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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11656) Classpath isolation for downstream clients
Date Tue, 03 Mar 2015 21:13:08 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14345741#comment-14345741
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Steve Loughran commented on HADOOP-11656:
-----------------------------------------

I hadn't come across the ApplicationClassloader/HADOOP-10893 myself: It''s **a** classloader;
I'd actually have to spend time using it to be confident it worked.

As Vinod noted, YARN apps can bundle up their entire tarball of dependencies, client-side
and have them deployed in-cluster, picking up the core-site.xml & c from the live cluster
by setting up their classpath of the AM right. This is what we do in slider to ensure that
there are no signature-compatibility problems between our AM code and the hadoop JARs. We
don't isolate classpaths though, & make sure that we are consistent with the dependency
versions of the AM. I think if hadoop was OSGi-ready, then we'd consider deploying in an OSGI
runtime like felix.

One troublespot, even with that tactic, is shown by HADOOP-11064: "UnsatisifedLinkError with
hadoop 2.4 JARs on hadoop-2.6 due to NativeCRC32 method changes". Changes in the internal
JNI bindings meant that no hadoop-2.4 app (like HBase) would run in a Hadoop 2.6-alpha cluster.
We were lucky that I got to find that before 2.6 shipped, otherwise we'd have a lot of complaints.
The problem here is that even with HBase  isolated on classpath, it was picking up the hadoop-native
binaries from somewhere on PATH/LIB or whatever, and so failing to link. 

Classloader isolation & shading isn't going to be sufficient here. HADOOP-11127 proposes
some versioning, which will help —but I don't think it will let us load >1 hadoop.lib
into a JVM. As a result, the only version of hadoop-common.jar which can be reliably loaded
into a process is the one that is in sync with the version of the native library on the target
machine.

> 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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