Spark学习笔记：Pregel API

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import org.apache.spark.graphx._// Import random graph generation libraryimport org.apache.spark.graphx.util.GraphGenerators// A graph with edge attributes containing distancesval graph: Graph[Int, Double] =  GraphGenerators.logNormalGraph(sc, numVertices = 100).mapEdges(e => e.attr.toDouble)val sourceId: VertexId = 42 // The ultimate source// Initialize the graph such that all vertices except the root have distance infinity.val initialGraph = graph.mapVertices((id, _) => if (id == sourceId) 0.0 else Double.PositiveInfinity)val sssp = initialGraph.pregel(Double.PositiveInfinity)(  (id, dist, newDist) => math.min(dist, newDist), // Vertex Program  triplet => {  // Send Message    if (triplet.srcAttr + triplet.attr < triplet.dstAttr) {      Iterator((triplet.dstId, triplet.srcAttr + triplet.attr))    } else {      Iterator.empty    }  },  (a,b) => math.min(a,b) // Merge Message  )println(sssp.vertices.collect.mkString("\n"))

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