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Subject: Re: [sca-assembly] Misadventures exploring "@coupledTo" for proposedresolution of ASSEMBLY-227

Hi Mike,

On 12/9/10 11:29 PM, Mike Edwards wrote:
OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">

Some revealing comments - some responses inline....

Yours, Mike

On 12/8/10 8:10 AM, Mike Edwards wrote:


I find it VERY hard to see how this "double headed beast" termed "prosumer" (or use whatever other term you find more congenial) is

in any way simpler or avoiding of the problems laid at the door of either "groupID" or "coupledTo".

I boil it down to the difference between negative and positive assertions.  I find it far more useful to search a list of positive assertions about what I MUST do with a "prosumer" rather than to search a list of negative assertions about what I cannot do with a producer and consumer that are "coupled".  With the positive case, I can simply search the spec for references to "prosumer".  In the negative case, I must read everything to make sure I haven't missed a constraint.

If it is positive vs negative that worries you, I am sure I can cast the requirements on consumers / producers
using positive assertions

<eej>I probably should have pointed out as well that I found myself wanting to have a label for the coupling of a producer and consumer.  Then, of course, I want to have normative statements about that pairing.  Once I've got normative statements about the pairing, that begs the question as to why we're not simply representing the pairing in the XML form.

OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">

The trouble with "prosumer" is that I think it ends up having to include all the features and capabilities of both a producer and a consumer,

with the need for both @target and @source - and also allowing for promotion

(and in the worst case allowing for promotion to a
prosumer, to a consumer & to a producer).

That's nonsensical to me. I don't see any reason you ever allow for splitting of a prosumer's characteristics once you've brought them together.

That is one of the most revealing comments on this issue so far.

What that says, in my terms, is that linked producers/consumers CANNOT be used in ways that are
standard for non-linked ones.

<eej>To be precise, I said that I don't see any reason for doing such a thing</eej>
OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">
I asked about this at the F2F and didn't get a crisp answer.  But the comment above seems very crisp.  Let me paraphrase
how I read that comment:

"When there is a linked producer/consumer, the producer/consumer can only EVER be connected in such a way that the
channel(s) connecting them always send every event from the producer to the consumer."  In other words if I as the
Assembly happen to want to take the events from the producer and send them out to some group of consumers which
DOES NOT include the linked consumer, I can't do it.  Even if I did have a separate connection from the producer to
the linked consumer which ensures that the required linking is satisfied.

<eej>All right, but you've identified a feature, not a reason/scenario/use-case.  Trying to identify the use case:

A component (CompA) has producer (ProdA) which is wired to multiple channels (ChanA, ChanB).  One of those channels (in order to solve the use cases I've postulated) must be wired back to the coupled consumer (ConA).

The question, then is about the 2nd channel (ChanB).  Why wire the producer to it in the first place?  I presume that some producers (ProdB & ProdC) are sending messages to ChanB that aren't meant to arrive at the coupled consumer (ConA) ?  OK, but why not simply have the downstream clients (ConB, ConC) consume both ChanA and ChanB?

So it looks like that can't be the problem you're concerned about, because within a single composite, I can still solve it with the prosumer approach as we've discussed so far.

Perhaps then, you mean to wire the prosumer to a channel within the composite, but then promote the producer half of it past the edge of the composite?  Can we explore this more?  This involves at least three levels of composite nesting to be interesting - the inner composite (CompA) that defines a prosumer (or @coupledTo), a containing composite that wants to promote the producer side of the prosumer, and at least one final wrapping composite that will use the events of that promoted producer.  Can you draw a picture?

An odd realization: the "injected channels" approach solves just this scenario, by way of letting producers and consumers explicitly wire to the channel within the composite, or not, as they see fit, and letting the use of that channel beyond the edge of the composite happen at the whims of the subsequent assemblers.  But then, I'm confused, because you seem to have rejected the possibility of solving this problem that way.

However, whether or not we pursue injected channels, I think the analysis highlights a key point. To achieve your approach with injected channels, obviously, I would promote ProdA to the edge of the composite.  To promote the channel to the edge of the composite as well, I have to make an affirmative decision to *also* indicate said channel at the edge of the composite.

Poking at the premise of your question, one question jumps into my mind, why not simply require that the component CompA promote an equivalent to ProdA uncoupled from the consumer ConA?  I could ask the same question with the injected channels approach - if, as a component I chose to promote the channel, but not an independent producer, is that OK?

Circling back, this seems to be one nugget of potential discussion then - if I promote a coupled producer & consumer (via @coupledTo, element prosumer, or injected channel - doesn't matter, the problem is always the same), then are the events of the producer separately available?  (Note: I don't think this problem applies equally to consumers, because a component implementation can already decide whether or not to receive events outside of those coupled to a producer by simply defining another consumer, so this is already under the control of the component, rather than the assembler).

I can think of at least two ways I might extend the prosumer model to encompass this requirement:
  • Whenever a prosumer appears in a componentType, that implies the existence of distinct producer with a related name
  • Allow composite promotion of just the producer side of a component prosumer
Care to comment on whether either seems better to you?
OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">
If my paraphrase is correct, then I am now parting company from this whole notion of linking.  This is undermining the
capabilities that I want to see Assemblers having at their fingertips.  What's the point of that??

That seems somewhat draconian to throw the baby out with the bath-water.  Rather than reject the policy model outright, because it might impose constraints on higher-level composites or unpleasant complexity, we've started trying to figure out how that might work.  Same thing applies here - this notion of bottom-up constraints matters, and we should have a discussion about what it means.
OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">

 AND the rule has to be that some combination of these things ensures that there is a "path"
that connects the prosumer to itself via some channel.

Since the prosumer has a single identity, defining it at the edge of a composite, and then wiring that to a channel in the containing composite results in wiring a *single* thing.  How could the path you refer to actually be broken?

This tends to confirm my paraphrase above.  The restriction implied is too great.

To avoid those problems, you end up having to place restrictions on what a prosumer can do - and any such restrictions are equally

applicable to "groupID" or "coupledTo" (etc).

As for avoiding the notion of a producer explicitly naming a consumer - that IS the game we are in here.  You can't hide it, no matter

how you try.  For sure, the producer and the consumer involved may be connected via some (as yet unspecified) channel, but the

fact of them BEING connected to one another IS the point of this function.

You want simplicity?  Then don't have this "this producer must be connected to this consumer" type of function in the model at all.

Clearly an Assembler can achieve this function if they so wish - it's a piece of cake for the assembler.  It's just that putting this metadata

into the model in a way that it can be POLICED by the runtime (or by tooling) is what generates all this complexity.  Leave it as

descriptive metadata in the implementations/composites.  Job done.

I think the complexity of the current spec is somewhat manufactured.  Take this:
"One technique which enables component producers to send events outside the composite and for component consumers to receive events from outside the composite is to configure producers and/or consumers of components inside the composite to use domain channels – that is, channels at the Domain level."

Suppose we define "event endpoint" as encompassing "producers" and "consumers".  I can then rewrite the above as:
"One technique which enables event endpoints of components to send and receive events outside of the composite is to configure those event endpoints to use domain channels - that is, channels at the Domain level."

Now, notice that if I define "event endpoint" as encompassing "producers", "consumers", and "prosumers", the the above statement stays exactly the same.

Consider the following revision:
An event endpoint declares where the messages it produces are sent through a list of one or more target URIs in its @target attribute.  The event endpoint declares the sources for the messages it receives through a list of one or more source URIs in its @source attribute.  The form of these URIs include:
  • The URI of a channel in the same composite as the producer, in the form channelName
  • The URI of of a channel at the Domain level in the form //channelName
The above is simpler than the current text.

To bear the above assertions out, I'm currently going through and trying to make a draft which uses the abstract notion of an event endpoint.

This isn't simplifying anything other than the language.  The concepts remain.  A producer ain't the same thing as a consumer.

Indeed, I argue that it is an attempt to sweep complexity under the carpet.  But the big bulge is still showing.  In fact, it is probably made worse.

<eej>As I've pointed out with the various diagrams that I've drawn, and with various discussions of specific scenarios, when trying to get to the bottom of the problems that arise when coupling composition with eventing, we discover complexity.  At the moment, the 1.2 WD completely punts on addressing that complexity, by either resorting to "use global channels", or creating hopelessly confused wiring diagrams.

Ignoring that complexity doesn't make it go away, it just pushes it onto our customers, or into implementation-specific extensions that solve it, but which won't be portable.

OFF00C6CF3.500B6030-ON802577F5.00271C00-802577F5.0028A925@uk.ibm.com" type="cite">


Yours, Mike

Dr Mike Edwards  Mail Point 146, Hursley Park
STSM  Winchester, Hants SO21 2JN
SCA & Services Standards  United Kingdom
Co-Chair OASIS SCA Assembly TC  
IBM Software Group  
Phone: +44-1962 818014  
Mobile: +44-7802-467431 (274097)  
e-mail: mike_edwards@uk.ibm.com  

From: Eric Johnson <eric@tibco.com>
To: Anish Karmarkar <Anish.Karmarkar@oracle.com>
Cc: sca-assembly@lists.oasis-open.org
Date: 07/12/2010 00:11
Subject: Re: [sca-assembly] Misadventures exploring "@coupledTo" for proposed resolution of ASSEMBLY-227

Hi Anish,

On 12/6/10 3:57 PM, Anish Karmarkar wrote:
> I think this is fine.
> Although, I do find the minimal change to CTs via Mike's proposal
> attractive. But certainly understand the complications that come with
> it wrt policy/intent and constrains like everything has to be promoted
> or connected.
> A minor difference, perhaps inconsequential at this stage is, wrt how
> I envisioned Mike's proposal to work:
> I didn't think of a @mustConnectTo or @coupledTo as the attribute we
> would use but an attribute such as @label or @groupID, which would be
> any arbitrary string. The @mustConnectTo or @coupledTo identified
> other consumers (or producers). I would rather the producers not point
> to consumers (or vice versa). Instead, the producers and consumers
> that are to be connected together would be identified with a common
> label/group id.

In one of the later emails, Mike and I discussed the problems with a
generic label, and I think we both agreed that a "groupID" kind of
notion allowed for an extra axis of flexibility (promoting to two
consumers on the composite, for example, but assigning the same group
ID) that isn't required by the use case, and simply introduces an extra
opportunity for confusion and mis-wiring.  So we had agreed on
@mustConnectTo in the email referenced below (00100).


> -Anish
> --
> On 11/30/2010 1:47 PM, Eric Johnson wrote:
>>   As per my action item, I've been trying to write up the approach that
>> I agreed to, the one that Mike suggested:
>> Mike labeled this as "@mustConnectTo" in his proposal, but I thought it
>> more natural to call this "@coupledTo".
>> Rather than spell out the changes I was making, let me instead describe
>> the corner I found myself in.
>> Just about the time I wrote this:
>> "The */coupled consumer and producer of a component/* is defined as a
>> consumer and producer from a component, where the componentType of the
>> component in question defines a consumer that has been @coupledTo a
>> producer from the same component."
>> ... I realized I was in trouble.
>> To make this notion work, when talking about composites, we have to make
>> normative statements to the effect of "whenever you promote a consumer
>> coupled to a producer, or vice-versa, the "coupled" consumer or producer
>> MUST also be promoted, and the resulting consumer and producer MUST be
>> reflected into the componentType of the composite as being coupled." To
>> that end, I wanted to define a notion of "coupled consumer and producer
>> of a component." That way, I could say more simply:
>> "Either both parts of a coupled consumer and producer of a component
>> MUST be promoted and remain coupled, or neither is."
>> We've also discussed that a "coupled consumer and producer of a
>> component" must also both be connected to a channel, if either of
>> them are.
>> I can only begin to imagine the verbal knots we're going to get into
>> when we start applying policies, and have to introduce gems like "the
>> coupled consumer and producer of a component" must share the same policy
>> intents and policy sets.
>> The difficulty here stems from a simple problem - producers and
>> consumers, so far at least, have independent existence, and now we want
>> to add text that couples them together tightly while still giving them
>> an independent identity.
>> Having tried to write it up that way, I conclude it is far more natural
>> to reflect the notion of "prosumer"
>> (
http://en.wiktionary.org/wiki/prosumer) (or conducer
http://en.wiktionary.org/wiki/conduce?) as Anish has stated, because:
>>     * That creates a thing with a single identity, to which specific
>>       rules can be applied
>>     * You don't have to create normative rules about how coupled
>>       consumers and producers must be both promoted or neither is
>>       promoted, and likewise about how they're both wired to a channel
>>       or not. With a single prosumer, there's no question of a split, so
>>       fewer normative constraints are required.
>>     * The policy questions, as applied to a prosumer, are likely
>>       different than those applied to consumers and producers
>> independently
>> The beauty of the "coupledTo" approach is that it leaves the
>> componentType almost untouched - with just a single additional attribute
>> on either the consumer or producer. Unfortunately, I think adds huge
>> conceptual cost, and based on what I've seen from what I've tried to
>> write up, it obscures the underlying intended model.
>> Having come to this conclusion, and considering that I want something
>> ready for our next call, I'm going to take a run at writing up the
>> "prosumer" approach starting Thursday or Friday of this week. That is,
>> unless I hear from others enough that convinces me I'm jumping the gun.
>> -Eric.
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