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From "Vinod Kumar Vavilapalli (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11656) Classpath isolation for downstream clients
Date Tue, 03 Mar 2015 18:24:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14345439#comment-14345439
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Vinod Kumar Vavilapalli commented on HADOOP-11656:
--------------------------------------------------

bq. Ignore the amount of the work that would take us there, I would argue that creating a
client jar is the correct approach based on technical merits. It's silly to force users to
access HDFS to depend on Jetty 6, jersey-server, even if you manage to load them in a perfect
classloader.
This cannot be more true. Having a separate hdfs client JAR would vastly reduce the amount
of classpath conflicts. We have seen that in practice when we moved from Hadoop-1 MR to YARN
- having a leaner client JAR avoided a whole lot of problems we had before even if wasn't
perfectly done. A lean client JAR is also a major help in how we rationalize stack wide rolling
upgrades - today NameNode is on the classpath of ResourceManager and RegionServer even if
it doesn't get used and so it is very hard to layout and upgrade bits easily. Honestly, I
can't believe we are debating the benefits of having a clean client JAR.

> 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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