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Subject: Re: [sca-assembly] Fw: [sca-assembly-comment] NEW ISSUE: (1.2) Promotionof consumers and producers undermines composability

Hi Peter,

On 05/11/2010 04:24 AM, Peter Niblett wrote:
OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">

I have read your correspondence with Dave Booz, and taken a quick look at your revisions. I have a number of things to ask about that, but first I would like to understand the issues here.

I think your points relative to the Feb 12 draft can be summarized as

1. Components within a composite can be directly wired to a domain-level channel (see the example in figure 2-6). As Dave points out, this can be viewed as breaking encapsulation as there is nothing explicit in the component types of components D1 and D2 that tells you that this is going on (since you can't reconfigure it when assembling the composite). You have to look inside the definitions of composites X and Y to see this, and if there is a deep nesting of composites you have to look down all the levels.  

2. The mechanism of linking up producing and consuming components within a composite, e.g. in the case shown in figure 2-5, is awkward. You have to configure the @sources of the consumers and the @targets of the producers to point to the same channel

Not just awkward - it breaks encapsulation.  It hides a critical detail.

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
3. At the moment channels are either completely private (scoped only within a single composite), or completely global (when they are in the domain). There is no mechanism for promoting a channel so that it is visible only to a given composite and its subcomponents.

4. Domain level channels don't seem to be necessary since we could do the same thing by binding to JMS topics.
On this last point, you're construing my point as being overly specific.  Any binding could do, not just a JMS binding.  UDP, AMQP, TIBCO Rendevous, Apple's Bonjour are others that I can think of.  We could equally well have an "binding.scaeventing".

Some additional points:
  1. Policies assigned to producers and consumers are likely to need to be the same, or to align.  It makes more sense to assign those policies to a shared concept of a "channel", rather than treat the producers and consumers as independent.
  2. SCA already has the means to "break encapsulation" by way of a well understood pattern - bindings.  The existing eventing draft allows "sources"/"targets" that provide an alternate means of breaking encapsulation without justifying why an alternate means is necessary.
  3. Channels have bindings, not producers and consumers.  Suppose I have a producer and a consumer in a composite - on both I "target/source" the same transport destination, but here's a weirdness - I could put a binding on both, and perhaps it isn't the same.  Does that imply, behind the scenes, that the runtime has to do automatic bridging from one transport to the other?  That no longer sounds like we're modeling eventing, but modeling a system in which we hope transport level details can be modeled as if they don't exist.  My gut tells me that hope will be squashed by an unfriendly reality.

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
Please correct me if this is wrong.

In "defence" of the OSOA spec, what we were trying to do is

a) Allow true decoupling of event-centric components, so that it is possible to author a component as a pure consumer and/or producer of events without knowing how these components are going to be assembled. This means keeping details of the channels out of the component types. This is the main motivation for having <producer> and <consumer> elements in the component type and for declaring filters as part of the consumer (if there's no way a component can handle a particular kind of event, it should say so)

b) Allow an assembler - if he/she wishes - to define a tight coupling of components so that they can be wired up without "leakage" of events outside a composite, and without external events coming into the composite uninvited. This is needed when you are modelling event processing applications. This is why we have private channels scoped within components, as shown in figures 2-4, 2-5 and 2-7.

c) Allow loose binding of components, where producers are not aware of the identities of the consumers and vice versa, and where there is some kind of global event distribution mechanism (an event bus if you will) that spans all application components. This is done by having global channels at the domain level

d) Provide an abstraction that allows event distribution to be modelled in a protocol neutral fashion. This is the reason for having channels in the model at all.

Again, I maintain, as I stated on the conference call, that my proposal only adds additional use-cases, it does not remove any.

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
Maybe the confusion is caused by accommodating both b) and c) simultaneously.

Returning to the points

1. It is true that you can do this, see figure 2-6, but you don't have to. The alternative is to promote the producers and consumers, so they do become visible at the next level up and then can be wired to a channel (or promoted further) by the person assembling that composite. That's what you would do if you were designing libraries of reusable nested event processing components. On the other hand if you really do have a uniform event bus concept that spans all your components, we felt it would be unnecessarily tedious for people to have to do this all the time.

Well, except if my use case is that some of the producers and consumers from a composite are listening on the same logical destination, and they want to share that destination outside of the composite, the only way they can do that is to specify "source" & "target".  At which point, I can no longer compose as freely.

With the proposal I've submitted, I have added the ability to directly associate producers and consumers, and model that they share the underlying transport destination, without having to specifically tie that to any "target" or "source" for that destination.  In other words, it preserves composability without sacrifice.

Further, in my proposal, a binding is associated with a channel, not with an individual producer or consumer, which means that the additional bullet point #7 above of having to automatically generate a bridge goes away.  Designers would have to intentionally create a bridge.  This strikes me as an important feature.

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
2. I agree that this is awkward. I have always favoured being able to wire a producer directly to a consumer (effectively having an implicit channel). You should also be able to choose whether you establish this by setting @target on the producer or @source on the consumer. That same mechanism could be used if you have an explicit channel, so you could set the @targets/@sources on the channel to point at the producers/consumer (effectively what Eric does in his example of figure 2-4) rather than the other way round.

I think wiring directly is a mistake, and implies a different kind of transport.  I think, if you're talking about producers & consumers, you want to graphically depict that as wiring via an indirection.  The semantics and behavior of publish/subscribe are dramatically different from direct point-to-point communications, and shouldn't be confused by identical/very similar graphics.

My proposal has a simple unification.  Producers and consumers go away - components always "wire" to channels when they want to produce/consume.   The difference with my proposal is that channels are either private to the composite, or promoted.

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
3. At the moment we accommodate both use cases (completely private or completely global). Intermediate cases sound interesting, but I am not sure whether anyone really needs them. Just to be clear on how we'd expect things to work at the moment... if you look at figure 2-4 you would normally either wire a producer/consumer (the yellow triangles) to a channel or promote them. That's what the figure shows. You could promote the consumer for component 4, in which case it would receive events from the outside AND events from channel A (which can only have come from components 1 and 2). One other thing one could consider is allowing the input or output of the channel (which are in effect producers and consumers themselves) to be promoted, to allow events from the outside to go directly into channel A without having to go via components 1 or 2, but again I'm not sure why that would be useful.

You've fundamentally hit the nail on the head here, perhaps unintentionally.  The notion of completely private or completely global simply isn't composable.

I think it is fair to turn this question around - why wouldn't I want to abstractly model and promote the channel that my composite wishes to send and receive on, rather than be forced to early bind to some notion of a global "source/target" for where those messages are coming from.  The essential value of an abstract concept of channel is the model of which events are being sent and received over it, not its name!

OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">
4. I think modelling the distribution mechanism is a good thing.

And yet, where do I model a notion of a channel that crosses composite boundaries, but for which I don't yet have a specific concrete name for the destination?  We model services and references abstractly in this manner as part of the component type, yet for some reason you seem to be resistant to including the channel in the component type.

My take on the above:
  • You agree that it isn't composable (see your #3), so shouldn't we open the issue?
  • For the notion of promoting channels, I don't see any problems/objections identified above, except that it is different from promoting producers and consumers.
  • My proposal doesn't remove any use cases/functionality, and it adds composability
I admit that my proposal is *not* perfect - it needs work on the artwork, and the policy details probably need more detail, and perhaps a discussion of channel hopping (channel bridging), but I do think it adds functionality, and solves problems that the current draft specifically identifies as areas needing work.

All of this discussion, I note, is in service of the question: should we open the issue?  Seems like you've acknowledge that the current approach isn't composable.  Further, I've provided an approach that covers additional use-cases, without taking any away, while simultaneously solving the problems raised by the issue.  So maybe we don't take my solution, but the issue ought to be opened.


OF2A680098.9414579C-ON80257720.003672D3-80257720.003EA5F8@uk.ibm.com" type="cite">


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: OASIS Assembly <sca-assembly@lists.oasis-open.org>
Date: 12/04/2010 17:56
Subject: Re: [sca-assembly] Fw: [sca-assembly-comment] NEW ISSUE: (1.2) Promotion of consumers and producers undermines composability

Follow up....

I've just posted to the OASIS documents repository a proposal for ASSEMBLY-227.


I posted in PDF and Word format:


These documents are *not* in anything close to final form:
  • OpenOffice did a number of things to slightly mangle the word document
  • We need new pictures to show the new channel promotion notion.  I've included my notion of what it would look like below, in the hopes that it will help, although I should add that so far within TIBCO, I've gotten the some disagreement about the proposed artwork.
  • I haven't vetted the schema changes.
  • Obviously this is still a fork of the current spec, based off of the copy that Anish started from.
However, before I spent many hours trying to get everything perfect, I figured I should at least know whether the intent is even close to acceptable to the TC.


P.S. My current conception of a channel might look like - see diamond at the bottom edge of the component.

On 04/09/2010 01:19 AM, Mike Edwards wrote:


Forwarding to the main sca-assembly TC list....

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
----- Forwarded by Mike Edwards/UK/IBM on 09/04/2010 09:18 -----
From: Eric Johnson <eric@tibco.com>
To: sca-assembly-comment@lists.oasis-open.org
Date: 08/04/2010 18:18
Subject: [sca-assembly-comment] NEW ISSUE: (1.2) Promotion of consumers and producers undermines composability

Target: sca-assembly-1.2-spec-wd01.doc

Title: Promotion of SCA consumers and producers undermines composibility


In the assembly 1.2 WD 01, consumers and producers are identified as part of the "component type" of a component, whereas "channels" are limited in scope to the boundaries of a composite.  This is contrary to the rest of SCA, where the indication of the communication between components surfaces in the component type, currently as a service or reference.  When needed, services or references can establish concrete bindings, but otherwise communication needs are exposed at the boundary of a composite.  In the case of producers, consumers, and channels, not only can bindings be applied, but also "targets", which then either hide endpoints within a composite, or expose them globally as part of the "global domain."

This makes composition of applications using eventing more difficult.

Further, although producers and consumers can refer to the same target, this is an awkward way for these two constructs to establish that they intend to operate on the same "destination".  When building a composite the composite developer may wish for one component to produce for a channel, and a different component to publish on the same channel, and then promote the combination of producer and consumer.  If the developer only promotes the consumer, or only promotes the publisher, that would be misleading to the composites using that component.  Perhaps it is even an error.

Here, then, are two problems:
  • "target" channel references are a weak way to couple the use of the same destination.
  • target channel references can be to a domain channel, thus undermining composability - either by collisions in the naming of target channels, or by forcing special knowledge of which channels are used where.

Instead of promoting consumers and producers, promote channels.  Move the filter and eventType information on consumers and producers into the channel.


Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

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