Sean Busbey created HADOOP-11656:
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             Summary: 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


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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