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From David Bosschaert <david.bosscha...@gmail.com>
Subject Re: [SPI Fly] Interest for a more dynamic plug-in/service discovery API?
Date Fri, 05 Oct 2012 12:04:06 GMT
Hi Jeremias,

I took some time to look at your code, just echoing what I think that
it does to make sure that I understand things correctly.
I think it implements a dynamic services mechanism that works both
inside OSGi as well as outside of OSGi. Correct?

Services are declared using the META-INF/services resources, just like
they are done with ServiceLoader. The client side doesn't use the
java.util.ServiceLoader.load() API.

In the case we're outside of OSGi there is a custom non-OSGi
ServiceTracker/ServiceListener that reacts to services arriving /
disappearing.

When you're inside OSGi you can look up these services from the
Service Registry as SPI-Fly has put them there.

Am I correctly summarizing? Did I miss anything?

Next question is would it make sense to add this functionality to Aries?
I think it does. To me many of the ideas in here match with the OSGi
Connect RFP 145 (http://www.osgi.org/bugzilla/show_bug.cgi?id=145) and
I think that, besides its practical use today, this code could be a
valuable input to the standardization process of OSGi Connect. Overall
the charter of OSGi Connect is to create a dynamic services
environment that works both inside OSGi and out. To me the overall
goal of your code seems similar.
If we all agree that it would be suitable for this component to reside
in Aries, I think we should strive to make it ultimately compliant
with the OSGi Connect spec, when that's available.

Does this make sense to you?

Best regards,

David

On 1 October 2012 14:24, David Bosschaert <david.bosschaert@gmail.com> wrote:
> Thanks for reminding me, Jeremias. I downloaded it but then I got
> distracted. Hope to have a look soon.
>
> David
>
> On 1 October 2012 03:27, Jeremias Maerki <dev@jeremias-maerki.ch> wrote:
>> Hi David,
>> did you have any chance to look at it already?
>>
>> BTW, I've just found a possible memory leak in the OSGi case when the
>> client doesn't (or can't easily) remove the ServiceListener. Usually,
>> there's only one ServiceListener per client class (static context) so
>> it's not totally serious, but it prevents GC'ing the bundle. I'm working
>> on that now.
>>
>> Cheers,
>> Jeremias Maerki
>>
>>
>> On 30.08.2012 17:29:47 David Bosschaert wrote:
>>> Thanks Jeremias!
>>>
>>> I'll have a look at this soon.
>>>
>>> Cheers,
>>>
>>> David
>>>
>>> On 23 August 2012 15:39, Jeremias Maerki <dev@jeremias-maerki.ch> wrote:
>>> > Hi David
>>> >
>>> > Sorry for the delay. As life plays, you get side-tracked all the time,
>>> > but since this is important to me I had to revisit eventually.
>>> >
>>> > I've simplified and cleaned up the code some more and added some tests.
>>> > It's now ready for review:
>>> > http://www.jeremias-maerki.ch/download/osgi/ch.jm.util.services-2.0.0.dev-src.zip
>>> >
>>> > So, if you still think this could be a good addition to Aries, I can
>>> > rename the packages, switch to a Maven build (sigh) and prepare a
>>> > package to vote upon for inclusion in Aries.
>>> >
>>> > What could we name it?
>>> > - org.apache.aries.spifly.consumer
>>> > - org.apache.aries.spifly.discovery
>>> > - org.apache.aries.discovery
>>> > - org.apache.aries.plugin.discovery
>>> > - org.apache.aries.spi.catch ;-)
>>> >
>>> > You mentioned earlier that you don't see it as part of SPI-Fly. In its
>>> > current state it actually relies on a working implementation of RFP 167
>>> > (SPI Service Loader support) because I removed the extender that looks
>>> > for META-INF/services from 1.0 to 2.0. So, while it is not part of RFC
>>> > 167, it is still thighly connected to SPI-Fly, although not through
>>> > package imports.
>>> >
>>> > On RFP 143: like I already mentioned, I do find it interesting. It can
>>> > serve as a different approach to solving the same problem. However, my
>>> > approach gets away with a 20KB JAR, compared to the 220KB of PojoSR
>>> > (still small!). But my main argument is actually this: I need to find a
>>> > way to sell an easy way to the Apache XML Graphics project to achieve
>>> > OSGi compatibility including plug-in support without throwing everything
>>> > upside down. Practically none of the committers are familiar with OSGi.
>>> > Introducing an OSGi-compatible service registry is generally cool for
>>> > someone who knows OSGi already but the others would probably ask: why do
>>> > we need to learn about this? After all, the service registry can be a
>>> > bit verbose for a newbie with ServiceReference, BundleContext and such.
>>> > I believe that selling OSGi Connect would be more difficult in such a
>>> > case than my small proposal.
>>> >
>>> > In the end, I'm looking for the best home for it. I think Aries is a
>>> > logical place.
>>> >
>>> > When looking at PojoSR I was wondering about use cases. Personally, I'd
>>> > rather have a full OSGi environment any time even if it means that I
>>> > have to be able to maintain a bundle cache/storage folder on the local
>>> > file system (I actually wonder why these are not pluggable and
>>> > virtualizable, yet, but that's a different story). Use case 4.2 (WARs)
>>> > in the RFP draft makes a lot of sense, though. There you don't usually
>>> > have any service dynamics. Or a command-line application that uses some
>>> > code that usually runs in a OSGi-enabled server. I have such a case that
>>> > I might explore eventually. But frankly, OSGi Connect doesn't create too
>>> > much attraction for me since I will usually want to work in a full OSGi
>>> > environment. And when I can't do that I'm likely happy with plain
>>> > META-INF/services. I guess I can't correctly imagine the possibilities
>>> > that are mentioned in use case 4.3 (Application Frameworks).
>>> >
>>> > Cheers,
>>> > Jeremias Maerki
>>> >
>>> >
>>> > On 13.07.2012 16:07:58 David Bosschaert wrote:
>>> >> Hi Jeremias,
>>> >>
>>> >> Just FYI OSGi RFP 143 has just been published which relates precisely
>>> >> to the issue of using a Service Registry without going full-blown on
>>> >> the modularity. Ultimately the idea is that there will be an OSGi
>>> >> specification around this.
>>> >> Your work might fit with this idea as well and I would be very
>>> >> interested in your thoughts around this topic.
>>> >>
>>> >> See here: http://blog.osgi.org/2012/07/new-rfps-available-for-feedback.html
>>> >> and here: https://www.osgi.org/bugzilla/show_bug.cgi?id=145
>>> >>
>>> >> Best regards,
>>> >>
>>> >> David
>>> >>
>>> >> On 13 June 2012 08:02, Jeremias Maerki <dev@jeremias-maerki.ch>
wrote:
>>> >> > Hi David
>>> >> >
>>> >> > Thanks a lot for your feedback! PojoSR is interesting. You could
say
>>> >> > that there is a certain overlap. If I understand this right, PojoSR
is
>>> >> > suggesting to detect not-quite-OSGi bundles via
>>> >> > ClassLoader.getResources("META-INF/MANIFEST.MF") rather than to
detect
>>> >> > individual services via "META-INF/services/xy". It has a focus
on
>>> >> > maintaining a service registry that is based on OSGi principles
(bundle
>>> >> > activator and all) rather than working off the old JAR service
providers.
>>> >> > It brings OSGi to plain Java rather than plain Java to OSGi. ;-)
As an
>>> >> > OSGi addict, that is very appealing to me. The problem is selling
it, I
>>> >> > guess. Even selling OSGi compatibility in a plain Java project
requires
>>> >> > addressing things like: "no, you don't have to change your application
>>> >> > to OSGi."
>>> >> >
>>> >> > So, both approaches have their strong points. Let's try to put
them next
>>> >> > to each other:
>>> >> >
>>> >> > PojoSR:
>>> >> > - ability to work OSGi-like in plain Java, no changes running in
real OSGi
>>> >> > - runtime dependency on PojoSR, OSGi core and compendium in plain
Java
>>> >> > - existing applications using META-INF/services have to change
their
>>> >> > plug-in approach to OSGi-style services
>>> >> > - existing plug-ins need to get OSGi metadata
>>> >> >
>>> >> > my approach:
>>> >> > - developers are shielded from OSGi except for the build-time dependency
>>> >> > (OSGi core) and the build changes for OSGi metadata
>>> >> > - one runtime dependency in plain Java (the ServiceListener
>>> >> > infrastructure)
>>> >> > - existing applications have to switch from ServiceLoader (or Services
>>> >> > in Apache XML Graphics) to a ServiceListener (ServiceTracker-like)
>>> >> > for obtaining plug-ins
>>> >> > - existing plug-ins need to add OSGi metadata as per RFC 167.
>>> >> > - probably has a considerably lower learning curve for people that
>>> >> > haven't had contact with OSGi, yet.
>>> >> >
>>> >> > I guess one question is whether PojoSR works with SPI-Fly in plain
Java.
>>> >> > That would make this very nice. But I suspect that might not be
so
>>> >> > simple. If it doesn't work, all plug-ins have to be changed to
publish
>>> >> > services through a BundleContext. That would make it impossible
to
>>> >> > retro-fit an older plug-in just by adding OSGi metadata to the
JAR.
>>> >> >
>>> >> > Anyway, I'll put together a submission in the next few days (probably
>>> >> > next week due to other obligations). We can then see where to go
from
>>> >> > there. I'd be happy to help with the spec however I can. I guess
my code
>>> >> > will at least be a useful additional discussion base.
>>> >> >
>>> >> > Thanks,
>>> >> > Jeremias Maerki
>>> >> >
>>> >> >
>>> >> > On 12.06.2012 14:47:01 David Bosschaert wrote:
>>> >> >> Hi Jeremias,
>>> >> >>
>>> >> >> Many thanks for the suggestion. You are highlighting a valid
point:
>>> >> >> namely that people often have the need for services, like the
ones
>>> >> >> supported in OSGi, but are unable to (or don't want to) modularize
>>> >> >> their project in order to be able to use OSGi.
>>> >> >> There is currently some work underway in the OSGi expert groups
that
>>> >> >> relates to this.
>>> >> >>
>>> >> >> The PojoSR project (http://code.google.com/p/pojosr/) is a
relatively
>>> >> >> mature project that implements most of the OSGi Service Registry
and
>>> >> >> also supports things like the BundleActivator without doing
the
>>> >> >> modularity part of OSGi. So in a way it is similar to what
you have
>>> >> >> written, if I'm not mistaken.
>>> >> >>
>>> >> >> In the OSGi Core Platform Expert Group there is currently an
effort
>>> >> >> underway to create a specification inspired by PojoSR. I will
ensure
>>> >> >> that during the specification work your work is also considered
as
>>> >> >> input, if you wish [1].
>>> >> >>
>>> >> >> So in short - there will be a specification around this in
the not too
>>> >> >> distant future, your work can be considered a stepping stone
to this
>>> >> >> specification and it could even possibly become a compliant
>>> >> >> implementation in the future. I assume that PojoSR will also
become
>>> >> >> compliant but I think that there is always room for multiple
>>> >> >> implementations of a spec.
>>> >> >>
>>> >> >> If you are interested in updating your implementation to comply
with
>>> >> >> the spec in the future then I think that Apache Aries would
be a good
>>> >> >> place to put your implementation right now from where it can
mature. I
>>> >> >> think that it should not be a subcomponent of SPI-Fly but rather
a
>>> >> >> top-level component of its own, within Aries.
>>> >> >>
>>> >> >> Best regards,
>>> >> >>
>>> >> >> David
>>> >> >>
>>> >> >> [1] or, if you'd rather do that yourself, let me know :)
>>> >> >>
>>> >> >> On 8 June 2012 10:42, Jeremias Maerki <dev@jeremias-maerki.ch>
wrote:
>>> >> >> > Hi there,
>>> >> >> >
>>> >> >> > due to my involvement with the Apache XML Graphics project
(FOP/Batik)
>>> >> >> > where we make heavy use of META-INF/services, I've got
a big interest in
>>> >> >> > how best to bring FOP and Batik into OSGi. My particular
requirement
>>> >> >> > here is that FOP and Batik have to continue working in
plain Java but
>>> >> >> > profit from the OSGi service registry for plug-in discovery
when
>>> >> >> > possible.
>>> >> >> >
>>> >> >> > So, two years ago I came up with this:
>>> >> >> > http://www.jeremias-maerki.ch/development/osgi/jar-services.html
>>> >> >> >
>>> >> >> > That works half-way good enough although I've never felt
comfortable
>>> >> >> > enough to push it back to the XML Graphics project as
I've had a few
>>> >> >> > flaws in the above implementation. So, by now SPI Fly
does a lot of
>>> >> >> > things much better than my approach and it is following
an RFC that I
>>> >> >> > find very useful.
>>> >> >> >
>>> >> >> > However, on the client side, RFC 167 is a bit heavy on
J2SE-1.6's
>>> >> >> > ServiceLoader. Apache XML Graphics is still on J2SE-1.5
(yes, I know, I
>>> >> >> > know) so it contains its own Services.java to find plug-ins
via
>>> >> >> > META-INF/services. Furthermore, with ServiceLoader you
basically have to
>>> >> >> > poll for changes.
>>> >> >> >
>>> >> >> > Finally getting to my point: I'd like to offer a little
API that brings
>>> >> >> > some of the service dynamics you get with OSGi services.
This can
>>> >> >> > already be seen on the page indicated above although I've
slightly
>>> >> >> > modified the API. Essentially, to get notified about new
or disappearing
>>> >> >> > plug-ins/services, you can register a service listener.
And that would
>>> >> >> > look like this (client code):
>>> >> >> >
>>> >> >> >        ServiceTracker<ImageWriter> tracker = Plugins.getServiceTracker(ImageWriter.class);
>>> >> >> >        tracker.addServiceListener(new ServiceListener<ImageWriter>()
{
>>> >> >> >
>>> >> >> >            public void added(ImageWriter writer) {
>>> >> >> >                register(writer);
>>> >> >> >            }
>>> >> >> >
>>> >> >> >            public void removed(ImageWriter writer) {
>>> >> >> >                unregister(writer);
>>> >> >> >            }
>>> >> >> >
>>> >> >> >        });
>>> >> >> >
>>> >> >> > Where Plugins is:
>>> >> >> >
>>> >> >> > public class Plugins {
>>> >> >> >
>>> >> >> >    /**
>>> >> >> >     * This is the services singleton.
>>> >> >> >     */
>>> >> >> >    private static final Services SERVICES = new Services();
>>> >> >> >
>>> >> >> >    public static void setServicesBackend(ServicesBackend
backend) {
>>> >> >> >        SERVICES.setServicesBackend(backend);
>>> >> >> >    }
>>> >> >> >
>>> >> >> >    public static <T> ServiceTracker<T> getServiceTracker(Class<T>
providerIntf) {
>>> >> >> >        return SERVICES.getServiceTracker(providerIntf);
>>> >> >> >    }
>>> >> >> >
>>> >> >> > }
>>> >> >> >
>>> >> >> > And the BundleActivator for the client looks like this:
>>> >> >> >
>>> >> >> > public class Activator implements BundleActivator {
>>> >> >> >
>>> >> >> >    private volatile ServicesOSGi services;
>>> >> >> >
>>> >> >> >    /** {@inheritDoc} */
>>> >> >> >    public void start(BundleContext context) throws Exception
{
>>> >> >> >        this.services = new ServicesOSGi(context);
>>> >> >> >        Plugins.setServicesBackend(this.services);
>>> >> >> >    }
>>> >> >> >
>>> >> >> >    /** {@inheritDoc} */
>>> >> >> >    public void stop(BundleContext context) throws Exception
{
>>> >> >> >        Plugins.setServicesBackend(null);
>>> >> >> >        if (this.services != null) {
>>> >> >> >            this.services.close();
>>> >> >> >        }
>>> >> >> >    }
>>> >> >> >
>>> >> >> > }
>>> >> >> >
>>> >> >> > What happens is this: by default "Services" starts up
with a plain-Java
>>> >> >> > backend that simply looks up services through classic
means. When the
>>> >> >> > BundleActivator is called, the backend is replaced by
ServicesOSGi that,
>>> >> >> > instead, gets the plug-ins from the service registry.
In the background,
>>> >> >> > any previously discovered plug-ins are "removed()" and
the new ones from
>>> >> >> > the registry "added()".
>>> >> >> >
>>> >> >> > In the case of classic Java, the "removed()" method is
never called
>>> >> >> > (probably doesn't ever need to be), i.e. no dynamics there.
>>> >> >> >
>>> >> >> > To conclude: the goal is to profit from OSGi service dynamics
when
>>> >> >> > discovering plug-ins while preserving the possibility
to run without
>>> >> >> > OSGi API runtime dependencies and still in J2SE-1.5. Furthermore,
the
>>> >> >> > OSGi-specific code shall be as minimal as possible to
shield those with
>>> >> >> > no OSGi knowledge in the team from the learning curve.
>>> >> >> >
>>> >> >> > And now, I'm wondering if you're interested to adopt this
as a new part
>>> >> >> > in SPI Fly as a useful alternative to using ServiceLoader.
If you don't
>>> >> >> > want it, I'm going to publish it under Apache Extras.
>>> >> >> >
>>> >> >> > Thanks,
>>> >> >> > Jeremias Maerki
>>> >> >> >
>>> >> >
>>> >
>>

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