Hi Mike,
On 12/8/10 8:10 AM, Mike Edwards wrote:
Folks,
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.
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
Yes.
(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.
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?
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.
-Eric
Yours, Mike
Dr
Mike Edwards
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From:
Eric Johnson
<>
To:
Anish Karmarkar
<>
Cc:
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).
-Eric.
>
> -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:
>>
>> 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.
>>
>
>
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