方法，意味，如果newOpt存在，返回newOpt,不存在返回old
        print(g.vertices.take(10).mkString("\n"))
        g.cache();//新的graph cache起来，下一次迭代使用

       val oldMessages = messages;//备份，同prevG = g操作一样
      //Send new messages.Vertices that didn't get any message do not appear in newVerts.so
      //don't send messages.We must cache messages.so it can be materialized on the next line.
      //allowing us to uncache the previous iteration.
　　　 //⑤下一次迭代要发送的新的messages,先cache起来
      messages = g.mapReduceTriplets(sendMsg,mergeMsg,Some((newVerts,activeDirection))).cache()
      print("下一次迭代要发送的messages:"+messages.take(10).mkString("\n"))
      activeMessages = messages.count();//⑥
      print("下一次迭代要发送的messages的个数："+ activeMessages)//如果activeMessages==0，迭代结束
      logInfo("Pregel finished iteration" + i);
　　　 //原来，旧的message和graph不可用了，unpersist掉
      oldMessages.unpersist(blocking= false);
      newVerts.unpersist(blocking=false)//unpersist之后，就不可用了
      prevG.unpersistVertices(blocking=false)
      prevG.edges.unpersist(blocking=false)
      i += 1;
    }
    g//返回最后的graph
  }

}
输出的调试信息：(距离v0节点最近的节点)
第一次跌代：

messages:(11342,1.0)
(824020,1.0)
(867923,1.0)
(891835,1.0)
newVerts:(11342,1.0)
(824020,1.0)
(867923,1.0)
(891835,1.0)
下一次迭代要发送的messages:(302284,2.0)
(760842,2.0)
(417728,2.0)
(322178,2.0)
(387543,2.0)
(846213,2.0)
(857527,2.0)
(856657,2.0)
(695578,2.0)
(27469,2.0)
下一次迭代要发送的messages的个数：29

下一次迭代要发送的messages:(754862,3.0)
(672426,3.0)
(320258,3.0)
(143557,3.0)
(789355,3.0)
(596104,3.0)
(118398,3.0)
(30115,3.0)
下一次迭代要发送的messages的个数：141
依次不断类推，直到activeMessages = 0跌代结束。

上面需要cache的有：graph，messages，newVertis.spark中的创建RDD和transformation操作都是lazy的，存储的只是内存地址，并非真正创建对象，当进行action时，需要从头至尾运行一遍，所以cache之后，重复利用RDD，再次进行action时，速度会大大提升。unpersist之后，就不能用了，所以需要把旧的备份。

一般情况使用mapReduceTriplets可以解决很多问题，为什么Spark GraphX会提供Pregel API?主要是为了更方便地去做迭代操作。因为在GraphX里面，Graph这张图并没有自动cache，而是手动cache。但是为了每次迭代更快，需要手动去做cache，每次迭代完就需要把没用的删除掉而把有用的保留，这比较难以控制。因为Graph中的点和边是分开进行Cache的，而Pregel能够帮助我们。例如，PangeRank就非常适合用Pregel来做。

web-Google.txt.gz文件下载地址：http://snap.stanford.edu/data/web-Google.html

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