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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11656) Classpath isolation for downstream clients
Date Mon, 02 Mar 2015 20:01:05 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14343673#comment-14343673

Steve Loughran commented on HADOOP-11656:

I'm not trying to stop this work, I do agree that it needs fixing, just wondering how to do
this in a way which has (a) tangible immediate benefits in 2015 (b) keeps Hadoop 3.x a low-cost,
low-risk update, not a Perl 6 or python 3.

Maybe there are multiple strategies to take here, short term and long term

Short term (2.x)
# Hadoop works across all shipping guava versions, so update it in 2.8 (giving a warning in
2.7 that this is the last)
# get the OSGI patches in, so that anyone who wants to use Hadoop 2.x code within an OSGi-enabled
JVM, can.

Longer term (3.x)

# split client/server artifacts with a leaner client (which can still use guava, protobuf,
SLF4J &c), just strip out the pure-server side stuff from HDFS, so at least introduce
less there.
# maybe a pure-REST client built on Jersey (and its dependencies), supporting SPNEGO authed
interaction with WebHDFS, YARN, other apps. This will underperform compared to in-cluster
HDFS apps, but should be sufficient for remote interaction.
# classpath isolation as proposed here (somehow)

> 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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