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Subject: Action item: 01-18-2: Distinction between global and local channels
As per my action item 2011-01-18-2: "Johnson to propose more
concretely what the different roles of global vs local channels
In the discussion of ASSEMBLY-250, Peter and I found ourselves looping back to ASSEMBLY-227. Specifically, I highlighted a problem with understanding the meaning of a "channel". Consider that a channel might stand for:
One problem we've identified with global logical scope is that it undermines composability (ASSEMBLY-227). For example, many different components of type implementation.composite might be deployed to the domain, and all of them might reference a global domain channel. Either we're imposing a significant top-down burden on those components, in that before they can be deployed, they must adhere to the constraints applied to the global channel, or we have a myriad different bottom-up perspectives, and they don't necessarily mesh in obvious ways.
For example, in my //Sales/North America channel, I might be producing information gleaned from public sources about sales by competitors. I might also be producing information about internal sales successes, and key information like the value, which must be kept extremely confidential. Do we intend that //Sales/North America encompass both? If not, why not?
When I go to deploy a component that references a global domain channel, what if it hasn't been defined? What does that mean? Must that be an error? Why?
What if the global domain channel is redeployed after some components that use it have already been deployed? What does that mean?
Now, assuming the global domain channel has been defined & deployed, the problems we face:
Policies: Some deployed components might use the global channel (a "scope"), and indicate a policy of confidentiality. Others might not care. Since the actual implementation can assign these two different scenarios to different JMS topics, this doesn't seem to be a problem. But is the policy on a global domain channel actually meaningful? One hypothetical policy intent I think is of use - something like "jms", or "apple-bonjour", "tibco-rv", or "amqp". That is, I don't care to bind to specific JMS details, but I want to indicate JMS.
Binding: I suspect this is utterly useless on a global domain channel. Why would I want to allow a global domain channel to bind to a single JMS topic, when I've defined myriad components that only want to listen for specific kinds of events....
Filters: Seriously bad - I've defined all sorts of components that reference the global domain channel, and in its initial definition it has no filter. Then it gets redeployed with a filter that blocks some of the events that used to get through. Conversely, the GDC is already deployed, but now I go to deploy a component that sends messages to that channel and those events will be filtered out. Error or no?
Events: Can't really declare which events end up on a global domain channel. At least not usefully. After they are *global*.
Conclusion: Global domain channels are really a notion of "scope". It makes no sense to assign most policy intents to them (possible exception of transport intents). A concrete binding is likely a mistake. Filters make no sense. Declaring events makes no sense. In other words, a global domain channel is merely a scoping label.
Case #2: A specific binding to an existing system.
This scenario supposes a channel definition within a composite, where the assembler puts a specific binding on the channel.
Very useful - within a composite.
Policies: In the most general case, policy intents and policy sets are unlikely to be unenforceable in practice, because it will be difficult or impossible to verify that an existing JMS destination meets the constraints dictated by SCA, since the SCA runtime is not deploying the destination. We've dealt with this in the context of existing SCA bindings by punting to the implementation. Same considerations likely apply here, so this concern is a wash.
Filters: Not clear what this really means. Since the existing use of the JMS topic likely sends messages not modeled in the SCA space, what do event filters mean? They can't mean filtering out messages for all recipients - at best only those recipients defined by SCA. The filtering can only be done once the message is received by the consumer. Certainly possible to apply filters, but certainly of odd utility.
Events: Provides incomplete information, but then it has always been thus.
Case #3: Bottom up definition of a transport concept, such as a destination
In this scenario, I'm defining an application that delivers messages via logical placeholders for destinations in the ultimate transport (such as JMS Topics, or RV subjects). In my composite, I define one or more "channels" - the placeholders, if you will - and for each producer or consumer I connect to them, I want to fully specify which events I produce or consume.
If the sending and receiving is not limited to my particular composite, I may want to promote the logical information about the channel, rather than the producers and consumers separately - see the diagrams I've drawn with respect to 227 for the weird cases one might run into.
Policies: Useful, as this constrains how others might also use a promoted channel, and provides meaningful information about how the channel should be used.
Filters: Possibly useful for consumers. Unlikely to be relevant to the channels themselves, as I wouldn't create a channel in a composite, wire it to producers, and then block some of those messages from going through.
Binding: Perhaps very useful. Early binding of where messages should be sent, but sometimes appropriate.
Events: Critical meta-data about what's being sent and processed via this logical construct. Helps identify places where producers and consumers might not be talking the same details - helps find errors, and further, if promoted past the edge of the composite, helps identify the purpose of the logical construct.
Case #1 seems to differ radically from cases #2 & #3. Perhaps I've explained them poorly above, and I should go into more detail, or that others see solutions that I don't?
Contemplating case #1, I wonder why we bother to call the "global domain channels" channels. They seem like message scopes. I don't know why we'd require that they exist before the first producer/consumer that uses them is deployed. If we assign policies to them, I don't know what that means. I cannot imagine ever allowing them to be redeployed with any meaningful changes to policies, filters, or events, which makes them seem quite static. I don't know what a binding means for them.
In addition, the naming seems arbitrary. Is "//Sales/North America" different from "//North America/Sales"?
My suggestion is that we simply don't refer to global domain channels as channels any more. For now, let me call them "scopes".
I would want to see a componentType expose the "scopes" that it uses, either via producers or consumers - whether or not the producers and consumers on those scopes are themselves "promoted" is somewhat silly, because by binding to the scopes, the producers/consumers are promoted.
A scope, it seems to me, is just a collection of names. The distinction between "//North America/Sales", and "//Sales/North America" is arbitrary and capricious - our systems are difficult enough as it is. So why not just "sales, north america"
In a radical departure from what we've currently got, I don't see why scopes have a definition at the domain level, rather they are an emergent property of what has been deployed. If someone starts putting intents on uses of "scopes", then what that simply means is that only consumers and producers with matching intents will get paired up.
As for channels, I see those as being useful within a composite, as they've been defined today. However, as per the proposals for 227, I want to see it as possible to expose the information about the defined logical message source/destination in the component, so that it can be used by higher up composites, as components are composed.
Sorry, that turned out to be a lot, but it seems there's a lot to the discussion.