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https://issues.apache.org/jira/browse/JENA-820?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14227846#comment-14227846
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ASF subversion and git services commented on JENA-820:
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Commit b65a26f445ef8c840c150ce19855e17f3e7ca5a6 in jena's branch
refs/heads/hadoop-rdf from [~rvesse]
[ https://git-wip-us.apache.org/repos/asf?p=jena.git;h=b65a26f ]
Initial workaround for JENA-820
Currently only works correctly when the output is line based, need to
add more test cases and research further into how to implement this for
block and whole file based modes.
> Blank Node output under Hadoop can cause identifiers to diverge in
> multi-stage pipelines
> ----------------------------------------------------------------------------------------
>
> Key: JENA-820
> URL: https://issues.apache.org/jira/browse/JENA-820
> Project: Apache Jena
> Issue Type: Improvement
> Components: RDF Tools for Hadoop
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Fix For: Jena 2.12.2
>
>
> In writing up the documentation on the RDF Tools for Hadoop and enumerating
> the possible issues that blank nodes imply I discovered an issue that I
> hadn't previously considered.
> For a single job the input and output formats all ensure that blank nodes are
> consistently given the same identifiers if they had the same syntactic ID and
> were in the same file. This is done even when a file is being read in
> multiple chunks by multiple map tasks. However by its nature each reduce
> task will create an output file so potentially you can end up with blank
> nodes spread over multiple files.
> However if we then read these files into a subsequent job the blank nodes may
> now be spread across multiple files so even though they were the same node
> originally our allocation policy will cause the identifiers to diverge and
> become distinct blank nodes which is incorrect behaviour.
> Since there is no clear universal fix for this what I am considering doing is
> instead introducing a configuration setting that will allow the file path to
> be ignored for the purpose of blank node identifier allocations within a job.
> This will mean that identifiers are purely allocated on the basis of the Job
> ID and thus the same syntactic ID in any file will result in the same blank
> node identifier. As the user will hopefully will have left this turned off
> for the first job even if we start with the same syntactic ID but in
> different files the normal allocation policy for the first job should ensure
> unique IDs for the later jobs.
> My next step on this is to implement a failing unit test (and then
> temporarily ignore it) which demonstrates this issue.
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