Misadventures exploring "@coupledTo" for proposed resolution of ASSEMBLY-227

[Eric Johnson <eric@tibco.com>]

As per my action item, I've been trying to write up the approach that I agreed to, the one that Mike suggested:

http://lists.oasis-open.org/archives/sca-assembly/201011/msg00100.html

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.