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From "Luke Lu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-1700) User supplied dependencies may conflict with MapReduce system JARs
Date Wed, 05 Sep 2012 16:57:09 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13448882#comment-13448882
] 

Luke Lu commented on MAPREDUCE-1700:
------------------------------------

bq. The conflict might require a user jar that is not compatible with one needed by the framework,
either order breaks something

You can always change the client framework and make it work with user code, per job, with
class path ordering. There is currently always a way in both Hadoop 1 and 2 to submit a job
with arbitrary dependencies, even though it might not be pretty (may require change to client
framework).

bq. The user might override a system jar and alter functionality in a way that breaks the
framework, or subverts security.

The client framework code can always be changed per job to accommodate new dependencies. MR
security is done at protocol level, i.e. no amount class path ordering can subvert security.

I agree with Arun that this is a nice to have feature to improve usability. Advanced users
can already achieve whatever that can be achieved (including running an OSGi container) per
job.
 
                
> User supplied dependencies may conflict with MapReduce system JARs
> ------------------------------------------------------------------
>
>                 Key: MAPREDUCE-1700
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1700
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>            Reporter: Tom White
>            Assignee: Tom White
>         Attachments: MAPREDUCE-1700.patch, MAPREDUCE-1700.patch
>
>
> If user code has a dependency on a version of a JAR that is different to the one that
happens to be used by Hadoop, then it may not work correctly. This happened with user code
using a different version of Avro, as reported [here|https://issues.apache.org/jira/browse/AVRO-493?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12852081#action_12852081].
> The problem is analogous to the one that application servers have with WAR loading. Using
a specialized classloader in the Child JVM is probably the way to solve this.

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