[jira] [Comment Edited] (OOZIE-2714) Detect conflicting resources during class loading

["Peter Bacsko (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OOZIE-2714?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16185895#comment-16185895 ] 

Peter Bacsko edited comment on OOZIE-2714 at 9/29/17 2:23 PM:
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Ping [~andras.piros], [~gezapeti], [~asasvari], [~rkanter] - what do you guys think about this approach?


was (Author: pbacsko):
Ping [~andras.piros], [~gezapeti], [~rkanter] - what do you guys think about this approach?

> Detect conflicting resources during class loading
> -------------------------------------------------
>
>                 Key: OOZIE-2714
>                 URL: https://issues.apache.org/jira/browse/OOZIE-2714
>             Project: Oozie
>          Issue Type: New Feature
>          Components: core
>            Reporter: Peter Bacsko
>            Assignee: Peter Bacsko
>         Attachments: ClassLoaderTest.java, OOZIE-2714-POC01.patch
>
>
> There are a bunch of issues in Oozie which are related to class loading. 
> The main problem is that the classpath is constructed in a way which is very specific to Oozie:
> - Hadoop lib jars
> - Sharelib jars
> - User-defined jars
> Sometimes there is a conflict between sharelib and hadoop lib version. Also, users can add their own jars which sometimes contain a different version of popular libraries such as Guava, Apache commons, etc.
> We should be able to detect these conflicts and print exact error message so that Oozie users can take appropriate actions to resolve the problem.
> A possible approach is the following:
> * start the execution of an action on a different thread
> * replace the thread's context classloader with a classloader which can detect conflicts
> * when the JVM invokes the {{loadClass()}} method of the classloader, it  scans through the jars (which are available as {{URLClassPath}} objects). If it finds the given resource in at least two jars, it can do different things depending on the setup:
> ** throws an error immediately, mentioning the conflicting jars (this is probably too strict - but still an option)
> ** loads the two resource into a byte array and compares them - it only throws an error if there is difference
> ** compares the jars but only emits an error message if there is a conflict
> ** something else (user defined action?)
> Implementing such a classloader is not difficult and would greatly enhance the supportability of Oozie. It could work in multiple modes depending on the setup - perhaps being able to control it from a workflow config is desirable. If there's any problem, we should be able to turn it off completely, too.



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