Github user manuzhang commented on a diff in the pull request:

    https://github.com/apache/incubator-gearpump/pull/223#discussion_r138883853
  
    --- Diff: core/src/main/scala/org/apache/gearpump/util/Graph.scala ---
    @@ -243,13 +259,34 @@ class Graph[N, E](vertexList: List[N], edgeList: 
List[(N, E, N)]) extends Serial
        * The node returned by Iterator is stable sorted.
        */
       def topologicalOrderIterator: Iterator[N] = {
    -    val newGraph = copy
    -    var output = List.empty[N]
    -
    -    while (!newGraph.isEmpty) {
    -      output ++= newGraph.removeZeroInDegree
    +    tryTopologicalOrderIterator.get
    --- End diff --
    
    Then return a `Try` is better since the caller will be aware this method 
may fail then.


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