[jira] [Commented] (TINKERPOP-1925) Don't apply PathRetractionStrategy recursively

["Daniel Kuppitz (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TINKERPOP-1925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16402032#comment-16402032 ] 

Daniel Kuppitz commented on TINKERPOP-1925:
-------------------------------------------

A quick thought on this: What if {{PathRetractionStrategy}} would just find the last child traversal and then only kick in when it's applied to this traversal?

Something like this:

{noformat}
apply(traversal) {
  // collect all child traversals, starting from the root traversal
  if (traversal != last child traversal in list) return;
  // apply strategy to root traversal
}
{noformat}

Unfortunately, this child traversal scan has to be done over and over again, as other strategies could change the result. However, it's just a scan not involving any mutations, I don't think it would cost too much time.

> Don't apply PathRetractionStrategy recursively
> ----------------------------------------------
>
>                 Key: TINKERPOP-1925
>                 URL: https://issues.apache.org/jira/browse/TINKERPOP-1925
>             Project: TinkerPop
>          Issue Type: Improvement
>          Components: process
>    Affects Versions: 3.2.7
>            Reporter: Daniel Kuppitz
>            Priority: Major
>
> {{PathRetractionStrategy}} should not be applied recursively, if possible. This is considered an anti-pattern and just recently it caused a lot of trouble. This commit fixed the recent issue:
> https://github.com/apache/tinkerpop/commit/b8eb8a02f1675a1b22b86ccc54d22453c65f406f
> {{PathRetractionStrategy}} collects requirements of child traversals, before other strategies have a chance to mutate them. When other strategies get to the child traversals, mutate them and thus add or remove requirements, a few things can go crazy, especially because traverser requirements get cached when they're first requested.
> Since I do not fully understand the {{PathRetractionStrategy}} code, my fix was actually a workaround. As you can see in the commit linked above, I implemented the same anti-pattern in {{SubgraphStrategy}} and {{IncidentToAdjacentStrategy}}; these are the 2 strategies that can potentially change the traverser requirements.
> It would be nice to revert these changes and instead make {{PathRetractionStrategy}} work like all others. However, I'm not sure if that's even possible.



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