I recommend that we update the compatibility guide with some text that explicitly addresses subclassing/interface inheritance stability for classes/interfaces annotated Stable. This is for our own benefit too. (I often refer back to that doc when I'm coding a patch that might have a chance of being backwards-incompatible.)
--Chris Nauroth On 5/31/16, 9:46 AM, "Karthik Kambatla" <[email protected]> wrote: >Argh! Totally my bad on YARN-2882. Kept missing the changes to >ContainerStatus even after you pointed out. > >Filed YARN-5184 to fix this before we release it. Thanks for pointing it >out, Steve. > >On Tue, May 31, 2016 at 6:00 AM, Steve Loughran <[email protected]> >wrote: > >> >> On 31 May 2016, at 05:44, Karthik Kambatla <[email protected]<mailto: >> [email protected]>> wrote: >> >> Inline. >> >> On Sat, May 28, 2016 at 11:34 AM, Sangjin Lee <[email protected]<mailto: >> [email protected]>> 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 ³Private². 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" >> --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
