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Subject: ISSUE-227 - Promotion of consumers and producers undermines composability
Hi Eric, I'm not quite following the argument so maybe we can clarify in email before the telecon. I'm not seeing the distinction that you're trying to draw. I can't tell if you're pushing on a problem in the vertical composition case (promotion) or horizontal composition (within a composite). >> communication needs are exposed at the boundary of a composite Not quite. Components in the Domain are wired together (services and references), across composite boundaries, without the use of promotion. Promotion (services and references) is really about exposing the configurable aspects of a composite that you wish to use as a component implementation. If your composite is not a component implementation then there's not much use for service/reference promotion. I think the same is true for consumers and producers. >> 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." The same thing happens inside a composite with services and references. A component service that is wired-to (by a reference) and not promoted is an endpoint that is hidden within the composite. Likewise, in the Domain (which is a composite) a component service is globally visible even if it is already wired-to by another component in the Domain. In this case it is not hidden...but that's because the Domain is treated as a composite. It is true however that a component reference cannot target a Domain level service unless that component reference is also in the Domain. Perhaps it's this last bit that you want to make consistent in the event model? >> target channel references can be to a domain channel, thus undermining composability This point I'm clear on, so I'll just make a comment. I prefer to think of it as breaking encapsulation, but the net effect is the same given the comparison to service and references. The Domain channel is an interesting beast because you can imagine configuring your system (or defaulting your authoring tools) such that ALL events flow through it, which makes it easy to add new consumers who can pick up any event they want from anywhere in the vertical composition without needing to promote producers just to enable visibility of the events. It really raises the question of whether or not we should have something similar for the ref->service model. This was a hotly debated topic in the past, one in which I was only an observer so there are others who will probably jump in here. >> Instead of promoting consumers and producers, promote channels. Move the filter and eventType information on consumers and producers into the channel. What does it mean to promote a channel? Does it mean that the consumer and producer aspect of the channel are promoted either together or somehow jointly? Is it different from allowing a composite consumer (and provider on the other end) to point to a channel as promoting the consumer aspect of the channel? My mental model of a channel is that it's a builtin implementation type, which is probably getting in my way of understanding what you're pushing on. thanks Dave Booz STSM, BPM and SCA Architecture Co-Chair OASIS SCA-Policy TC and SCA-J TC "Distributed objects first, then world hunger" Poughkeepsie, NY (845)-435-6093 or 8-295-6093 e-mail:booz@us.ibm.com |------------> | From: | |------------> >--------------------------------------------------------------------------------------------------------------------------------------------------| |Mike Edwards <mike_edwards@uk.ibm.com> | >--------------------------------------------------------------------------------------------------------------------------------------------------| |------------> | To: | |------------> >--------------------------------------------------------------------------------------------------------------------------------------------------| |"OASIS Assembly" <sca-assembly@lists.oasis-open.org> | >--------------------------------------------------------------------------------------------------------------------------------------------------| |------------> | Date: | |------------> >--------------------------------------------------------------------------------------------------------------------------------------------------| |04/09/2010 04:19 AM | >--------------------------------------------------------------------------------------------------------------------------------------------------| |------------> | Subject: | |------------> >--------------------------------------------------------------------------------------------------------------------------------------------------| |[sca-assembly] Fw: [sca-assembly-comment] NEW ISSUE: (1.2) Promotion of consumers and producers undermines composability | >--------------------------------------------------------------------------------------------------------------------------------------------------| 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 ----- From: Eric Johnson <eric@tibco.com> To: sca-assembly-comment@lists.oasis-open.org Date: 08/04/2010 18:18 Subje [sca-assembly-comment] NEW ISSUE: (1.2) Promotion of consumers and ct: producers undermines composability 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. 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