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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11656) Classpath isolation for downstream clients
Date Wed, 04 Mar 2015 17:49:08 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14347234#comment-14347234
] 

stack commented on HADOOP-11656:
--------------------------------

bq. To add, I think we can and should strive for doing this in a compatible manner, whatever
the approach.

Sure. Sounds good if possible at all as well as being a load of work proving changes are indeed
compatible.

bq. Marking and calling it incompatible before we see proposal/patch seems premature to me.

I'd suggest you open a new issue to do classpath isolation in a 'compatible manner' rather
than add this imposition here. In this issue, the reporter thinks it a breaking change ("At
a minimum we'll break dependency compatibility and operational compatibility."). The two issues
can move along independent of each other.

And to be clear when we talk 'compatible manner', the expectation is that a downstream apps,
for example hbase, should be able to move from hadoop-2.X to hadoop-2.Y without breakage,
right? That is, in spite of shading, new locations for dependencies, cleaned up exposure of
libs likely transitively included, etc., there will be no need for downstreamers to add in
new compensatory code, no need of our having to release special versions to work with hadoop-2.Z,
and no need of callouts in code or for us to do educate our community's that "if on hadoop-2.X
do this...but if on hadoop-2.Y...." do that? Or are we talking something else (And "downstreamers,
you are doing it wrong" is not allowed).

> Classpath isolation for downstream clients
> ------------------------------------------
>
>                 Key: HADOOP-11656
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11656
>             Project: Hadoop Common
>          Issue Type: New Feature
>            Reporter: Sean Busbey
>            Assignee: Sean Busbey
>              Labels: classloading, classpath, dependencies
>
> Currently, Hadoop exposes downstream clients to a variety of third party libraries. As
our code base grows and matures we increase the set of libraries we rely on. At the same time,
as our user base grows we increase the likelihood that some downstream project will run into
a conflict while attempting to use a different version of some library we depend on. This
has already happened with i.e. Guava several times for HBase, Accumulo, and Spark (and I'm
sure others).
> While YARN-286 and MAPREDUCE-1700 provided an initial effort, they default to off and
they don't do anything to help dependency conflicts on the driver side or for folks talking
to HDFS directly. This should serve as an umbrella for changes needed to do things thoroughly
on the next major version.
> We should ensure that downstream clients
> 1) can depend on a client artifact for each of HDFS, YARN, and MapReduce that doesn't
pull in any third party dependencies
> 2) only see our public API classes (or as close to this as feasible) when executing user
provided code, whether client side in a launcher/driver or on the cluster in a container or
within MR.
> This provides us with a double benefit: users get less grief when they want to run substantially
ahead or behind the versions we need and the project is freer to change our own dependency
versions because they'll no longer be in our compatibility promises.
> Project specific task jiras to follow after I get some justifying use cases written in
the comments.



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