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From Steve Loughran <ste...@hortonworks.com>
Subject Re: Hadoop Code of Incompatible Changes
Date Wed, 30 Jul 2014 11:17:24 GMT
On 29 July 2014 22:14, Sandy Ryza <sandy.ryza@cloudera.com> wrote:

>
> http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/Compatibility.html


Sandy is correct is a semantic compatibility issue. Back then the notion of
"interface" was defined in the early 1970s by D.L Parnas (see:
http://www.cs.mun.ca/~harold/Courses/Old/CS3718.F08/Diary/p330-parnas.pdf),
he made clear from the outset that an interface was a combination of
(binary signature, functional semantics) -it's just that C++ derivative
languages have only focused on the signature, which is what can be checked
at compile and link times


Policy there is "The behavior of API may be changed to fix incorrect
behavior"


If an AM is now expected to handle new things, then it is a change in the
semantics...though if it was due to a bug fix -things didn't work before-
then it could maybe get in.

With YARN, there's now a (wide) interface between apps and Hadoop, and
right now it is fairly intimate -you do tend to end up using things marked
as private because its how you get stuff like the web proxy/filter and
other things that I can't remember); I think it's going to take a while to
stabilise.

Ideally for this patch, I'd like an AM (including my own) to at least
degrade to its existing behaviour, even if it doesn't pick up any new
features. Is this the case?

-steve

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