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Subject: Re: [sca-assembly] Pointer to SCA event processing presentation
Hi Martin,
I will try my best to attend in September. I have been tied up with
other commitments over the summer that overlap with the SCA calls. In
the meantime, it may be beneficial if we can continue some of the
discussion through email as a way to make forward progress.
Thanks,
Jim
On Aug 27, 2009, at 5:45 PM, Martin Chapman wrote:
> Jim,
>
> On yesterdays call we agreed to have bi-weekly sessions, starting
> 9th Sep, focussing on eventing.
> We need to collect comments, and address them as appropriate.
> You contribution below is valuable to this effort and I would
> encourage you to participate in these calls.
>
> Martin
>
>> -----Original Message-----
>> From: Jim Marino [mailto:jim.marino@gmail.com]
>> Sent: 27 August 2009 15:36
>> To: Martin Chapman
>> Cc: 'OASIS Assembly'
>> Subject: Re: [sca-assembly] Pointer to SCA event processing
>> presentation
>>
>> OK. Not having been privy to the initial drafting of the
>> specification
>> outside of OASIS (i.e. in OSOA), I may raise some questions that have
>> already been answered. But for those who are new to this such as
>> myself, I'll ask them anyway.
>>
>> My first reaction - and this is based on incomplete knowledge - is
>> that eventing seems to be dragging in a boat-load of additional
>> concepts and technologies. For example, as you mention below, there
>> is
>> the possibility that an interoperable eventing protocol may be
>> required.
>>
>> There are also: consumers, producers, events, event types, EDL,
>> channels, filters, and channel promotion. Many of those concepts
>> appear to simply mirror concepts we already have. For example:
>>
>> consumers ----> services
>> producers ----> references
>> events ----> messages
>> EDL ----> a type system such as XML Schema or Java
>> channels ----> binding metadata
>> filters ----> binding metadata
>> channel promotion ----> service/reference promotion
>>
>> The reason why I asked if JMS could be used to implement eventing is
>> that I wanted to see if this apparent duplication of concepts could
>> be
>> eliminated by accommodating eventing capabilities using existing
>> concepts, in particular bindings. Since JMS appears to capture the
>> semantics of eventing, this leads me to believe an "eventing" binding
>> and a set of intents could be defined to achieve this.
>>
>> I noticed the deck contained a slide outlining why these additional
>> concepts are needed. Before responding to that, I'll outline what I
>> was thinking first.
>>
>> To start, I was thinking there would be an eventing binding which
>> could be configured in the following way:
>>
>> <component name="MyProducer">
>> <implementation.java ..../>
>> <reference name="channel1">
>> <binding.eventing channel="foo"/>
>> </reference>
>> <reference name="channel1">
>> <binding.eventing channel="bar"/>
>> </reference>
>> </component>
>>
>> <component name="MyFooConsumer">
>> <implementation.java ..../>
>> <service>
>> <binding.eventing channel="foo" filter=".."/>
>> </service>
>> </component>
>>
>> <component name="MyBarConsumer">
>> <implementation.java ..../>
>> <service>
>> <binding.eventing channel="bar" filter=".."/>
>> </service>
>> </component>
>>
>>
>> public class MyProducerImpl implements SomeService {
>>
>> @Reference
>> protected MyChannel channel1;
>>
>> @Reference
>> protected MyChannel channel2;
>>
>> public void doSomething(...) {
>> Event event1 == //...
>> Event event2 == //...
>> channel1.onEvent(event1);
>> }
>>
>> }
>>
>> public interface MyChannel {
>>
>> @OneWay
>> void onEvent(Event event) ;
>> }
>>
>>
>> Based on this example, I've included some comments with respect to
>> the
>> slide from the deck mentioned above:
>>
>> 1. No decoupling
>>
>> I think the above example is fairly decoupled. It requires a
>> reference
>> to be bound (but not wired), which I think is the correct behavior
>> given that allowing 0..n would require code to check for null
>> pointers
>> making it complex. If there was no consumer, the binding would
>> swallow/
>> ignore the event.
>>
>> 2. Requires interfaces for typing
>>
>> I'm not sure I follow this. My understanding is that for strongly
>> typed languages such as Java, some interface is required. If an
>> interface is not required, how is an event sent in Java? Even other
>> language, some shared type system or type mapping is required.
>>
>> 3. Interface/operations have processing semantics, events don't
>>
>> I'm not sure I agree with the assumption that interfaces have
>> processing semantics. How does MyChannel.onEvent() imply processing
>> semantics? One thing that would help is if someone could provide a
>> Java code example that did not imply processing semantics so a
>> comparison can be mode. Note that the MyChannel example was strongly
>> typed, but the following could also be defined:
>>
>> public interface MyUntypedChannel {
>>
>> void event(Object event);
>> }
>>
>> 4. Doesn't allow addition of producers/consumers dynamically
>>
>> I think the proposal above addresses this concern. It would be up to
>> the binding infrastructure to handle this. Note that I think there
>> may
>> be an incorrect assumption related to references/wires as well. Wires
>> can be updated dynamically.
>>
>> 5. No ability to set filters
>>
>> This would be handled in the binding configuration. In the example, I
>> used an attribute but it could also be modeled as an element for
>> complex expressions. I'm hoping the filter language will be left open
>> as opposed to inventing one.
>>
>> 6. Various issues crop up because of the model mismatch. For example,
>> sca:reference(0..n) is mapped to an array in Java
>>
>> This would be addressed by using a bound required (1..1) reference.
>> Also, note that 0..n does not have to map to an array in Java, it can
>> also map to certain collection types. Are there other issues along
>> these lines?
>>
>> 7. Subscribers do not offer services they offer callbacks
>> 8. Services do not subscribe they make software functionality
>> available for others to use.
>>
>> I think these two are debatable. For example, the JMS binding works
>> this way. Why can't we use the same principle and keep things
>> consistent with JMS?
>>
>> 9. A subscribe message will be sent from a service to a reference: a
>> direction reversal
>>
>> I think this is conflating services/references and runtime
>> infrastructure. The latter is setting up subscriptions, not a service
>> or reference. This is consistent with how the JMS binding is
>> designed.
>>
>> 10. Changes are hard to manage
>>
>> Could you provide examples?
>>
>>
>> -------------------------------
>>
>> What would help me is if someone can provide examples of how eventing
>> works in a concrete programming language. My own preference is Java,
>> but C++, BPEL, etc,. would be fine.
>>
>> Thanks for reading this far :-)
>>
>> Jim
>>
>> On Aug 27, 2009, at 3:24 PM, Martin Chapman wrote:
>>
>>> We have not addressed that question yet. The current Assembly spec
>>> does state in the conformance section that a web services binding
>>> is required, and there is a PR comment on that. Depending on how we
>>> resolve that comment, we may have to add an analogous
>>> interoperable protocol for eventing (or not as the case may be.)
>>>
>>> Martin.
>>>
>>>
>>>> -----Original Message-----
>>>> From: Jim Marino [mailto:jim.marino@gmail.com]
>>>> Sent: 27 August 2009 14:14
>>>> To: Martin Chapman
>>>> Cc: 'OASIS Assembly'
>>>> Subject: Re: [sca-assembly] Pointer to SCA event processing
>>>> presentation
>>>>
>>>> OK I'm currently just interested in the API. From your response, it
>>>> sounds as if the JMS API can be used to capture the semantics of
>>>> the
>>>> eventing proposal.
>>>>
>>>> Given that, your response does raise an additional question. Is the
>>>> proposal mandating an interoperable wire protocol such as WS-
>>>> Eventing
>>>> or AMQP be used? In other words, would it be considered compliant
>>>> to
>>>> build eventing capabilities using a JMS provider that did not
>>>> support
>>>> an interoperable wire protocol? I'm hoping your answer is "yes" :-)
>>>>
>>>> Thanks,
>>>> Jim
>>>>
>>>>
>>>>
>>>> On Aug 27, 2009, at 2:48 PM, Martin Chapman wrote:
>>>>
>>>>> Everything but a standard wire protocol;-)
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Jim Marino [mailto:jim.marino@gmail.com]
>>>>>> Sent: 27 August 2009 00:06
>>>>>> To: OASIS Assembly
>>>>>> Subject: Re: [sca-assembly] Pointer to SCA event processing
>>>>>> presentation
>>>>>>
>>>>>> Hi Anish,
>>>>>>
>>>>>> I have a very naive question about the eventing proposal: is
>>>>>> there a
>>>>>> capability that cannot be implemented using JMS? My impression is
>>>>>> the
>>>>>> JMS API has everything that is needed to be the foundation of an
>>>>>> eventing system.
>>>>>>
>>>>>> As an aside, I'm not asking this as a way of implying that JMS
>>>>>> should
>>>>>> be used and/or required.
>>>>>>
>>>>>> Jim
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Aug 26, 2009, at 6:57 PM, Anish Karmarkar wrote:
>>>>>>
>>>>>>> Hi Sanjay,
>>>>>>>
>>>>>>> Here is the link to the presentation:
>>>>>>> http://www.oasis-open.org/apps/org/workgroup/sca-assembly/download.php/32653/SCA-
>>>>>> EventingProcessing.pdf
>>>>>>>
>>>>>>> It does talk about the motivation and has use cases.
>>>>>>>
>>>>>>> HTH
>>>>>>>
>>>>>>> -Anish
>>>>>>> --
>>>>>>>
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>>>>>>
>>>>>>
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>>>>>
>>>>>
>>>>>
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>>>
>>>
>>>
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>>
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>
>
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