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From Steve Loughran <ste...@hortonworks.com>
Subject Re: Compatibility in Apache Hadoop
Date Tue, 23 Apr 2013 16:00:03 GMT
On 22 April 2013 18:32, Eli Collins <eli@cloudera.com> wrote:

> On Mon, Apr 22, 2013 at 5:42 PM, Steve Loughran <stevel@hortonworks.com>
> wrote:
> >
> > There's a separate issue that says "we make some guarantee that the
> > behaviour of a interface remains consistent over versions", which is hard
> > to do without some rigorous definition of what the expected behaviour of
> an
> > implementation should be.
> Good point, Steve.  I've assumed the semantics of the API had to
> respect the attribute (eg changing the semantics of FileSystem#close
> would be an incompatible change, since this is a public/stable API,
> even if the new semantics are arguably better).  But you're right,
> unless we've actually defined what the semantics of the APIs are it's
> hard to say if we've materially changed them.  How about adding a new
> section on the page and calling that out explicitly?


Maybe we should list which bits we consider both well specified and covered
with tests that verify the implementations in our svn match that

> In practice I think we'll have to take semantics case by case, clearly
> define the semantics we care about better in the javadocs (for the
> major end user-facing classes at least, calling out both intended
> behavior and behavior that's meant to be undefined) and using
> individual judgement elsewhere.  For example, HDFS-4156 changed
> DataInputStream#seek to throw an IOE if you seek to a negative offset,
> instead of succeeding then resulting in an NPE on the next access.

I'd seen that the DFS seek was the best implementation, but hadn't seen the
cause. The other ones (especially the Buffered one that goes in front of
most others) is much weaker

> That's an incompatible change in terms of semantics, but not semantics
> intended by the author, or likely semantics programs depend on.

That's a key problem: what do people depend on? A lot of the junit tests
depended on ordering of methods, after all

> However if a change made FileSystem#close three times slower, this
> perhaps a smaller semantic change (eg doesn't change what exceptions
> get thrown) but probably much less tolerable for end users.

You know that the blobstores all buffer their data so that

   1. flush() is a no-op
   2. the write takes place on close()

#1 changes durability expectations, while #2 means the time to close() is
O(data)*O(latency); P(fail) scales with time and distance, and as lots of
code swallows exceptions on close, those failures may even miss.

then there's the assumption that rename is atomic, which MapReduce depends

> In any case, even if we get an 80% solution to the semantics issue
> we'll probably be in good shape for v2 GA if we can sort out the
> remaining topics.   See any other topics missing?   Once the overall
> outline is in shape it make sense to annotate the page with the
> current policy (if there's already consensus on one), and identifying
> areas where we need to come up with a policy or are leaving TBD.
> Currently this is a source of confusion for new developers, some
> downstream projects and users.
How about

"semantic compatibility" : we strive to ensure that the behavior of APIs
remains consistent over versions, though changes for correctness may result
in changes in behavior  That is: if you relied on something which we
consider to be a bug, it may get fixed.

We are in the process of specifying some APIs more rigorously, enhancing
our test suites to verify compliance with the specification, effectively
creating a formal specification for the subset of behaviors that can be
easily tested. We welcome involvement in this process, from both users and
implementors of our APIs.

If you are concerned about compatibility at any level, we strongly
encourage you follow the Hadoop developer mailing lists, and track on JIRA
issues that may concern you. You are also strongly advised to verify that
your code works against beta releases of forthcoming Hadoop versions, as
that is a time in which identified regressions can be corrected rapidly -if
you only test when a new final release ships, the time to fix is likely to
be at least three months. "

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