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https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14349350#comment-14349350
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Colin Patrick McCabe commented on HADOOP-11656:
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bq. Steve wrote: 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.
Yeah. It's certainly true that we can't currently load multiple versions of
the native libraries, and this project would not improve that situation. There
are a lot of problems involved in making that work... just off the top of my
head, dlopen + dlsym could be helpful in accomplishing this, combined with
perhaps different names for different versions of libhadoop. However, that is
a separate project and I think well outside the scope discussed here.
> 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, scripts, shell
>
> 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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