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From Steve Loughran <ste...@hortonworks.com>
Subject Re: [DISCUSS] what exactly are the stability guarantees of the YARN APIs
Date Tue, 31 May 2016 13:00:29 GMT

On 31 May 2016, at 05:44, Karthik Kambatla <kasha@cloudera.com<mailto:kasha@cloudera.com>>
wrote:

Inline.

On Sat, May 28, 2016 at 11:34 AM, Sangjin Lee <sjlee0@gmail.com<mailto:sjlee0@gmail.com>>
wrote:
I think there is more to it. The InterfaceStability javadoc states:
    Incompatible changes must not be made to classes marked as stable.

And in practice, I don't think the class annotation can be considered a simple sum of method
annotations. There is a notion of class compatibility distinct from method stability. One
key example is interfaces and abstract classes as in this case. The moment a new abstract
method is added, the class becomes incompatible as it would break all downstream subclasses
or implementing classes. That's the case even if *all methods are declared stable*. Thus,
adding any abstract method (no matter what their scope/stability is) should be considered
in violation of the stable contract of the class.

Fair point. I was referring to them in the context of adding @Evolving methods to @Stable
classes. Our policy states that "Classes not annotated are implicitly “Private”. Class
members not annotated inherit the annotations of the enclosing class." So, the annotation
on a method overrides that of the enclosing class. This seems pretty reasonable to me.



My code wouldn't even compile because new abstract methods were added to a class tagged as
stable.

As far as I'm concerned, it doesn't meat the strict semantics of "stable", unless there is
some nuance I'm missing.

Therefore, I'm with Sangin: adding new abstract methods to an existing @Stable class breaks
compatibility. Adding new non-abstract methods —fine. It would have been straightforward
to add some new methods to, say ContainerReport, which were no-ops/exception raising, but
which at least didn't break compilation. (though they may have broken codepaths which required
the methods to act as getters/settes)

Do you think there is reason to revisit this? If yes, we should update this for Hadoop 3.

I'm not sure about revisiting. I'm raising the fact that changes to classes marked as stable
have broken code, and querying the validity of such an operation within the constraints of
the 2.x codebase.

And I'm raising it on yarn-dev, as that's where things broke. If we do want to revisit things,
that'll mean a move to common-dev.



Regarding interfaces and abstract classes, one future enhancement to the InterfaceStability
annotation we could consider is formally separating the contract for users of the API and
the implementers of the API. They follow different rules. It could be feasible to have an
interface as Public/Stable for users (anyone can use the API in a stable manner) but Private
for implementers. The idea is that it is still a public interface but no third-party code
should not subclass or implement it. I suspect a fair amount of hadoop's public interface
might fall into that category. That itself is probably an incompatible change, so we might
have to wait until after 3.0, however.

Interesting thought. Agree that we do not anticipate users sub-classing most of our Public-Stable
classes.

There are also classes which we do not anticipate end-users to directly use, but devs might
want to sub-class. This applies to pluggable entities; e.g. SchedulingPolicy in fairscheduler.
We are currently using Public-Evolving to capture this intent.

Should we add a third annotation in addition to Audience and Stability to capture whether
a class can be extended? Given the few classes we anticipate being extended, this is likely
lesser work. :)


Some options.

-add a specific @PluginPoint extension with different stability requirements.(stable, unstable,
evolving). That tells implementors how likely things are to break.

-Add some interface to indicate really, really, unstable. That comes up more with things like
the Async FS APIs, where the discussion there is about how it may change radically.

Something like @Experimental could be that. That means not just "can change" but "can go away"
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