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From Andrew Wang <andrew.w...@cloudera.com>
Subject Re: [DISCUSS] Treating LimitedPrivate({"MapReduce"}) as Public APIs for YARN applications
Date Tue, 10 May 2016 21:40:04 GMT
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.


On Tue, May 10, 2016 at 2:26 PM, Chris Nauroth <cnauroth@hortonworks.com>

> +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> 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> wrote:
> >>
> >>
> >>> On May 10, 2016, at 8:37 AM, Hitesh Shah <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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> >
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