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https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14347188#comment-14347188
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Sean Busbey commented on HADOOP-11656:
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{quote}
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.
{quote}
Yes, native library support is an entire additional can of worms. For this
improvement I'd prefer to leave that to future work, if only because the JVM
doesn't really offer options. Perhaps docs that cover the limitations of what
isolation we offer would be a good start.
> 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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