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From Chris Nauroth <cnaur...@hortonworks.com>
Subject Re: [DISCUSS] Treating LimitedPrivate({"MapReduce"}) as Public APIs for YARN applications
Date Tue, 10 May 2016 21:46:51 GMT
Yes, I agree with you Andrew.

Sorry, I should clarify my prior response.  I didn't mean to imply a blind s/LimitedPrivate/Public/g
across the whole codebase.  Instead, I'm +1 for the intent of HADOOP-10776: a transition to
Public for UserGroupInformation, and by extension the related parts of its API like Credentials.

I'm in the camp that generally questions the usefulness of LimitedPrivate, but I agree that
transitions to Public need case-by-case consideration.

--Chris Nauroth

From: Andrew Wang <andrew.wang@cloudera.com<mailto:andrew.wang@cloudera.com>>
Date: Tuesday, May 10, 2016 at 2:40 PM
To: Chris Nauroth <cnauroth@hortonworks.com<mailto:cnauroth@hortonworks.com>>
Cc: Hitesh Shah <hitesh@apache.org<mailto:hitesh@apache.org>>, "yarn-dev@hadoop.apache.org<mailto:yarn-dev@hadoop.apache.org>"
<yarn-dev@hadoop.apache.org<mailto:yarn-dev@hadoop.apache.org>>, "mapreduce-dev@hadoop.apache.org<mailto:mapreduce-dev@hadoop.apache.org>"
<mapreduce-dev@hadoop.apache.org<mailto:mapreduce-dev@hadoop.apache.org>>, "common-dev@hadoop.apache.org<mailto:common-dev@hadoop.apache.org>"
<common-dev@hadoop.apache.org<mailto:common-dev@hadoop.apache.org>>
Subject: Re: [DISCUSS] Treating LimitedPrivate({"MapReduce"}) as Public APIs for YARN applications

Why don't we address these on a case-by-case basis, changing the annotations on these key
classes to Public? LimitedPrivate{"YARN applications"} is the same thing as Public.

This way we don't need to add special exceptions to our compatibility policy. Keeps it simple.

Best,
Andrew

On Tue, May 10, 2016 at 2:26 PM, Chris Nauroth <cnauroth@hortonworks.com<mailto:cnauroth@hortonworks.com>>
wrote:
+1 for transitioning from LimitedPrivate to Public.

I view this as an extension of the need for UserGroupInformation and
related APIs to be Public.  Regardless of the original intent behind
LimitedPrivate, these are de facto public now, because there is no viable
alternative for applications that want to integrate with a secured Hadoop
cluster.

There is prior discussion of this topic on HADOOP-10776 and HADOOP-12913.
HADOOP-10776 is a blocker for 2.8.0 to make the transition to Public.

--Chris Nauroth




On 5/10/16, 11:34 AM, "Hitesh Shah" <hitesh@apache.org<mailto:hitesh@apache.org>>
wrote:

>There seems to be some incorrect assumptions on why the application had
>an issue. For rolling upgrade deployments, the application bundles the
>client-side jars that it was compiled against and uses them in its
>classpath and expects to be able to communicate with upgraded servers.
>Given that hadoop-common is a monolithic jar, it ends up being used on
>both client-side and server-side. The problem in this case was caused by
>the fact that the ResourceManager was generating the credentials file
>with a format understood only by hadoop-common from 3.x. For an
>application compiled against 2.x and has *only* hadoop-common from 2.x on
>its classpath, trying to read this file fails.
>
>This is not about whether internal implementations can change for
>non-public APIs. The file format for the Credential file in this scenario
>is *not* internal implementation especially when you can have different
>versions of the library trying to read the file. If an older client is
>talking to a newer versioned server, the general backward compat
>assumption is that the client should receive a response that it can parse
>and understand. In this scenario, the credentials file provided to the
>YARN app by the RM should have been written out with the older version or
>at the very least been readable by the older hadoop-common.jar.
>
>In any case, does anyone have any specific concerns with changing
>LimitedPrivate({"MapReduce"}) to Public?
>
>And sure, if we are saying that Hadoop-3.x requires all apps built
>against it to go through a full re-compile as well as downtime as
>existing apps may no longer work out of the box, lets call it out very
>explicitly in the Release notes.
>
>‹ Hitesh
>
>> On May 10, 2016, at 9:24 AM, Allen Wittenauer
>><allenwittenauer@yahoo.com<mailto:allenwittenauer@yahoo.com>> wrote:
>>
>>
>>> On May 10, 2016, at 8:37 AM, Hitesh Shah <hitesh@apache.org<mailto:hitesh@apache.org>>
wrote:
>>>
>>> There have been various discussions on various JIRAs where upstream
>>>projects such as YARN apps ( Tez, Slider, etc ) are called out for
>>>using the above so-called Private APIs. A lot of YARN applications that
>>>have been built out have picked up various bits and pieces of
>>>implementation from MapReduce and DistributedShell to get things to
>>>work.
>>>
>>> A recent example is a backward incompatible change introduced ( where
>>>the API is not even directly invoked ) in the Credentials class related
>>>to the ability to read tokens/credentials from a file.
>>
>>      Let¹s be careful here.  It should be noted that the problem happened
>>primarily because the application jar appears to have included some
>>hadoop jars in them.   So the API invocation isn¹t the problem:  it¹s
>>the fact that the implementation under the hood changed.  If the
>>application jar didn¹t bundle hadoop jars ‹especially given that were
>>already on the classpath--this problem should never have happened.
>>
>>> This functionality is required by pretty much everyone as YARN
>>>provides the credentials to the app by writing the credentials/tokens
>>>to a local file which is read in when
>>>UserGroupInformation.getCurrentUser() is invoked.
>>
>>      What you¹re effectively arguing is that implementations should never
>>change for public (and in this case LimitedPrivate) APIs.  I don¹t think
>>that¹s reasonable.  Hadoop is filled with changes in major branches
>>where the implementations have changed but the internals have been
>>reworked to perform the work in a slightly different manner.
>>
>>> This change breaks rolling upgrades for yarn applications from 2.x to
>>>3.x (whether we end up supporting rolling upgrades across 2.x to 3.x is
>>>a separate discussion )
>>
>>
>>      At least today, according to the document attached to YARN-666 (lol),
>>rolling upgrades are only supported within the same major version.
>>
>>>
>>> I would like to change our compatibility docs to state that any API
>>>that is marked as LimitedPrivate{Mapreduce} implies LimitedPrivate{YARN
>>>Applications}.
>>>
>>> Comments/concerns?
>>
>>
>>      a)  That isn¹t good enough.  No one reads the compatibility guidelines
>>as it is given how many times someone says ³X² isn¹t covered when it
>>clearly is.
>>
>>      b) LimitedPrivate{³YARN Applications²} makes zero sense.  At that
>>point it¹s Public and the source should be changed to reflect that.
>>Especially given those flags impacts things like how the javadocs are
>>generated.
>
>
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