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From "Sean Busbey (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11656) Classpath isolation for downstream clients
Date Mon, 24 Apr 2017 18:39:05 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15981645#comment-15981645

Sean Busbey commented on HADOOP-11656:

This is an issue IMO. I didn't realize the shaded classes were bundled into the hadoop jars
themselves and published to the maven central. I wonder if it is feasible to keep shaded dependencies
as individual jars but only provide it in the dist tarball. That way, we don't republish shaded
dependencies to maven central. Sean Busbey, what do you think?

No, this isn't possible. We're talking about client side dependencies. Maven central is exactly
where the vast majority of downstream users will get the artifacts they need to test that
things work as expected.

To me this means the following when using the shaded jar :
* Can't take a stack trace, drop it into an IDE, and then start clicking on line numbers to
see source. Can easily get around this by removing prefixes in stack trace.
* When stepping through code in a debugger, will not have source for relocated dependencies.
Not sure of an easy way around this one other than using non-shaded jars for debugging.

This is an issue if we don't make source jars. HBase is working on this for some of their
stuff. We could work to do the same, presuming the simple decompiling the IDE does when there
isn't source available isn't sufficient.

> 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
>            Priority: Blocker
>              Labels: classloading, classpath, dependencies, scripts, shell
>         Attachments: HADOOP-11656_proposal.md
> 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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