Github user greghogan commented on a diff in the pull request: https://github.com/apache/flink/pull/2527#discussion_r80311778 --- Diff: flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/library/Summarization.java --- @@ -226,11 +247,15 @@ public void setGroupRepresentativeId(K groupRepresentativeId) { } public VGV getVertexGroupValue() { - return f2; + return f2.isLeft() ? f2.left() : null; } public void setVertexGroupValue(VGV vertexGroupValue) { - f2 = vertexGroupValue; + if (vertexGroupValue == null) { + f2 = new Either.Right<>(NullValue.getInstance()); --- End diff -- I am tracing `VertexGroupReducer.reduce` calling `createGroupRepresentativeTuple` which calls `setVertexGroupValue` which creates a new `Either.Left` or `Either.Right`. Then `VertexGroupReducer.reduce` calls `reset` which creates a new `Either.Right`.
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