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Subject: Re: [sca-assembly] Misadventures exploring "@coupledTo" for proposedresolution of ASSEMBLY-227
- From: Mike Edwards <mike_edwards@uk.ibm.com>
- To: sca-assembly@lists.oasis-open.org
- Date: Fri, 10 Dec 2010 07:29:33 +0000
Eric,
Some revealing comments - some responses
inline....
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:
| Mike Edwards/UK/IBM@IBMGB
|
Cc:
| sca-assembly@lists.oasis-open.org
|
Date:
| 08/12/2010 23:09
|
Subject:
| Re: [sca-assembly] Misadventures exploring
"@coupledTo" for proposed resolution of ASSEMBLY-227 |
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.
<mje>
If it is positive vs negative
that worries you, I am sure I can cast the requirements on consumers /
producers
using positive assertions
</mje>
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.
<mje>
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.
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.
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??
</mje>
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?
<mje>
This tends to confirm my
paraphrase above. The restriction implied is too great.
</mje>
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.
<mje>
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.
</mje>
-Eric
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
|
|
|
|
|
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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