Packages

object Pregel extends Logging

Implements a Pregel-like bulk-synchronous message-passing API.

Unlike the original Pregel API, the GraphX Pregel API factors the sendMessage computation over edges, enables the message sending computation to read both vertex attributes, and constrains messages to the graph structure. These changes allow for substantially more efficient distributed execution while also exposing greater flexibility for graph-based computation.

Example:

  1. We can use the Pregel abstraction to implement PageRank:

    val pagerankGraph: Graph[Double, Double] = graph
  // Associate the degree with each vertex
  .outerJoinVertices(graph.outDegrees) {
    (vid, vdata, deg) => deg.getOrElse(0)
  }
  // Set the weight on the edges based on the degree
  .mapTriplets(e => 1.0 / e.srcAttr)
  // Set the vertex attributes to the initial pagerank values
  .mapVertices((id, attr) => 1.0)

def vertexProgram(id: VertexId, attr: Double, msgSum: Double): Double =
  resetProb + (1.0 - resetProb) * msgSum
def sendMessage(id: VertexId, edge: EdgeTriplet[Double, Double]): Iterator[(VertexId, Double)] =
  Iterator((edge.dstId, edge.srcAttr * edge.attr))
def messageCombiner(a: Double, b: Double): Double = a + b
val initialMessage = 0.0
// Execute Pregel for a fixed number of iterations.
Pregel(pagerankGraph, initialMessage, numIter)(
  vertexProgram, sendMessage, messageCombiner)
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  4. def apply[VD, ED, A](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)(implicit arg0: ClassTag[VD], arg1: ClassTag[ED], arg2: ClassTag[A]): Graph[VD, ED]

    Execute a Pregel-like iterative vertex-parallel abstraction.

    Execute a Pregel-like iterative vertex-parallel abstraction. The user-defined vertex-program vprog is executed in parallel on each vertex receiving any inbound messages and computing a new value for the vertex. The sendMsg function is then invoked on all out-edges and is used to compute an optional message to the destination vertex. The mergeMsg function is a commutative associative function used to combine messages destined to the same vertex.

    On the first iteration all vertices receive the initialMsg and on subsequent iterations if a vertex does not receive a message then the vertex-program is not invoked.

    This function iterates until there are no remaining messages, or for maxIterations iterations.

    VD

    the vertex data type

    ED

    the edge data type

    A

    the Pregel message type

    graph

    the input graph.

    initialMsg

    the message each vertex will receive at the first iteration

    maxIterations

    the maximum number of iterations to run for

    activeDirection

    the direction of edges incident to a vertex that received a message in the previous round on which to run sendMsg. For example, if this is EdgeDirection.Out, only out-edges of vertices that received a message in the previous round will run. The default is EdgeDirection.Either, which will run sendMsg on edges where either side received a message in the previous round. If this is EdgeDirection.Both, sendMsg will only run on edges where *both* vertices received a message.

    vprog

    the user-defined vertex program which runs on each vertex and receives the inbound message and computes a new vertex value. On the first iteration the vertex program is invoked on all vertices and is passed the default message. On subsequent iterations the vertex program is only invoked on those vertices that receive messages.

    sendMsg

    a user supplied function that is applied to out edges of vertices that received messages in the current iteration

    mergeMsg

    a user supplied function that takes two incoming messages of type A and merges them into a single message of type A. This function must be commutative and associative and ideally the size of A should not increase.

    returns

    the resulting graph at the end of the computation

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