[jira] [Created] (TINKERPOP-2396) TraverserSet should be extendable for GraphDB provider

["Norio Akagi (Jira)" <ji...@apache.org>]

Norio Akagi created TINKERPOP-2396:
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             Summary: TraverserSet should be extendable for GraphDB provider
                 Key: TINKERPOP-2396
                 URL: https://issues.apache.org/jira/browse/TINKERPOP-2396
             Project: TinkerPop
          Issue Type: Improvement
          Components: process
            Reporter: Norio Akagi


Currently in many TinkerPop steps, TraverserSet is internally created and used. 
However GraphDB provider may want to use their own TraverserSet to change the logic  how it populates an inner Map.

Just let me give one example

Say internally we have a unique internal Id for each element and to get a user facing Id we need to retrieve it from our Storage using the internal Id. 
For example Element#id returns the user facing Id so one Storage API call is required on our end. It is ok.

What is unexpected for us was that when TraverserSet adds a Traverser<Element> , the element's id is used for hashCode.
So whenever some step with TraverserSet handles solution, as many Storage API calls are required as a number of Traverser<Element>. As you may see, some query like .count() at the end doesn't require the user facing value so we want to avoid that if we can.


We can simply achieve this by using internal id as a hashCode, but we can't overwrite ElementHelper's behavior so first we overwrote Element#hashCode but after that we saw that this test effectively fails.
https://github.com/apache/tinkerpop/blob/cff4c161615f2b50bda27b6ba523c7f52b833532/gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/structure/util/detached/DetachedVertexTest.java#L75
Because v here is our own custom vertex instance which uses internal id for hashCode but DetachedVertex just uses a user facing id copied from v, so they are not considered as equal when used as hash key.

We may skip this test but it's better to keep all Tinkerpop tests passed as GraphDB provider I think.

So instead, I will propose to change so that we can add a capability to swap TraverserSet as needed.
The change should be simple, 
1. ExpandableIterator should have constructor like ExpandableIterator(final Step<S, ?> hostStep, final TraverserSet<S> traverserSet) and use the traverserSet as its own traverserSet value.
2. AbstractStep should have construct like AbstractStep(final Traversal.Admin traversal, final Supplier<TraverserSet<S>> traverserSetSupplier) and use traverserSetSuppler.get() for all places that currently we instantiate new TraverserSet.
 Also this supplier.get() may be passed when AbstractStep instantiates ExpandableIterator.

So still GraphDB provider needs to extends all Steps that deals with TraverserSet and overwrite their constructor to have their own traverserSetSupplier, but after that they can freely use their own TraverserSet, so the logic how the key of TraverserSet is determined is up to GraphDB provider.
Or if we add some helper method like getTraverserSetSupplier under, for instance, Graph class, then instead of extending all steps we can just override that method under our own Graph class. But I am not sure which is better in this case, but either way the goal is the same.

Any thoughts or objections on this idea ?



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