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https://issues.apache.org/jira/browse/HADOOP-11656?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14344194#comment-14344194
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Haohui Mai commented on HADOOP-11656:
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bq. Honestly, I don't see a lot of value in that work. It's a lot of
refactoring and restructuring, and at the end of the day, you end up with
(almost) as many library dependencies in the client as you had before (if you
keep guava, protobuf, jackson, etc. etc. in the client)
Just to want to make sure that the value of a HDFS client jar (HDFS-6200) is
appreciated. I'm not an expert in YARN, thus I only provide my two cents from
the HDFS prospective here.
Ignore the amount of the work that would take us there, I would argue that
creating a client jar is the correct approach based on technical merits. It's
silly to force users to access HDFS to depend on Jetty 6, jersey-server, even
if you manage to load them in a perfect classloader.
bq. The split would also introduce more complexity in the build... you can
expect that a lot of "class not found" errors would be popping up for a while.
The point of creating a HDFS client jar is not just simply moving the bits from
A to B. A more important aspect is to pick the more "stable" dependency in the
process -- for example, the client jar should at least avoid using any Guava
features as we know that Guava APIs are not stable.
bq. It's nice to have fewer jars on the classpath, but that's a pretty minor
benefit compared with the amount of work it would take to split this out. The
split would also introduce more complexity in the build... you can expect that
a lot of "class not found" errors would be popping up for a while.
What HDFS-6200 shows is that the approach is feasible. Indeed there are quite a
bit of work but I don't think it is insurmountable. I'm also concerned that the
complexity of the classloader approach is underestimated. Errors will show up
only at run-time until the classloader attempts to load the dependency.
bq. since nearly every Hadoop install co-locates the DNs and the clients.
This was true in the old days, but as new use cases emerge it might not be the
case in the future. As more data are being pumped into HDFS, there are more and
more terminals that access the data of HDFS. People are creating libraries
(e.g., Snakebite, libhdfs3) just to HDFS. People will certainly benefit it a
thin HDFS client is available.
> 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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