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From Simon Kitching <Simon.Kitch...@smartstream-stp.com>
Subject RE: Proposal : remove references to guava library from public APIs
Date Wed, 01 Apr 2015 16:04:41 GMT
Hi All,

This problem would have occurred with or without OSGi. However without OSGi, it wouldn't be
solvable at all :-)

I need to write code that:
* uses a library that needs a very old version of guava, ie is not compatible with guava 16.0.1.
* uses curator which relies on guava 16.0.1 or later.

With a "flat" classpath, this just cannot be solved. With OSGi, it can be solved _if_ one
or both of these libraries only relies "internally" on guava.

If curator had used guava everywhere throughout its public API, I wouldn't bother asking here.
But curator is _very close_ to merely using guava as an internal detail, and so it seemed
worth asking if the dev team were interested in making the few changes necessary.

Note that this problem is partially self-inflicted due to my employer's requirement to use
this ugly library that relies on very old guava (actually, "com.google.collections:google-collections"
which is the earlier name for guava and sadly guava uses the same com.google.common.* package-names).
However every library makes incompatible changes from time to time, so this will probably
also bite someone else sometime.

Thanks for at least discussing this. As there's apparently no great enthusiasm for the idea,
I'll need to solve my immediate problem some other way (eg by forking curator locally or using
maven-shade-plugin or similar) for now. I'll try to find time next month (May) to see if I
can come up with some patches that might be acceptable (as mentioned in "compatible API" below).

Regards,
Simon

________________________________
From: Jordan Zimmerman [jordan@jordanzimmerman.com]
Sent: Wednesday, April 01, 2015 17:47
To: dev@curator.apache.org; Sean Busbey
Cc: Mike Drob; Simon Kitching
Subject: Re: Proposal : remove references to guava library from public APIs

The problem, though, was with OSGi. OSGi support for Curator was added by Curator users and
not the core team. Curator has never committed to OSGi support. Of course, we could change
that.

-JZ




On April 1, 2015 at 10:41:51 AM, Sean Busbey (busbey@cloudera.com<mailto:busbey@cloudera.com>)
wrote:

This thread started with Simon running in to a Guava problem while trying
to use Curator.

On Wed, Apr 1, 2015 at 10:33 AM, Jordan Zimmerman <
jordan@jordanzimmerman.com> wrote:

> I know Guava has caused problems elsewhere. But, I don’t recall any
> problems with Curator.
>
> -JZ
>
>
>
> On April 1, 2015 at 10:32:04 AM, Mike Drob (madrob@cloudera.com) wrote:
>
> Respectfully disagree. Guava issues have plagued Hadoop in the past (and to
> an extent, still do). Guava versions tend to iterate relatively quickly and
> don't always have the keenest eye on backwards compatibility. When you
> expose your guava implementation, that locks all of your users into the
> same version because newer versions may no longer work with your library
> (an issue which osgi seeks to address).
>
> The JDK, on the other hand, goes to painstaking lengths to ensure backwards
> compat for the past 20+ years.
>
> On Wed, Apr 1, 2015 at 9:55 AM, Jordan Zimmerman <
> jordan@jordanzimmerman.com
> > wrote:
>
> > I consider Guava to be part of the JDK so I disagree. We haven’t had many
> > issues with Guava compatibility. In fact, I can’t think of one Jira
> > reported on it. So, my vote would be to leave things as they are.
> >
> > -JZ
> >
> >
> >
> > On April 1, 2015 at 3:09:49 AM, Simon Kitching (
> > simon.kitching@smartstream-stp.com) wrote:
> >
> > Thanks Jordan.
> >
> > The root cause of the problem isn't really anything osgi-specific; it's
> > the fact that curator uses another library (guava) as part of its
> _public_
> > API.
> >
> > Imagine you wanted to change from using guava to some other collections
> > library - it wouldn't be possible without breaking the public API of
> > curator. The question is whether guava really should be part of the
> curator
> > API, or should be just an implementation detail. I would suggest that the
> > use of guava is really an implementation detail that should be
> > private/hidden - unlike use of jaxws for example, which really is an
> > externally-defined abstract API and is reasonable to include as part of
> the
> > public API of curator.
> >
> > This difference between used-in-the-impl and used-in-the-api doesn't
> > matter so much in a java application with one big classloader that has
> > every single jarfile in it; if you need the guava library in the
> classpath
> > for internal use by curator, then it will automatically also be visible
> to
> > all other classes and so it is impossible to have a different version of
> > the library also present. Using OSGi (which creates multiple
> classloaders)
> > can allow multiple versions of the same lib - but only when the lib is
> only
> > used-in-the-impl (ie is for "internal" usage by a jarfile).
> >
> > Re keeping a compatible API: possibly all classes in package
> > "org.apache.curator.framework.listen" could be copied into a new package,
> > and then the ListenerContainer class updated to not expose guava. All
> > classes in "org.apache.curator.framework.listen" could then be
> deprecated.
> > As long as OSGi code avoids using any code from the old package, there
> > would be no binding to the guava library used by curator. I don't know if
> > that would be better than simply changing the API for a couple of methods
> > or not.
> >
> > I will create a JIRA issue to update the maven-build-plugin version;
> > that's trivial and is not a binary incompatibility.
> >
> > Unless somebody objects within the next few days, I will also create a
> > JIRA issue regarding the APIs that expose guava. I might have time to
> work
> > on this myself next month (may).
> >
> > By the way, if you're interested in how OSGi classloading works, this may
> > be helpful: http://moi.vonos.net/java/osgi-classloaders/
> >
> > Thanks & Regards,
> > Simon (aka skitching at apache.org)
> >
> > ________________________________
> > From: Jordan Zimmerman [jordan@jordanzimmerman.com]
> > Sent: Tuesday, March 31, 2015 17:46
> > To: dev@curator.apache.org; Simon Kitching
> > Subject: Re: Proposal : remove references to guava library from public
> APIs
> >
> > I don’t have an objection in general. The biggest problem for me is that
> I
> > know very little about OSGI. All of the OSGI work has been contributed so
> > it’s hard to make sure that we keep it working. That said, changing
> > existing APIs is very disruptive to the Curator community. I’d like to
> see
> > someone (Simon?) commit to maintaining the OSGi compatibility of Curator
> > and make sure releases don’t introduce issues. Also, can the existing
> APIs
> > remain and new, OSGi compatible parallel APIs be added?
> >
> > -JZ
> >
> >
> >
> > On March 31, 2015 at 7:39:08 AM, Simon Kitching (
> > simon.kitching@smartstream-stp.com<mailto:
> > simon.kitching@smartstream-stp.com>) wrote:
> >
> > Hi,
> >
> > I've noticed that several curator classes expose the use of classes from
> > google's guava library [1] as part of their *public* api.
> >
> > [1] maven artifact "com.google.guava:guava" which contains java packages
> > com.google.common.*
> >
> > In an OSGi environment, it is possible to load multiple different
> versions
> > of the same library, as long as that library is a purely internal
> > implementation detail. Unfortunately, as curator exposes its use of
> guava,
> > this makes it impossible for code that uses curator to also use a
> different
> > version of Guava for its own purposes. Unfortunately, this has just
> bitten
> > me : I need to write code that uses both curator (requires guava 16.0 or
> > later) and a third-party library that requires an earlier version of
> guava.
> >
> > Are there any objections to me raising an enhancement issue in JIRA for
> > this? Note that this change would be a binary incompatibility (though
> > fairly limited).
> >
> > The problem classes that I have found are:
> > * curator-framework:
> org.apache.curator.framework.listen.ListenerContainer
> > : method forEach takes a parameter of type
> com.google.common.base.Function
> > * curator-framework:
> > org.apache.curator.framework.api.transaction.CuratorTransactionResult :
> > method ofTypeAndPath returns com.google.common.base.Predicate
> > * curator-x-discovery-server:
> > org.apache.curator.x.discovery.server.contexts.GenericDiscoveryContext :
> > constructor takes param of type com.google.common.reflect.TypeToken
> > * curator-x-discovery: org.apache.curator.x.discovery.InstanceFilter :
> > inherits from com.google.common.base.Predicate
> >
> > And by the way, I noticed that org.codehaus.jackson types are also used
> in
> > public APIs (at least, GenericDiscoveryContext). It may also be worth
> > looking into whether it is really necessary to expose this dependency.
> >
> > The goal of the change would be to ensure that in the MANIFEST.MF file
> for
> > each curator bundle (jarfile), the Export-Packages line minimises the
> > "uses:=" entries which refer to non-curator packages. A uses-constraint
> on
> > a package should only be needed when something in the package being
> > exported uses an external type in its public API.
> >
> > As a separate problem, I have noticed that with the 2.7.1 release (at
> > least), the "bnd" tool (via maven-bundle-plugin) is adding entries to the
> > "uses" entries even when the referenced library is purely used
> internally.
> > I have found a reference (https://github.com/emlun/bnd-uses-strange)
> that
> > suggests this is a bug which is fixed in later bnd releases.
> Unfortunately
> > I can find no release-notes for bnd, nor any source-code repository so
> > cannot confirm this. However updating curator/pom.xml to specify the
> > following fixes the "uses" clauses:
> > <maven-bundle-plugin-version>2.5.3</maven-bundle-plugin-version>
> >
> > Thanks & Regards,
> > Simon
> >
> > ________________________________
> > The information in this email is confidential and may be legally
> > privileged. It is intended solely for the addressee. Access to this email
> > by anyone else is unauthorised. If you are not the intended recipient,
> any
> > disclosure, copying, distribution or any action taken or omitted to be
> > taken in reliance on it, is prohibited and may be unlawful. SmartStream
> > Technologies GmbH, Vienna Twin Tower, Wienerbergstrasse 11, 1100 Vienna,
> > Austria, FN 194340w, HG Wien
> > ________________________________
> > The information in this email is confidential and may be legally
> > privileged. It is intended solely for the addressee. Access to this email
> > by anyone else is unauthorised. If you are not the intended recipient,
> any
> > disclosure, copying, distribution or any action taken or omitted to be
> > taken in reliance on it, is prohibited and may be unlawful. SmartStream
> > Technologies GmbH, Vienna Twin Tower, Wienerbergstrasse 11, 1100 Vienna,
> > Austria, FN 194340w, HG Wien
> >
>



--
Sean
________________________________
The information in this email is confidential and may be legally privileged. It is intended
solely for the addressee. Access to this email by anyone else is unauthorised. If you are
not the intended recipient, any disclosure, copying, distribution or any action taken or omitted
to be taken in reliance on it, is prohibited and may be unlawful. SmartStream Technologies
GmbH, Vienna Twin Tower, Wienerbergstrasse 11, 1100 Vienna, Austria, FN 194340w, HG Wien

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