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Subject: Re: [sca-assembly] Re: [ASSEMBLY-249]: Need some notion of "callback"address in conjunction with eventing

Hi Mike,

It is possible we're vehemently agreeing on many points.  Which should make this an interesting discussion....

On 11/4/10 9:11 AM, Mike Edwards wrote:
OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

This message from Eric starts to get into one of the things that Danny raised on he call earlier this week, namely the
inability of SCA to model the various messaging patterns describable in WSDL.

I DISAGREE that this is the case, and I will describe how SCA can deal with these cases.

> * message out, maybe message back
> * message out, N messages back
> * message out *, message back

Currently, SCA can do ALL of these using references & services, using Callbacks.

I agree, to the extent that you're talking about a single consumer at the destination.  Concretely mapping this to JMS (or email), all I know is that I send messages to a destination, and I don't know if I get a "message" back at all, whether I get five "messages" back, or just one, and I further don't know if those "messages" will come from the same source.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">
Where a service has both an interface AND a callback interface, then it is saying:

- for any forward invocation of the service
-- there may be 0, 1 or many invocations of any one of the operations in the callback
   interface (which can also be read as "response messages")
- the invocation of an operation on the callback interface can occur in response to one forward invocation
-- or to some sequence of multiple forward invocations

Yes, all true.  The way I read it, you've assumed a single "responder", whereas I have not.

I perceive an important semantic distinction between "here's a message", and "invoke particular operation", even if they're called with the same data.  Eventing systems move data, whereas "services" systems perform actions based on parameters.  I think they align about as much as REST does with SOAP.  You can accomplish the same ends with both, but with REST you think primarily about resources, representations, and state, whereas with SOAP you think about verbs and parameters.  You can end up in radically different places, with radically different architectural implications.  In my head at least, that analogous distinction extends to messaging/eventing vs. web-services/SOAP.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

> * message out, message back to different component

- this is supported through an override of the callback binding HOWEVER this is then rightly an aspect
of the configuration of the ASSEMBLY and not the result of some individual implementation artifact
deciding on the wiring - ie there is no way that the "client" component should itself be dictating
the target for the response - that MUST be part of the composition/assembly.

I agree completely.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

Now, I think the real debate here is whether "replyTo" has **ANY** place in the style of Event Processing
that the SCA spec tries to describe.

Looking forward in your email, you through out a notion of "relatesTo".  That *exactly* what I'm trying to get at with this issue.

In certain circumstances, the "relatesTo" meta-data exists as part of the application-defined payload of a message, and that's probably unavoidable.  I don't want to aim to boil the ocean, as it were.

I'd like however, if the binding can handle the "relatesTo" notion whenever it makes sense.  That raises exactly the question I'm trying to get at - how does the binding even *know* that it is supposed to manufacture a unique identifier so that a subsequently delivered message can be correlated via some notion of "relatesTo"?  When that subsequent message does arrive, how does the binding layer even know to associate it with whatever state it might have stored earlier?  How did the first, second, or third consumer know where to send the "relatesTo" information?

How can the consuming binding of a message receive the necessary information from the incoming message and stash it, such that when the consumer then produces a message that relates to the originally received message, it will work?  The code calling the producer must either get at the "relatesTo" information, or tell the producer it's invoking to look up that previously saved state....

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">
The real question that Eric & Danny need to address is how an SCA event producer can make **any** assumption
that the receiving component actually produces any kind of response message.  A subsidiary question is whether
the event producer can also make some assumption that the receiving component will **only** send some message
back to the event producer and nowhere else.
I don't think I've made either assumption.  The only assumption I want to make is that an assembler can *choose* to do so with event producers and consumers that declare that they have an implementation that leverages the notion of "relatesTo" or "replyTo".

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">
I think that our current model of event processing is simply more loosely coupled than this.  If an event
producer anticipates that some new events may be generated as a result of some events that it produces, then
I would say that the component with the producer should also have a consumer for that type (or types) of event.
I can think of use-cases where that doesn't work.  For example, assume I have a component A that produces messages on some global domain channel //foo.  At some point, some set of components show up in the system, and start receiving messages on //foo, and perhaps some of those components (call them B, C, D) start sending "relatesTo" information about messages originally sent to //foo.

How can component A catch these "relatesTo" messages?  Option (1) - listen to a predetermined channel //bar (or //foo?) where those messages are expected to be sent.  Fails if the assembler doesn't wire producers on B, C, and D properly, and unfortunately, this key bit of information *isn't* in the model.  Option (2) - A consumes on the default domain channel "//".  Problematic in that hosting environment of A, if not A itself now sees far more messages than it needs to, in order to find the ones it does want.  Option (3) - as part of the originally produced message, send an indication of where the "relatesTo" messages ought to be sent.  This last approach works even if A, B, C, and D are deployed in different composites and at different times.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">
It is then up to the assembler to decide to connect both the producer and the consumer separately to appropriate
channels - similarly for any "receiving" component's consumer & producer.

I absolutely agree that the actual choice of how to handle "relatesTo" messages should be addressed by the assembler.  I'm just noting that by not supporting the notion of "replyTo", we've taken away one option for loosely coupling.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">
Any "correlation" needed between an initial event and some future "response" event should be managed through
some aspect of the response event (eg a "relatesTo" field).

Indeed yes, if we choose not to model these details, then SCA users will need to force such correlation information into the application/"business" layer, rather than letting it be handled at the binding layer.  Or they'll choose a vendor implementation that extends SCA to enable some amount of correlation to be done at the binding layer.
OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

My other reaction to this material is to say "if you want a response, use services/references, not event processing"

I understand that reaction.  I can see how what I'm saying feels like it falls into the category of "if all you have is a hammer...".  But I think what I've offered above makes what I'm arguing for different in several ways:

1) Eventing is about moving data, not invoking operations, which I see as a key semantic difference
2) I'm not assuming a single "responder", nor a single message back, nor a response that comes back even to where it started from.  We're talking about choreographed message exchanges, rather than individual message exchanges.  Trying to model this with WSDL, or even with two WSDLs, is an effort in futility.  Even worse, trying to pretend that WSDL captures these choreographies obscures the fact that the actual message flows aren't doing the standard request/reply exchanges that WSDL typically models.
3) By not supporting some model notion of "relatesTo"/"replyTo", end-user applications will be forced to move such data into event messages, mixing transport and business data unnecessarily, rather than having the binding take care of it.
4) By not offering some way of indicating a "reply" destination, that moves a tight coupling elsewhere in the system - in the form of assumed knowledge about where those events will be sent, or assumed knowledge about the types of events will be sent, or both.  Or you build in unnecessary inefficiencies.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

As to the question of why the various protocols listed below have "replyTo" - I say it is because those protocols have
to be able to deal with request/response message patterns of the type listed above - the current SCA event processing
model AIN'T REQUEST/RESPONSE.  It is *much* more loose coupled than that...

As I point out in #4 above, I think the association of request/reply with tight coupling is a mental trap of sorts.  Just because I've indicated a reply to address, doesn't mean that I've tightly coupled anything.  Rather, I've provided a means for late binding.  The world of HTML over HTTP is anything but tightly coupled, and it is built entirely on a request/response paradigm.  Some uses of HTML over HTTP are tightly coupled, but that just demonstrates that you can abuse any system.  I think I can assemble some fairly interesting message choreographies that will leverage a "replyTo" notion, but that in no way implies that the components involved are tightly coupled.

I, for one, question why JMS singled out "JMSReplyTo" as the only way to send a Destination to a recipient.  It ought to have been more open ended than it is.  WS-Addressing doesn't repeat this mistake.  But generically, the ability of the event producer to indicate one or more interesting "Destinations" for sending follow-on messages seems like a way to further decouple systems, as it saves the recipient of those messages from having to pre-emptively "wire" to those destinations.

OF64D5243A.D5CCB758-ON802577D1.005642AC-802577D1.005886CD@uk.ibm.com" type="cite">

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com

Eric Johnson <eric@tibco.com> wrote on 01/11/2010 20:57:15:

> [image removed]

> Re: [sca-assembly] NEW ISSUE: Need some notion of "callback" address
> in conjunction with eventing

> Eric Johnson

> to:

> ashok.malhotra

> 01/11/2010 21:38

> Cc:

> Martin Chapman, Anish Karmarkar, sca-assembly

> Hi Ashok,
> Just about every "broadcasting" eventing system that I can think of
> supports some notion of "reply-to"
> JMS: JMSReplyTo
> SOAP WS: WS-Addressing ReplyTo (I admit this is a little bit of a stretch)
> AMQP: reply-to (I think - I'm not very familiar with this)
> UDP: source port number
> PGM: (piggy-backing on UDP, it looks like)
> TIBCO's Rendevous: (yes - don't know the details)
> Email: "Reply-To"
> ... : ??? (anyone know anything about Apple's Bonjour?)
> Perhaps we could come up with a more complete list of this "broadcast"
> mechanisms, and what they support?
> In any case, the notion of using services and references doesn't even
> come close to mapping on to various cases that you might relate to
> eventing.  Services and references are tightly tied to the
> interoperability and known semantics of WSDL 1.1, whereas eventing might
> use a whole bunch of messaging patterns that you might not even be able
> to define in WSDL:
> * message out, maybe message back
> * message out, N messages back
> * message out *, message back
> * message out, message back to different component
> All of these might take advantage of a "reply to" capability that has
> little or nothing to do with the request/reply semantics associated with
> services and references.  Also, the message back might effectively be
> "any" - there might not be a specific "response" message with a single
> specific type.
> This question does bring me back to one question that I've been raising
> for a while.  What bindings do we anticipate people using with SCA
> Eventing, and how do we think they're going to be used?  Absent that
> information, it strikes me as very difficult to address questions like
> this one.
> As to whether we open this issue, since all the existing transports I
> mentioned above support the functionality, I think closing it with no
> action would be premature. Yet, I agree that we may need much discussion
> to establish what this "reply to" notion means, and maybe we won't get
> there.
> -Eric.

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