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Subject: Re: [sca-assembly] Issue 227: what does promotion of channels meanwrt intents/policies


In service/reference promotion, if you promote a service/reference 
outside a composite *and* the same service/reference is also connected 
within that composite to a local reference/service using a wire, then 
that local wire is not affected by any intent/policy that may be set by 
a higher-level composite via the promoted service/reference.

Channels are different (at least the way I view it in my head at the 
model-level). They are an abstraction to provide the disconnection 
between producers and consumers. Any "matching" that may happen between 
producers and consumers happen via the channel. For example, if I have 
set 'reliability' intent on the channel then any producer/consumer 
connected to that channel would have to provide reliability, otherwise 
it would not be a valid connection. As a producer I'm now assured that 
any consumer receiving the message via that channel will receive it 
reliably. However, if there are consumer that are connected to the 
"same" channel at various levels, as a producer I cannot make that 
assumption anymore. Which makes me question whether channels when 
promoted are really the same channels?  The difference arises out of the 
fact that wires in service/reference are one-to-one and connections 
between producers/consumers are many-to-many mediated by channel(s).

Hope that makes it clearer.


On 6/14/2010 4:48 PM, Eric Johnson wrote:
> Hi Anish,
> Am I missing something here?
> I don't see how this is any different from a service or reference that is promoted to the boundaries of a containing composite.
> For that matter, the current eventing proposal promotes consumers and providers, and those in turn could be promoted to the boundary of the next containing composite.  The *only* difference I see is that my proposal leaves open the possibility that consumers and providers would be subject to the same policies set in the same place - for example an expectation of "confidentiality" set on a channel - and that seems more appropriate to me.
> In any case, I don't see why you perceive that this question around policies is specific to channels.
> Eric.
> On Jun 8, 2010, at 8:24 AM, Anish Karmarkar wrote:
>> One of the things that has been proposed by Eric is promotion of channels outside of a composite. I'm assuming that such promotions will allow for intents/policies to be added on promoted channels by composites at higher levels. I'm not sure what that means to consumers/subscribers that are deeper down in the hierarchy.
>> I imagine that channel policies/intents will impact all the consumers/producers connected to it. When a channel is promoted with additional/different intents/policies on it, what happens to consumers/producers that are connected to the same channel at a lower level. Are they affected by the additional intents/policies? If they are that would be very counter-intuitive. If they are not affected, is it still the same channel? For example, if it is a JMS topic, it is going to be the same topic for the consumers/producers at all levels. The topic has to be created with particular guarantees/persistence that are going to affect all consumers/producers. This changes the intent model. The intents would now flow up *and* down the hierarchy.
>> -Anish
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