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From Timothy Ward <timothyjw...@hotmail.com>
Subject RE: Spring DM/Blueprint Services
Date Thu, 26 Aug 2010 18:53:51 GMT


Kirk,

You could try something like the following, which I'll try to keep as brief as possible:


<blueprint....>

<bean id="defaultImpl" class=..../>

<service ref="defaultImpl" interface="IFace1"/>

<reference id="updatedImpl" availability="optional" interface="IFace1">
   <reference-listener bind-method=”available” unbind-method=”unavailable” ref="usingBean">
</reference>

<bean id="usingBean" class=....>
  <property name="defaultService" ref="defaultImpl"/>
  <property name="overrideService" ref="updatedImpl"/>
</bean>

</blueprint>


Using the above xml, a bundle will export a service using IFace1, and optionally consume a
service using IFace1. The "usingBean" will have the implementation bean for the default service
injected and an override service proxy.

When the proxy is bound to a backing service the "usingBean" will have the available(IFace1
svc) method driven (there are some other allowable signatures too). This will allow the "usingBean"
to use the injected override service until such time as:

a) available is driven again, indicating that the original override service has been unregistered
and has been transparently replaced with a new service, no action is necessary

b) unavailable(IFace1 svc) is driven, indicating that the override service has been unregistered
and no replacement is available, at this point the default implementaion should be used again.


This is a reasonably simple model, which avoids some of the lifecycle issues around trying
to expose a service that needs to consume itself. I hope it is useful to you.

Regards,

Tim


----------------------------------------
> Subject: Re: Spring DM/Blueprint Services
> From: pragkirk@kirkk.com
> Date: Thu, 26 Aug 2010 12:56:15 -0500
> To: aries-user@incubator.apache.org
>
> Tim,
>
> That is an alternative solution I tried. But I didn't use a listener and so it had a
pretty bad smell.
>
> Essentially what I did was that when Bundle C started, it got a reference to the Interface2
service that was backed by the Interface2Impl2 class and I injected that into the client which
it turned called the set method on the Interface1 service. That just didn't seem right.
>
> Is there a way to use the listener approach you describe below and swap out the default
implementation bean (Interface2Impl1) with the service (Interface2Impl2) via blueprint configuration?
Or at least create a listener class that gets invoked when the Interface2 service backed by
Interface2Impl2 is registered, but where the listener class is not dependent on the OSGi framework?
>
> Kirk Knoernschild
> http://www.kirkk.com
> http://techdistrict.kirkk.com
> http://planet.kirkk.com
> twitter: pragkirk
>
>
>
>
> On Aug 26, 2010, at Aug 26, 12:43 PM, Timothy Ward wrote:
>
>>
>> I would like to ask, why would you expose a service to provide a default implementation
within a bundle? The default implementation bean can be wired in directly using blueprint,
and a listener can be used to switch in the optional "replacement service" if it becomes available.
The default service can still be exposed in the service registry for use by other bundles.
>>
>> Regards,
>>
>> Tim
>>
>> ----------------------------------------
>>> Subject: Re: Spring DM/Blueprint Services
>>> From: pragkirk@kirkk.com
>>> Date: Thu, 26 Aug 2010 12:37:53 -0500
>>> To: aries-user@incubator.apache.org
>>>
>>> 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 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:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> id="bean1">
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> availability="optional">
>>>>>
>>>>>
>>>>>
>>>>> 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 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
>>>>>
>>>
>>
>
 		 	   		  
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