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https://issues.apache.org/jira/browse/TINKERPOP-1192?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15180115#comment-15180115
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ASF GitHub Bot commented on TINKERPOP-1192:
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Github user spmallette commented on the pull request:
https://github.com/apache/incubator-tinkerpop/pull/251#issuecomment-192346032
good with `mvn clean install`
VOTE +1
> TraversalSideEffects should support registered reducers (binary operators).
> ---------------------------------------------------------------------------
>
> Key: TINKERPOP-1192
> URL: https://issues.apache.org/jira/browse/TINKERPOP-1192
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.1.1-incubating
> Reporter: Marko A. Rodriguez
> Fix For: 3.2.0-incubating
>
>
> TinkerPop 3.2.0-SNAPSHOT has made leaps and bounds towards completely
> aligning Gremlin OLTP and Gremlin OLAP. What has got me really excited is
> that there is such a strong conceptual alignment between the following
> components:
> {code}
> VertexProgram <=> Traversal
> Iteration <=> Step
> Messages <=> Traversers
> MessageCombiner <=> TraverserSet ("bulking")
> BSP <=> Barrier
> Workers <=> Parallel Steps
> Master <=> Sequential Steps
> Memory <=> SideEffects
> {code}
> TraversalVertexProgram is very clean -- its lays atop the GraphComputer API
> in a natural, effortless way.
> However, there is one last pairing that needs some better alignment:
> GraphComputer Memory and Traversal SideEffects. A Memory slot has the notion
> of a key, a value, and a reducer (binary operator). A Traversal SideEffect as
> the notion of a key and a value. I think we should enable Traversal
> SideEffects to support registered reducers. If we do this, then there is
> perfect alignment between the two models and we won't have to have
> "if(onGraphComputer)"-type logic in our side-effect steps.
> Right now in GroupCountSideEffectStep we do this:
> {code}
> public void sideEffect(final Traverser<S> traverser) {
> Map<E,Long> groupCountMap =
> this.getTraversal().getSideEffects().get(this.sideEffectKey);
> MapHelper.incr(traverser.get(), traverser.bulk(), groupCountMap)
> }
> {code}
> We are explicitly getting the Map from the sideEffects and updating it. This
> model will not generally work in OLAP because groupCountMap is a distributed
> data structure and thus, local updates to a Map don't distribute. I have it
> working currently in master/, but at the cost of not being able to read the
> sideEffect, only write to it. To make TraversalSideEffects consistent across
> both OLTP and OLAP, I think we should express GroupCountSideEffectStep like
> this (*** analogously for GroupSideEffectStep, TreeSideEffectStep, etc.):
> {code}
> public void sideEffect(final Traverser<S> traverser) {
> this.getTraversal().getSideEffects().add(this.sideEffectKey,
> Collections.singletonMap(traverser.get(), traverser.bulk());
> }
> {code}
> Moreover, TraversalSideEffects should have the following method:
> {code}
> TraversalSideEffects.register(final String key, final Supplier<A>
> initialValue, final BinaryOperator<A> reducer)
> {code}
> Note that we already have:
>
> https://github.com/apache/incubator-tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/TraversalSideEffects.java#L88
> We can deprecate the current registerSupplier() in support of register().
> Moreover, for backwards compatibility, BinaryOperator<A> reducer would simply
> be Operator.assign.
>
> https://github.com/apache/incubator-tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/Operator.java#L59-L62
> Thus, this would not be a breaking change and it will ensure a natural
> congruence between these two related computing structures -- Memory and
> TraversalSideEffects.
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