[
https://issues.apache.org/jira/browse/JENA-763?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14094528#comment-14094528
]
Rob Vesse edited comment on JENA-763 at 8/12/14 7:09 PM:
---------------------------------------------------------
bq. I don't understand why you should have to override
Algebra.compileModifiers() which is working on syntax
In our real code we have extended the actual query parser to allow for custom
syntax extensions to a SPARQL query which are intended as special query
modifiers i.e. we are doing something along the lines of the {{CLUSTER BY}}
extension (http://link.springer.com/chapter/10.1007/978-3-642-05290-3_101 -
sorry I could not find a direct PDF link).
Therefore our actual custom algebra generator is looking at our extended Query
AST to see if these additional syntax constructs were present and if so adding
our custom operators as additional modifiers.
was (Author: rvesse):
bq. I don't understand why you should have to override
Algebra.compileModifiers() which is working on syntax
In our real code we have extended the actual query parser to allow for custom
syntax extensions to a SPARQL query which are intended as special query
modifiers i.e. we are doing something along the lines of the {{CLUSTER BY}}
extension (http://link.springer.com/chapter/10.1007/978-3-642-05290-3_101 -
sorry I could not find a direct PDF link) so what our actual custom algebra
generator is doing is looking at the extended Query AST to see if these
additional syntax constructs were present and if so adding our custom operators
as additional modifiers.
> Transforms should interact better with custom operators
> -------------------------------------------------------
>
> Key: JENA-763
> URL: https://issues.apache.org/jira/browse/JENA-763
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ
> Affects Versions: Jena 2.12.0
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Attachments: Jena673.java
>
>
> As already discussed briefly on the mailing list thread How to safely apply
> transforms to custom algebra operators?
> (http://s.apache.org/custom-algebra-transform) making some transforms pass
> correctly through custom algebra operators.
> {{TransformCopy}} defers the {{copy(OpExt ext)}} implementation back to the
> {{apply()}} method of {{OpExt}} which means a custom operator can do
> something simple like the following:
> {noformat}
> @Override
> public Op apply(Transform transform)
> {
> // This is required in order to not block optimizations
> return new CustomOperator(Transformer.transform(transform,
> this.subOp), this.customParams);
> }
> {noformat}
> Which will work correctly for stateless transforms but fails for transforms
> like {{Algebra.toQuadForm()}} which rely on external state. In the specific
> case of quad form transformation the external state is tracked by before and
> after visitors that are applied as the {{ApplyTransformVisitor}} works down
> the algebra with the state being used by the actual transform as it comes
> back up the algebra. However when passed through a custom operator there is
> no way to pass through the external state trackers and so inside the custom
> operator the transform may be accessing incorrect state.
> There are a couple of options for fixing this:
> # Fix this specific case by rewriting the quad form transform such that it
> does not rely on external state tracking (not sure that this is even feasible)
> # Revise the API for transforming {{OpExt}} so external state can also be
> passed where necessary
> Both options have difficulties and it may be possible to make simpler changes
> that allow the specific case of quad form transformations to be fixed without
> changing the public API.
> Another approach would be to have the quad form transform be a public class
> and provided public accessors to its external state such that a custom
> operator could specifically recognise it and special case it such that the
> external state tracking was passed onwards. More generally perhaps a marker
> interface {{StatefulTransform}} could be added which would provide a standard
> way to recognise transforms that may have this problem and provide access to
> the state trackers necessary to pass these through custom operators
> correctly. Additionally there could be overloads of
> {{Transformer.transform(Transform)}} i.e.
> {{Transformer.transform(StatefulTransform)}} that would wire things up
> appropriately allowing the existing basic approach for custom operators
> outlined above continue to work without special cases.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
