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Subject: Re: [sca-assembly] A look at use-cases for composition with eventing, alternateapproaches to make them work better
- From: Peter Niblett <firstname.lastname@example.org>
- To: Eric Johnson <email@example.com>
- Date: Tue, 24 Aug 2010 16:03:42 +0100
I have taken another look at PC #1 and
I think I understand your analysis. Underlying all this is the question
of what encapsulation / assembly is for and what it means for events.
My view is that its main purposes are
to simplify the depiction of complex event processing applications, and
to allow common subassemblies to be reused. The most common use case I
see is where an assembler groups together a set of components, with well
defined producers and / or consumers, that can be then treated as a single
component by some higher level user. The examples in the WD and your
"easy scenario" on page 1 are examples of this kind of use case.
The approach we have in the WD is well suited to this use case, and
I think it is important to preserve the simplicity that it provides for
this case. In this case the composite is a way of implementing a component
What makes the Pathological Case different
a) you have events spontaneously produced
by components in the contained assembly, and consumed by components in
the assembly as well as being emitted by the assembly
b) the consumer in the contained component
consumes events (on a single channel)
It's the combination of both these points
that makes it difficult, and I am still left wondering how important a
case it is.
Of all your options, the last one is
the cleanest, but it does raise a question, and that is whether the composite
implements a component, or a channel - or both. As you have drawn
it, it looks like it is a combination of both, and I am worried that complicates
things by muddling the concepts.
On the direct wiring point, I agree
with your "shorthand" comment. I see them as useful shorthand
for simple non-shareable channels used within an assembly.. You would
still use explicit channels for the more complex cases.. I am not suggesting
we get rid of explicit channels.
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Eric Johnson <firstname.lastname@example.org>
Anish Karmarkar <Anish.Karmarkar@oracle.com>
A look at use-cases for composition with eventing, alternate approaches
to make them work better
Let me take a stab at answering some of this. This might be repeating
some of what we discussed in the call today. Sorry about that.
On 08/16/2010 11:52 PM, Anish Karmarkar wrote:
> Thanks for taking the time to do this. I have a few questions/comments
> on the material (listed below). It would be a good idea to have you
> present this on one of our calls. I should also note that we should
> evaluate this from the POV of existing usecases that we want to solve.
> 1) page 1, last two bullets: Isn't that the same? IOW, if you promote
> channels won't they have to appear in the CT of the composite that
> they occur in?
The difference here is perhaps just how you think about it. Do you
primarily promote producers and consumers, and occasionally tie them
together (and that shows in the CT), or do you indicate channels in the
CT, but (we expect) mostly only use one half of the channel from the
> 2) page 2, diagram for PC#1, following current WD, but avoiding global
> channels and bindings:
> why are there two channels in the outermost and middle composite?
> that this is wrong, but one channel would suffice and be simpler.
Depends on how you think of compositions. If I build A & D, and
that I want to send messages from A & D, somehow the intent has to
carried to the developer/designer of the outermost composite to know
that A should be linked back to D. You've hit the fundamental question
- how do I enable composability, achieve a clear result, and also convey
As near as I can tell, from going through this exercise, if I cannot
convey the intent somehow in the CT, then you cannot address any of the
use-cases I drew up here, and still achieve a clear result, or
> Another alternative for the same is to not have any channels in the
> innermost and middle composite, promote all the consumer/producers
> the way to the outermost composite and have a single channel that
> connects C, F and the promoted consumer/producer.
And you've conveyed the intent that that needs to happen how?
> 3) page 3, direct wires: I see this as problematic. It does not scale
> well and has problems with dynamic systems. Consider domain-level
> components that want to use eventing and everyone connected to the
> same channel (as a producer and as a consumer). Any single component
> being undeployed (or deployed) causes changes to all other components.
I'm perfectly happy to treat the wires as a short-hand. Unlike wires
from references, I don't see multiple wires from references implying
multiple channels. That is, if component A wires to D, E, and F,
could perfectly well be a single channel/Destination when ultimately
deployed. If you go with the direct wires approach, you might question
whether or not that is an appropriate short-hand, but it seems possible,
> 4) page 4, PC#1: Option: Channels as components, coupled
> I'm not sure I understand this option. More explanation would help.
Let me try again. Drop the current notion of channel altogether.
Instead, replace it, in your head, with the notion that *any* component
type can couple a producer and consumer in a way that indicates
"pass-through". That is, "all" (approximately)
events sent to the
consumer will end up at the coupled producer.
Or, if you want to think about it in reverse, "channels" are
specialization of a component that follows specific characteristics in
the formation of its component type. For that matter, since it is
a specialization of a component type, there's no reason to exclude all
the other attributes of a component type, including services,
references, and properties.
> 5) page 6, PC#2, per WD: avoiding global channels and bindings:
> You don't need three channels in the outermost composite. One channel
> would suffice.
As we discussed in the call today, if you don't have the multiple
channels, what is implied, at least is that at least two copies of each
message would be delivered to certain destination components. Having
thought about it a while, at deployment time, even if all the "channels"
have exactly the same characteristics, I don't know for certain that a
runtime could infer that the multiple channels can be collapsed into
one, and then only one copy of a message would be received. I drew
that way to prevent the duplicate message scenario.
> 6) page 7, PC#2, Using channels as components, producer and consumer
> Not sure I understand this. Seems like direct wiring.
> 7) page 8, PC#3, per WD avoiding global channels and bindings:
> You can simplify this by using only one channel in the outer composite.
Again, you might, but that only makes sense if you've communicated that
intent somehow. If that communication on intent appears in the model,
presumably I'll find it in the component type? Otherwise, I think
just have to "know" what to do. And that just circles back
to the point
of the issue - that isn't composability, that's just a monolithic batch
of components/composites that happened to be built as many pieces, when
either of the two versions on the first page would be simpler.
> On 8/4/2010 10:29 PM, Eric Johnson wrote:
>> I've had an action item to pull together use cases relevant
>> discussing the various options around how to think about composing
>> Well, maybe it wasn't an official action item at first, but I
took it as
>> a useful exercise to apply some rigor to what I proposed, and
>> put my proposal through its paces, along with a bunch of other
>> to see how they all fare.
>> I definitely tried to do this fairly. it is possible that I've
>> overlooked some aspect of the current WD, or that I didn't implement
>> other options fairly, or that there are yet alternate ways that
>> try to tackle the problem.
>> Which, by way of introduction, I mean to say, please send back
>> arrows, darts, alternate use-cases/scenarios you'd like to see
>> in any of the different approaches. Or, please feel free
>> alternate approaches.
>> Unfortunately, I'm on vacation next week, so I'll be tardy in
>> up with whatever discussion we might have, at least any that happens
>> after Friday afternoon.
>> Attached, please find both the PDF and ODG format.
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