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From Jarek Jarcec Cecho <jar...@apache.org>
Subject Re: Hadoop as Compile time dependency in Sqoop2
Date Thu, 11 Dec 2014 22:50:19 GMT
Nope not at all Abe, I also feel that client and server changes should be discussed separately
as there are different reasons/concerns of why or why not introduce Hadoop dependencies there.

For the server side and for the security portion, I feel that we had good discussion with
Richard while back and I do not longer have concerns about using those APIs. I’ll advise
caution nevertheless. What we are trying to achieve by changing the scope from “provided”
to “compile” here? To my best knowledge [1] the difference is only that “provided”
means that the dependency is not retrieved and stored in resulting package and that users
have to add it manually after installation. I’m not immediately seeing any impact on the
code though.


1: http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html

> On Dec 11, 2014, at 8:41 AM, Abraham Elmahrek <abe@cloudera.com> wrote:
> Jarcec,
> Sorry to bud in... you make a good point on the client side. Would you mind
> if we discussed the server side a bit? Re-using the same mechanism on the
> server side does require "compile" scope dependencies on Hadoop. Would that
> be ok? Are the concerns mainly around the client?
> -Abe
> On Thu, Dec 11, 2014 at 10:30 AM, Jarek Jarcec Cecho <jarcec@apache.org>
> wrote:
>> Got it Richard, thank you very much for the nice summary! I’m wondering
>> what is the use case for delegation tokens on client side? Is it to support
>> integration with Oozie?
>> I do know that Beeline is depending on Hadoop common and that is actually
>> a very good example. I’ve seen sufficient number of users struggling with
>> this dependency - using various workarounds for the classpath issue, having
>> need to copy over Hadoop configuration files from real cluster (because
>> otherwise portion of the security didn’t work at all, something with
>> auth_to_local rules) and a lot of more. That is why I’m advising being
>> careful here.
>> Jarcec
>>> On Dec 11, 2014, at 12:17 AM, Zhou, Richard <richard.zhou@intel.com>
>> wrote:
>>> Hi Jarcec:
>>> Thank you very much for your clarification about the history.
>>> The root cause for why we want to change "provided" to "compile" is to
>> implement "Delegation Token Support" [1], review board [2]. The status in
>> Hadoop is showed below.
>>> Hadoop 2.5.1 or before: all classes used to implement Kerberos support
>> is in Hadoop-auth component, which depends only several libs with
>> non-Hadoop related lib. And it is added in Sqoop client side (shell
>> component [3]) as "compile" as we agreed before.
>>> Hadoop 2.6.0: There is a refactor to support delegation token in Hadoop
>> [4]. Most components in Hadoop, such as RM, Httpfs and Kms, have rewritten
>> authentication mechanism to use delegation token. However, all delegation
>> token related class is in Hadoop-common instead of Hadoop-auth, because it
>> uses UserGroupInfomation class.
>>> So if Sqoop need to support delegation token, it has to include
>> Hadoop-common lib, because I believe that copying code is an unacceptable
>> solution. Even using Hadoop shims, which is a good solution to support
>> different version of Hadoop (I am +1 on writing a Hadoop shims in Sqoop
>> like pig, hive etc.), the Hadoop-common is also a dependency. For example,
>> the client side (beeline) in hive depends on Hadoop-common lib [5]. So I
>> don't think it is a big problem to add Hadoop-common in.
>>> Additionally, I agree with Abe that wire compatibility is another reason
>> to change "provided" to "compile", since it is in "Unstable" state. There
>> will be a potential problem in the future.
>>> So I prefer to add Hadoop-common lib as "compile" to make "Delegation
>> Token Support" happen.
>>> Add intel-sqoop@cloudera.org.
>>> Links:
>>> 1: https://issues.apache.org/jira/browse/SQOOP-1776
>>> 2: https://reviews.apache.org/r/28795/
>>> 3: https://github.com/apache/sqoop/blob/sqoop2/shell/pom.xml#L75
>>> 4: https://issues.apache.org/jira/browse/HADOOP-10771
>>> 5: https://github.com/apache/hive/blob/trunk/beeline/pom.xml#L133
>>> Richard
>>> -----Original Message-----
>>> From: Jarek Jarcec Cecho [mailto:jarcec@gmail.com] On Behalf Of Jarek
>> Jarcec Cecho
>>> Sent: Thursday, December 11, 2014 1:43 PM
>>> To: dev@sqoop.apache.org
>>> Subject: Re: Hadoop as Compile time dependency in Sqoop2
>>> Hi Abe,
>>> thank you very much for surfacing the question. I think that there is a
>> several twists to it, so my apologies as this will be a long answer :)
>>> When we’ve started working on Sqoop 2 few years back, we’ve
>> intentionally pushed the Hadoop dependency as far from shared libraries as
>> possible. The intention was that no component in common or core should be
>> depending nor use any Hadoop APIs and those should be isolated to separate
>> modules (execution/submission engine). The reason for that is that Hadoop
>> doesn’t have particularly good track of keeping backward compatibility and
>> it has bitten a lot of projects in the past. For example every single
>> project that I know of that is using MR needs to have a shim layer that is
>> dealing with the API differences (Pig [1], Hive [2], …) . The only
>> exception to this that I’m aware of is Sqoop 1, where we did not had to
>> introduce shims is only because we (shamelessly) copied code from Hadoop to
>> our own code base. Nevertheless we have places where we had to do that
>> detection nevertheless [3]. I’m sure that Hadoop is getting better as the
>> project matures, but I would still advise being careful of using various
>> Hadoop APIs and limit that usage to the extend needed. There will be
>> obviously situations where we want to use Hadoop API to make our life
>> simpler, such as reusing their security implementation and that will be
>> hopefully fine.
>>> Whereas we can be pretty sure that Sqoop Server will have Hadoop
>> libraries on the class-path and the concern there was more about
>> introducing backward incompatible changes that is hopefully less important
>> nowadays, not introducing Hadoop dependency on client side had a different
>> reason. Hadoop common is quite important jar that have huge number of
>> dependencies - check out the list at it’s pom file [4]. This is a problem
>> because the Sqoop client is meant to be small and easily reusable wheres
>> depending on Hadoop will force the application developer to certain library
>> versions that are dictated by Hadoop (like guava, commons-*). And that
>> forces people to do various weird things such as using custom class loaders
>> to isolate those libraries from main application and making the situation
>> in most cases even worst, because Hadoop libraries assumes “ownership” of
>> the underlaying JVM and run a lot of eternal threads per class-loader.
>> Hence I would advise being double careful when introducing dependency on
>> Hadoop (common) for our client.
>>> I’m wondering what we’re trying to achieve by moving the dependency from
>> “provided” to “compile”? Do we want to just ensure that it’s always on
>> Server side or is the intent to get it to the client?
>>> Jarcec
>>> Links:
>>> 1: https://github.com/apache/pig/tree/trunk/shims/src
>>> 2: https://github.com/apache/hive/tree/trunk/shims
>>> 3:
>> https://github.com/apache/sqoop/blob/trunk/src/java/org/apache/sqoop/mapreduce/hcat/SqoopHCatUtilities.java#L962
>>> 4:
>> http://search.maven.org/#artifactdetails%7Corg.apache.hadoop%7Chadoop-common%7C2.6.0%7Cjar
>>>> On Dec 10, 2014, at 7:56 AM, Abraham Elmahrek <abe@cloudera.com> wrote:
>>>> Hey guys,
>>>> With the work being done in Sqoop2 involving authentication, there are
>>>> a few classes that are being used from hadoop auth and eventually
>>>> hadoop common.
>>>> I'd like to gauge how folks feel about including the hadoop libraries
>>>> as a "compile" time dependency rather than "provided". The reasons
>> being:
>>>> 1. Hadoop maintains wire compatibility within a major version:
>> http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/Compatibility.html#Wire_compatibility
>>>> 2. UserGroupInformation and other useful interfaces are marked as
>>>> "Evolving" or "Unstable":
>> http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/InterfaceClassification.html
>>>> .
>>>> I've been looking around and it seems most projects include Hadoop as
>>>> a compile time dependency:
>>>> 1. Kite -
>> https://github.com/kite-sdk/kite/blob/master/kite-hadoop-dependencies/cdh5/pom.xml
>>>> 2. Flume - https://github.com/apache/flume/blob/trunk/pom.xml
>>>> 3. Oozie - https://github.com/apache/oozie/tree/master/hadooplibs
>>>> 4. hive - https://github.com/apache/hive/blob/trunk/pom.xml#L1067
>>>> IMO wire compatibility is easier to maintain than Java API
>> compatibility.
>>>> There may be features in future Hadoop releases that we'll want to use
>>>> on the security side as well.
>>>> -Abe
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