[Issue 251] Some early thoughts

[Anish Karmarkar <Anish.Karmarkar@oracle.com>]

I took an AI to come up with a proposal for issue 251 [1].
Here are my initial thoughts:

Issue 251 points out that currently the cardinality (number of channel 
connections) of producers and consumers is 0..n. It also suggests that 
we allow 0..1 and 1..1.

It seems obvious that this is coming from the fact that in the 
service-reference model these possibilities exists. I believe that such 
a comparison isn't appropriate. In the service-reference model, a 
component-reference specifies an interface-based dependency. Sometimes 
such dependencies must to be satisfied to get the component/composite to 
work correctly. For example, an order processing component may have a 
dependency on a credit-card service. Unless that dependency is 
satisfied, the order processing component just won't work. Similarly, 
the same order processing component may want to allow the dependency to 
be satisfied by multiple credit-card services (requirement being, there 
be at least one). These dependencies get injected into the component and 
the component, based on its internal logic, may decide which services to 
call.

The pub-sub model is different than this. A consumer may express 
interest in certain events, but there is absolutely no guarantee that an 
event may ever be delivered to it. Similarly, a producer may produce 
events, but there is not guarantee that any consumer is either listening 
for those events or even if a consumer is listening, it may decide to 
just drop it on the floor and not take any action based on that event. 
Furthermore, these kind of connections are meant to be many-to-many. As 
far as cardinality goes, the cardinality has to be wrt how many channels 
the producer/consumer is connected to regardless of how many 
consumer/producers are on those channels. This makes cardinality in 
pub-sub tricky. As far as cardinality upper bound goes, what is the 
difference between a consumer connected to 2 channels each with 5 
producers and the same consumer connected to a single channel with 10 
producers?

Therefore, for the cardinality upper bound, I don't think it makes sense 
to have a "1" restriction. IOW, it should always be "n".

WRT cardinality lower bound, there are two possibilities "0" or "1". I'm 
leaning towards saying it is always "0", since there is no guarantee 
that even if you connect the producer/consumer to a channel that anyone 
else is participating. But I have a feeling that there *might* be good 
reasons to allow a "1" restriction on the lower bound

Thoughts?

-Anish
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[1] http://osoa.org/jira/browse/ASSEMBLY-251