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Subject: Re: [sca-assembly] Fw: [sca-assembly-comment] NEW ISSUE: (1.2) Promotion ofconsumers and producers undermines composability
- From: Peter Niblett <peter_niblett@uk.ibm.com>
- To: Eric Johnson <eric@tibco.com>
- Date: Tue, 11 May 2010 12:24:16 +0100
Eric
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
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.
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.
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.
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.
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.
4. I think modelling the distribution
mechanism is a good thing.
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:
| 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.
http://www.osoa.org/jira/browse/ASSEMBLY-227
I posted in PDF and Word format:
http://www.oasis-open.org/committees/document.php?document_id=37272
http://www.oasis-open.org/committees/document.php?document_id=37273
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.
-Eric.
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:
Folks,
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
Email: mike_edwards@uk.ibm.com
----- Forwarded by Mike Edwards/UK/IBM on 09/04/2010 09:18 -----
Target: sca-assembly-1.2-spec-wd01.doc
Title: Promotion of SCA consumers and producers undermines composibility
Description:
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.
Proposal:
Instead of promoting consumers and producers, promote channels. Move
the filter and eventType information on consumers and producers into the
channel.
-Eric
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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