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https://issues.apache.org/jira/browse/TINKERPOP-2187?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16832806#comment-16832806
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Daniel Kuppitz commented on TINKERPOP-2187:
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Also, see the JavaDoc for {{MessageCombiner}}:
{quote}
* A MessageCombiner allows two messages in route to the same vertex to be
aggregated into a single message.
* Message combining can reduce the number of messages sent between vertices
and thus, reduce network traffic.
* Not all messages can be combined and thus, this is an optional feature of a
VertexProgram.
{quote}
In other words, a {{GraphComputer}} should work in any case, whether the
message combiner is provided or not.
> Two issues with the ShortestPathVertexProgram
> ---------------------------------------------
>
> Key: TINKERPOP-2187
> URL: https://issues.apache.org/jira/browse/TINKERPOP-2187
> Project: TinkerPop
> Issue Type: Bug
> Components: process
> Affects Versions: 3.4.1
> Reporter: Florian Hockmann
> Assignee: Daniel Kuppitz
> Priority: Minor
>
> While trying to update JanusGraph to TinkerPop 3.4.0, I encountered two
> issues with the {{ShortestPathVertexProgramm}}:
> # {{ShortestPathVertexProgram.getMessageScopes()}} returns an empty
> collection which doesn't work for a graph computer that uses these scopes to
> execute the edge traversal. I think the scopes should be {{Global}} and
> {{Local}} with the edge traversal.
> # The {{ShortestPathVertexProgram}} defines no message combiner, but
> {{sendMessages()}} can apparently still send multiple messages for the same
> vertex.
> For some reason, this doesn't lead to any problems for the
> {{TinkerGraphComputer}}. (Maybe it receives each message before another one
> can be sent for the same vertex?)
> For {{FulgoraGraphComputer}} however some tests result in a situation where
> multiple messages are sent to the same vertex which fails because no combiner
> is defined.
> So, I'd say that the {{ShortestPathVertexProgram}} should have a message
> combiner.
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