Re: Application API

[Guillaume Nodet <gn...@gmail.com>]

Hi Lin, thx for this very good feedback.  See further comments inline.

On Wed, Mar 31, 2010 at 22:39, Lin Sun <li...@gmail.com> wrote:

> Hi Guillaume,
>
> Thanks for updating the APIs.  I have the following comments -
>
> 1. SubsystemAdmin.getSubsystem(id).  I wonder if we can provide a
> method like getSubsystem(String scope).   subsystemScope can be
> calculated using subsystem symbolic name + "_" + subsystem version,
> and it can be used to uniquely identify subsystems instead of (or in
> addition to) a long id.    Whenever a subsystem gets updated, we would
> need some way to tell which of the existing sybsystem it is updating.
>  To me, it is easy to tell that by the subsystem scope which can be
> simply calculated from the bundle location.  Another thought is to use
> bundle location, but I don't think that is guaranteed to be unique,
> when people are using install(location, inputstream) to install the
> subsystem.
>
>
The first question that comes to my mind is about the use of the
scope/identifier.
I don't have any problem adding such an identifier to a subsystem (or
replace the
long identifier by a string), however, I'm concerned about using  the
symbolic name
+ version as the identifier.  The problem is that when updating the
subsystem with
a more recent version, the id would either change (which would be
problematic) or
become inconsistent (which would be bad too).  In addition a user could
choose
to have the same subsystem (identified by its symbolic name) running in two
different versions.  The question becomes what would happen if the user try
to
update the first one using the more up to date subsystem.   For bundles, it
would
fail afaik.   In addition, the id would have been mapped to the underlying
composite
id, leading to a simple way to access the composite if needed.

So i'd like to understand how you see this scope being used.  If you want a
generated id,
then what about using a UUID ?  If what you want is something that can
easily be undestand
and linked to the symbolicname / version, then is that really an unique
identifier ? because it
may change over time.  Actually, I think it would be ok to update the
subsystem and have
the updated version have a different symbolic name.


> 2. SubsystemAdmin.getSubsystems() returns Collection<Subsystem>.  I
> wonder if it is more convenient to return Map<String, Subsystem> where
> the key is the subsystem scope.
>

Right, that makes sense.  I think I was trying to avoid having to build a
read-only map,
but there's no real point here.  Having the map would be sufficient, and we
should then
remove the getSubsystem(long) method imho.


>
> 3. Subsystem.install(location).  I think this should be install(String
> location, BundleContext bc) instead, because we would need the bundle
> context to get the CompositeAdmin service from OSGi service registry,
> assuming we would use that to install the composite.  We would also
> need bundle context to install constituents.
>
> 4. similar as No. 3, Subsyste.install(location, content) should be
> install(String location, InputStream content, BundleContext bc)
>
>
Ok, so I think those two points are related to nested subsystems.  My idea
was
to follow what composites do, not necessarily because we would use those,
but
because I think users will rarely have to deal with nested subsystems
directly.
Let me explain:  subsystems are defined by the user and deployed.  I don't
think
we should encourage users to manually install new bundles into the created
subsystem,
because i would assume those would be uninstalled as soon as the subsystem
is
updated (or do we want to actually track the changes done by the user so
that we don't
rollback manual changes?).  My idea was that if the user want to add /
remove bundles
from the composite, he needs to update the subsystem.  I don't think we
should forbid
such changes, just that we should not track and manage those changes.
Nested subsystems are imho the same problem.  Nested subsystems would be
installed
because a subsystem references another subsystem in its key content I think.
 So you
would not really want the user to manually install a subsystem inside an
existing subsystem.

Now, if what you were thinking was about installing subsystems from inside a
composite,
the problem is a bit different.  My idea was that the subsystem would be
installed from inside
the composite where the SubsystemAdmin service you use comes from.  So if
you have a composite
and you want to install a subsystem into it, you retrieve the SubsystemAdmin
from this composite
and simply call one of the install method.  This means that either:
   * the subsystem implementation has been installed directly in this
composite
   * or the subsystem impl has been installed in a parent composite and the
composite service policy has been
      written to allow the service goes through
>From an implementation pov, I think it's quite easy to achieve by using a
ServiceFactory and using the
bundle context from the requesting bundle.


> 5. Subsystem.getState().  I wonder if we should reinvent here.  Is
> there any reason why we don't make the return type of int, and just
> delegate this to the compositeBundle.getState().  Also, what if the
> composite bundle and constituents have different states.  Does this
> method only work when the states are consistent among these bundles?
>

My original idea was also to map closely to the composite state, so I don't
have any problem
using an int instead of an enum, though I think the state of composite is an
int because a composite
*is* a bundle and enums did not exist at the time the bundle interface has
been created.
However, Subsystem do not inherit the Bundle or CompositeBundle interface,
so we could decide to
use an enum (because the state is semantically an enum).

That being said, the question you raise about the constituents state is
important.  We need to choose
whether we want to represent the state of the subsystem as an aggregate of
the constituents state or not.
That would surely be interesting, because you would know the real state of
your subsystem much more
easily.   The problem is i'm not sure what would happen if one of the bundle
can't start or is manually
stopped by the user.  Would this leave the subsystem in the STARTING state
until the start or stop
method is called ?


> 6. Subsystem.start() or stop(), I think it should throw either
> BundleException or SubsystemException.
>

Yeah, SubsystemException would be fine.  Currently I've defined
SubsystemException as being
a RuntimeException though ...

>
> 7. The return type of Subsystem.getHeaders, do we want to use
> Dictionary<String, String> or Dictionary to be consistent with
> Bundle.getHeaders()?  when we implement this method, we can just
> delegate to compositeBundle.getHeaders.
>

Two things here.  First, I'm not sure we would want to return the composite
headers.
First because composite headers can't be localized, and second because i was
thinking
about returning the headers from the subsystem manifest as it was installed,
not of the
underlying composite (which includes computed headers for package and
service policies).
So if we return directly the composite headers, it would surely make more
sense to use the
same type, but if we don't, i think using Map is more natural.  OSGi is
really the only place
that uses Dictionary instead of Map ;-)


>
> 8. I'd like the add the following methods to Subsystem.java:
>
> +    void uninstall() throws SubsystemException;
> +
> +    String getScope();
> +
> +    void update() throws SubsystemException;
> +
> +    void update(InputStream content) throws SubsystemException;
>
> I think the uninstall(), update() are needed, because the
> SubsystemAdmin.uninstall/update can just delegate the work to the
> actual subsystem.  In fact, the SubsustemAdmin cannot really do the
> work, unless we make the composite bundle available out of the
> Subsystem.
>

I agree to your point from an implementation point of view.
I initially put those methods on the Subsystem, but later moved them
to the SubsystemAdmin.  I did that because it leads to a cleaner separation
between the actors and the responsibilities.  In my mind, the SubsystemAdmin
is responsible for managing the content of subsystems (installation / update
/ creation),
whereas the subsystem does not perform such modifications and the only
methods
available are start() and stop().    I had the exact same objection on the
application
api some time ago and Alasdair said the design was more service oriented
rather than
object oriented.  And now, I think it makes more sense to do it that way.


>
> I am also attaching a patch for stuff described above.  I'd love to
> help on implementing these APIs if that is ok with you.
>

Do you mind discussing the changes a bit more ?


>
> Thanks
>
> Lin
>
>
>
> On Mon, Mar 29, 2010 at 6:34 PM, Guillaume Nodet <gn...@gmail.com> wrote:
> > FWIW, I've slightly updated the API with some feedback here and mostly
> two
> > other changes:
> >  * moved some management methods from Subsystem to SubsystemAdmin (update
> > and uninstall)
> >  * make SubsystemConstants a non instantiable class instead of an
> interface
> >
> > On Fri, Mar 26, 2010 at 16:27, Guillaume Nodet <gn...@gmail.com> wrote:
> >
> >> Sorry, forgot to give a pointer:
> >>
> >>
> http://svn.apache.org/repos/asf/incubator/aries/trunk/subsystem/subsystem-api/src/main/java/org/apache/aries/subsystem/
> >>
> >>
> >> On Fri, Mar 26, 2010 at 15:27, Guillaume Nodet <gn...@gmail.com>
> wrote:
> >>
> >>> I've just checked in a user-oriented API for subsystems (which I hope
> can
> >>> be a superset of applications).
> >>> This is not the SPI part, but simply what the user needs to manipulate
> >>> subsystems at runtime.
> >>> The design has been chosen to be much more familiar to the OSGi API for
> >>> bundles.
> >>>
> >>> I'm planning to continue on this area when time permits, but I welcome
> >>> everyone to have a look and give feedback.
> >>>
> >>> On Thu, Mar 18, 2010 at 18:16, Jarek Gawor <jg...@gmail.com> wrote:
> >>>
> >>>> Hi all,
> >>>>
> >>>> I have a few questions / comments on the Aries Application API.
> >>>> Specifically, about AriesApplicationManager and
> >>>> AriesApplicationContextManager API.
> >>>>
> >>>> The application-management module provides a default implementation
> >>>> for the AriesApplicationManager API while the application-runtime
> >>>> module provides an example implementation of the
> >>>> AriesApplicationContextManager API. It is expected (as indicated by
> >>>> JavaDoc) that different server runtimes will provide their own
> >>>> implementations of the AriesApplicationContextManager API.
> >>>>
> >>>> So it seems to me like the AriesApplicationContextManager is more of a
> >>>> SPI - something that user shouldn't/wouldn't normally use. If so, the
> >>>> user would only interact with the AriesApplicationManager API to
> >>>> perform application operations. If that's true then the
> >>>> AriesApplicationManager is missing methods for getting all
> >>>> AriesApplicationContexts or finding one by name. And at the same time
> >>>> the AriesApplicationContextManager would probably need to be
> >>>> completely reworked. For example to be named
> >>>> AriesApplicationManagerProvider and have corresponding
> >>>> install/uninstall/getContexts/findContext operations.
> >>>> Or maybe we should just get rid off the AriesApplicationContextManager
> >>>> completely and let folks implement the AriesApplicationManager API
> >>>> directly?
> >>>>
> >>>> Btw, by 'user' I meant some code that uses these API to perform some
> >>>> application operations. For example, in my case, I would like to have
> >>>> some osgi shell commands that use these API to
> >>>> install/uninstall/start/stop applications.
> >>>>
> >>>> Jarek
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Cheers,
> >>> Guillaume Nodet
> >>> ------------------------
> >>> Blog: http://gnodet.blogspot.com/
> >>> ------------------------
> >>> Open Source SOA
> >>> http://fusesource.com
> >>>
> >>>
> >>>
> >>
> >>
> >> --
> >> Cheers,
> >> Guillaume Nodet
> >> ------------------------
> >> Blog: http://gnodet.blogspot.com/
> >> ------------------------
> >> Open Source SOA
> >> http://fusesource.com
> >>
> >>
> >>
> >
> >
> > --
> > Cheers,
> > Guillaume Nodet
> > ------------------------
> > Blog: http://gnodet.blogspot.com/
> > ------------------------
> > Open Source SOA
> > http://fusesource.com
> >
>



-- 
Cheers,
Guillaume Nodet
------------------------
Blog: http://gnodet.blogspot.com/
------------------------
Open Source SOA
http://fusesource.com