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https://issues.apache.org/jira/browse/FLINK-2716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14950500#comment-14950500
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Stephan Ewen commented on FLINK-2716:
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Comparators always refer to keys. Some operations thus only need serializers.
You can have two reducers consuming the result of a mapper for example. In that
case, the type and serializer between the mapper and both reducers is the same,
but the comparators may be different if the group on different keys.
> Checksum method for DataSet and Graph
> -------------------------------------
>
> Key: FLINK-2716
> URL: https://issues.apache.org/jira/browse/FLINK-2716
> Project: Flink
> Issue Type: Improvement
> Components: Gelly, Java API, Scala API
> Affects Versions: 0.10
> Reporter: Greg Hogan
> Assignee: Greg Hogan
> Priority: Minor
>
> {{DataSet.count()}}, {{Graph.numberOfVertices()}}, and
> {{Graph.numberOfEdges()}} provide measures of the number of distributed data
> elements. New {{DataSet.checksum()}} and {{Graph.checksum()}} methods will
> summarize the content of data elements and support algorithm validation,
> integration testing, and benchmarking.
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