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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 <peter_niblett@uk.ibm.com>
- To: Danny van der Rijn <dannyv@tibco.com>
- Date: Tue, 31 Aug 2010 12:57:58 +0100
Danny
I don't have a problem with a use case
that has A,B,C as various kinds of producer, and D,E,F as consumers
(two of them being loggers and one a visual display). Our discussion has
been around the grouping of these things into composites - and the rationale
for doing that.
As I said in my earlier note, the main
reasons for using composites are a) to simplify the resulting picture,
b) to allow someone to reuse a composite component by treating it as a
component in its own right, without having to understand its internal structure
(which may involve further nested composites).
So I think we are all happy with the
pictures on Eric's first page. The first one doesn't have composites at
all, and does what you would expect. The second one groups together all
the producers, so they become a "composite producer" which can
be used by an assembler just like a simple producer, and likewise groups
the consumers together to make a "composite consumer". So far
so good.
Now let's suppose you want to group
together producer A and consumer D into a composite, as suggested by the
picture on top of page 2. Why are you doing this? Well, several possible
reasons
i) You want to let people consume the
events produced by A, but also make sure that they are logged by
D. To do this you connect A to D by a channel that is private to the composite
and promote A. As far as the outside world is concerned the composite is
a producer, it just happens to log the events that it produces as well.
This use case works well with the current WD
ii) You want the composite to be able
to log things from the outside, as well as the events produced by A. In
this case you connect A to D by a local channel (as in the previous case),
but this time you promote D, so the users of the composite can view it
as a consumer. It just happens to consume events that are produced internally
as well. Again no problem with the current WD,
iii) You are grouping them together
for administrative convenience - a bit like an old 74XX integrated circuit
which would include (for example) 4 NAND gates in a single DIL package.
In this case you would promote both A and D, but not wire them internally.
The documentation for the component would say (in natural language) that
it contains a producer and a consumer, and it would then be up to the assembler
to decide whether to wire A to D or not, depending on the needs of the
application.
It is this last bit that you and Eric
are contesting, as I think your use case is along the lines of..
iv) A+D and B+E are siloed apps, that
should work either in isolation or when composed together. When composed
together cross-connects come in to play so that events from A have to go
to E, and events from B go to D. Doing this with local channels is a problem
(since you would want a local channel between A/D and B/E in the siloed
case, bit not in the cross-connect case since the connections there are
done at the next level up). However this scenario can be realised by using
global channels. Some time ago I suggested that what you are really asking
for is a way of parameterising the global channel name, so that it could
be modified by the assembler who uses the composite.
Regards
Peter Niblett
IBM Senior Technical Staff Member
Member of the IBM Academy of Technology
+44 1962 815055
+44 7825 657662 (mobile)
From:
Danny van der Rijn
<dannyv@tibco.com>
To:
OASIS Assembly <sca-assembly@lists.oasis-open.org>
Date:
25/08/2010 23:00
Subject:
Re: [sca-assembly]
A look at use-cases for composition with eventing, alternate approaches
to make them work better
Some responses inline
On 8/24/2010 8:03 AM, Peter Niblett wrote:
Hi Eric
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 is that
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.
<vdR>
IMO it's a pretty standard case. To repeat my use case from the meeting
yesterday, I'll make an analogy to slf4j. Consider some subsystem
that includes D as a log sink that writes to logfiles. 'A' represents
any number of components that emit log events. Compose that with
B/E where B represents more emitters, and E represents another sink, this
time to a database. And compose again with C/F where F filters some
of the events and shows them on a user console. The point of using
events for logging is exactly that you don't know who's producing the events,
and furthermore, there are so many ways and reasons to consume them that
you don't want to be tied down to using a service.
IMO the "fir tree" is exactly correct here: the A/D component
needs to say that A and D are coupled, but doesn't care to say on what
channel binding. B/E are coupled in a similar fashion, and
furthermore, B/E are coupled to the same channel binding. Etc.
</vdR>
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.
<vdR>
In the last two options, I think Eric is trying to show that the *coupling*
of the producer and consumer as part of the componentType. In this
last example that you're highlighting, since the consumer and producer
are shown separated, there's certainly the possibility of ambiguity in
what this might mean in the graphical form. If you overlay the semantics
that the runtime merely treats them as referring to the same thing, then
this is just constraining/delegating to the runtime to connect producers
and consumers, in the same way that the runtime connects services and references.
</vdR>
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.
Regards
Peter Niblett
IBM Senior Technical Staff Member
Member of the IBM Academy of Technology
+44 1962 815055
+44 7825 657662 (mobile)
From: Eric
Johnson <eric@tibco.com>
To: Anish
Karmarkar <Anish.Karmarkar@oracle.com>
Cc: sca-assembly@lists.oasis-open.org
Date: 17/08/2010
19:40
Subject: Re:
[sca-assembly] A look at use-cases for composition with eventing, alternate
approaches to make them work better
Hi Anish,
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:
> Eric,
>
> 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
component?
>
> 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?
Not
> 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
know
that I want to send messages from A & D, somehow the intent has to
be
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
that intent?
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
composability.
>
> Another alternative for the same is to not have any channels in the
> innermost and middle composite, promote all the consumer/producers
all
> 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,
that
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,
at least.
>
> 4) page 4, PC#1: Option: Channels as components, coupled
> producers/consumers:
> 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
just a
specialization of a component that follows specific characteristics in
the formation of its component type. For that matter, since it is
just
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
it
that way to prevent the duplicate message scenario.
>
> 6) page 7, PC#2, Using channels as components, producer and consumer
> tied:
> 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
I
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.
-Eric.
>
> -Anish
> --
>
>
> On 8/4/2010 10:29 PM, Eric Johnson wrote:
>> I've had an action item to pull together use cases relevant
for
>> discussing the various options around how to think about composing
with
>> eventing.
>>
>> 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
actually
>> put my proposal through its paces, along with a bunch of other
options,
>> 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
we could
>> try to tackle the problem.
>>
>> Which, by way of introduction, I mean to say, please send back
comments,
>> arrows, darts, alternate use-cases/scenarios you'd like to see
me pursue
>> in any of the different approaches. Or, please feel free
to suggest
>> alternate approaches.
>>
>> Unfortunately, I'm on vacation next week, so I'll be tardy in
catching
>> up with whatever discussion we might have, at least any that happens
>> after Friday afternoon.
>>
>> Attached, please find both the PDF and ODG format.
>>
>> -Eric.
>>
>>
>>
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