[jira] [Commented] (TINKERPOP3-679) Debuggable Traversal

["Marko A. Rodriguez (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TINKERPOP3-679?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15008935#comment-15008935 ] 

Marko A. Rodriguez commented on TINKERPOP3-679:
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Perhaps this is what an {{ExplainStep}} could do. I'm not too versed in what {{EXPLAIN}} does in other query languages, but I could see it as not a "real step" but a "terminal/action step" that looks like this:

{code}
g.V().outE("knows").inE().count().explain()
{code}

This would return a {{TraversalExplain}} object much like [~rjbriody]'s {{TraversalMetrics}} object. The API for {{TraversalExplain}} could be:

{code}
TraversalExplain.getStrategies() // the sorted list of strategies that were applied
TraversalExplain.getTraversal(int strategy) // the Traversal after being strategy i was applied, where getTraversal(0) is the user constructed concatenated traversal.
TraversalExplain.getTraversals() // the sorted list of traversals after each strategy was applied. 
{code}

You could then do this:

{code}
gremlin> g.V().outE("knows").inE().count().explain().getTraversals()
==>[NoStrategy,[GraphStep, VertexStep[edges,OUT,knows], VertexEdgeStep[IN],GlobalCountStep]]
==>[AdjacentIncidentStrategy,[GraphStep, VertexStep[vertices,OUT,knows], GlobalCountStep]]
==>[TinkerGraphStepStepStrategy,[TinkerGraphStep, VertexStep[vertices,OUT,knows], GlobalCountStep]] 
{code}

We could probably make a {{toString()}} which is basically the data in {{getTraversals}} but all pretty like {{ProfileStep}}. This seems pretty easy to do actually (a morning session to have a fully functional example).

> Debuggable Traversal
> --------------------
>
>                 Key: TINKERPOP3-679
>                 URL: https://issues.apache.org/jira/browse/TINKERPOP3-679
>             Project: TinkerPop 3
>          Issue Type: Improvement
>          Components: process
>    Affects Versions: 3.0.2-incubating
>            Reporter: stephen mallette
>
> I'd like to be able to put a {{Traversal}} into "debug mode".  {{TraversalStrategy}} application is a bit too mysterious, making it hard to figure out why a {{Traversal}} is failing.  In a recent debug session, the problem was only sorted out after we looked at the order in which strategies were applied and even then it was a bit tricky to really understand what was happening.
> If the {{Traversal}} were in this "debug mode" it could give us insight into that strategy application process which would mean at least the following:
> * Display the order in which strategies were applied
> * For each strategy applied, display the {{toString()}} of the {{Traversal}} at that stage.
> A developer could then logically see what's in the magic and recognize that strategy 7 is ovewriting what strategy 2 is doing or that strategy 4 is executing before strategy 3 somehow.  
> Not sure what the best way is to implement this.  A straightforward solution with an environment variable check and println in {{applyStrategies}} seemed too "cheap" though kinda nice because very little would change and that output would come as part of the standard {{Traversal}} execution.  Other (better) ways?



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