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From Alasdair Nottingham <...@apache.org>
Subject Re: Spring DM/Blueprint Services
Date Fri, 27 Aug 2010 10:41:06 GMT
We could do it only when the service properties are explicitly listed
as primitives in the blueprint. So if you include more complex
properties it wouldn't work. Matching shouldn't be a problem because
we can use the Filter class in OSGi.

The only catch would be whether the CT has a test for this scenario or not.

Alasdair

On 27 August 2010 11:27, Guillaume Nodet <gnodet@gmail.com> wrote:
> I think the spec says it's a programming error from the user, but i
> don't think it requires the implementation to throw any exception.
> I don't see why we could not enhance the aries blueprint
> implementation to support such a use case.  This would require to
> perform some kind of matching before the grace period to ignore
> references that could be satisfied by exported services.  However,
> this may not be possible by only looking at the metadata, as the
> reference could have a filter that would have to be matched against
> properties (which could be beans ...).
>
> On Fri, Aug 27, 2010 at 11:26, Alasdair Nottingham <not@apache.org> wrote:
>> My personal view is that a blueprint should be able to reference a
>> service it defines, however not to support the scenario you describe.
>> I think it should be supported to allow flexibility in refactoring, so
>> you can easily merge and split bundles. Alas the alliance disagrees.
>>
>> The defaulting scenario you describe doesn't really work in blueprint
>> because of the way service damping works. It will select an
>> implementation and then use it until it goes away. In your case should
>> it select the default it will never go away until the blueprint bundle
>> goes away. The only way to make this work would be to ensure that the
>> bundle with Interface2Impl2 in it is installed and processed by
>> blueprint first. This introduces an ordering constraint which given we
>> are in OSGi is, I think, a design flaw.
>>
>> That said it sucks that to make your scenario work the code is so yucky.
>>
>> Alasdair
>>
>> On 26 August 2010 18:37, Kirk Knoernschild <pragkirk@kirkk.com> wrote:
>>> Ah yes. The same phrase is found in the Spring DM documentation, and now I see
it's also in the blueprint spec. Thank you for pointing that out.
>>>
>>> So in general, my design below is not supported. However, I don't perceive it
as a design flaw.
>>>
>>> It seems reasonable that a bundle would define a default implementation for one
of it's services where that service is also used by that bundle. It also seems reasonable
that I might want to change the implementation at runtime so that the bundle now uses a different
implementation of that service. I can easily do that if I separate it all out into separate
bundles.
>>>
>>> Like this:
>>> Bundle A - Interface1, Interface1Impl (uses Interface2 service)
>>> Bundle A1- Interface2, Interface2Impl1
>>> Bundle A2 - Interface2Impl2
>>>
>>> Bundle C - uses Interface1service
>>>
>>> Upon deploying Bundle A and Bundle A1, Bundle C will use Interface1 with the
default Interface2Impl1 that backs Interface2. I can stop Bundle A1, deploy and start Bundle
A2 and now Bundle C uses Interface1 with the new Interface2Impl2 that backs Interface2. Works
fine.
>>>
>>> However, if I eliminate Bundle A1 and put those classes in Bundle A, it doesn't
work. Foremost, it doesn't work because the spec doesn't allow it. But also, since the service
lifecycle is tied to the bundle lifecycle, I suppose I wouldn't be able to easily substitute
Interface1Impl1 with Interface2Impl2. But that is what I would prefer because the approach
that works seem to cause an unnecessary proliferation of bundles.
>>>
>>> At least, that is what I'm seeing. Which is why I'm wondering if I'm missing
an alternative solution.
>>>
>>> Kirk Knoernschild
>>> http://www.kirkk.com
>>> http://techdistrict.kirkk.com
>>> http://planet.kirkk.com
>>> twitter: pragkirk
>>>
>>>
>>>
>>>
>>> On Aug 26, 2010, at Aug 26, 11:40 AM, Mark Nuttall wrote:
>>>
>>>> Enterprise 4.2 spec, section 121.7.9:
>>>>
>>>> "It is an error to declare a mandatory reference to a service that is
>>>> registered by the same bundle. Such
>>>> a definition could cause either deadlock or a timeout."
>>>>
>>>> Regards,
>>>> Mark
>>>>
>>>> On 26 August 2010 17:26, Alasdair Nottingham <not@apache.org> wrote:
>>>>> Hi,
>>>>>
>>>>> I can't comment on Spring DM, because I don't have any experience
>>>>> there, but if you use blueprint it can be possible if you define the
>>>>> reference to have an optional availability for example:
>>>>>
>>>>> <?xml version="1.0" encoding="UTF-8"?>
>>>>> <blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0">
>>>>>        <bean class="privatepackage.Interface1Impl" init-method="init"
>>>>>                id="bean1">
>>>>>                <property name="thing" ref="ref1"></property>
>>>>>        </bean>
>>>>>        <bean class="privatepackage.Interface2Impl" id="bean2"></bean>
>>>>>        <service ref="bean1" interface="publicpackage.Interface1"></service>
>>>>>        <service ref="bean2" interface="publicpackage.Interface2"></service>
>>>>>        <reference id="ref1" interface="publicpackage.Interface2"
>>>>>                availability="optional">
>>>>>        </reference>
>>>>> </blueprint>
>>>>>
>>>>> Having run this through the init method of Interface1Impl has the
>>>>> reference injected by the time it is called, but of course this isn't
>>>>> necessarily safe to assume.
>>>>>
>>>>> I'm going to head back to the blueprint spec to see if this cycle is
>>>>> prohibited from working for a mandatory reference, but if it isn't
>>>>> I'll raise a JIRA to allow this type of cycle.
>>>>>
>>>>> Alasdair
>>>>>
>>>>> On 26 August 2010 16:17, Kirk Knoernschild <pragkirk@kirkk.com>
wrote:
>>>>>> I've been using Spring DM, and one thing that I'm struggling with
is that a bundle that exports a service is unable to use that service. This seems to be a
feasible design option, and I'm wondering what others have done to work around it.
>>>>>>
>>>>>> For instance, let's say I have three bundles.
>>>>>>
>>>>>> BundleA with Interface1, Interface2, Interface1Impl, and Interface2Impl1.
>>>>>> BundleB with Interface2Impl2
>>>>>> BundleC with ClassC that uses Interface1.
>>>>>>
>>>>>> On start, BundleA registers two new services Interface1Service and
Interface2Service, using Interface1Impl and Interface2Impl1 as their implementations, respectively.
As it happens, the Interface1Impl requires an Interface2 type, so using Spring DM, I've tried
injecting the Interface2 service into the Interface1 service. It doesn't work because Spring
DM doesn't allow a bundle to use a service it registers, so I inject it as a regular bean.
>>>>>>
>>>>>> I want to do this because I install BundleB and register another
Interface2Service service, now using Interface2Impl2. Because I cannot inject the service
backed by Interface2Impl1 into the service backed by Interface1Impl, the service backed by
Interface1Impl won't be able to use Interface2Impl2.
>>>>>>
>>>>>> FWIW, I can move Interface2 and Interface2Impl1 to a separate bundle
and register it as a service. That does work, but it's not the application structure I want.
>>>>>>
>>>>>> Possibly there is an alternative design to this that is more suitable
to this situation. I'm just not sure what that is at this point. Any suggestions are appreciated.
>>>>>>
>>>>>> I can send code if it would be helpful to illustrate this.
>>>>>>
>>>>>> Kirk Knoernschild
>>>>>> http://www.kirkk.com
>>>>>> http://techdistrict.kirkk.com
>>>>>> http://planet.kirkk.com
>>>>>> twitter: pragkirk
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Alasdair Nottingham
>>>>> not@apache.org
>>>>>
>>>
>>>
>>
>>
>>
>> --
>> Alasdair Nottingham
>> not@apache.org
>>
>
>
>
> --
> Cheers,
> Guillaume Nodet
> ------------------------
> Blog: http://gnodet.blogspot.com/
> ------------------------
> Open Source SOA
> http://fusesource.com
>



-- 
Alasdair Nottingham
not@apache.org

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