Is SML to CIM what WS is to RPC?

The question I hear most often when talking about SML, is how it relates to CIM. The easy part of the answer is to explain that SML is a metamodel, like MOF, not a set of model elements/classes like CIM. SML per se doesn’t define what a blade server or a three tier application looks like. So the question usually gets refined to comparing SML to MOF, or comparing an SML-based model to the CIM model. Is it a replacement, people want to know.

Well, you can look at it this way, but whether this is useful depends on your usage model. Does your usage model include a distinction between observed state and desired state? You can use SML to model the laptop in front of you, but if all you’re doing is reading/writing properties of the laptop directly you don’t get much out of using SML rather than CIM, definitely not enough to justify modifying an existing (and tested) manageability infrastructure. But if you change your interaction model towards one where there is more automation and intermediation between you and the resource (at the very least by validating the requested changes before effectuating them), then SML starts to provide additional value. The question is therefore more whether you would benefit from the extra expressiveness of constraints (through schematron) and the extra transformability/validation/extensibility (through the use of XML) that SML buys you. If you’re just going to assign specific values to specific properties then CIM is just as good. And of course keep in mind that a lot of work has already gone into defining the domain-specific semantics of many properties in CIM and that work should, to the extent possible, be leveraged. Either directly by using CIM where SML doesn’t provide additional value, or indirectly by carefully surfacing CIM-defined elements inside an SML model. Finally, CIM defines operations while SML doesn’t have such a concept. The most natural way to do a “start” using SML is not to invoke a “start” operation as it is in CIM, it is to request the configuration to be changed to a state in which the resource is started. In conclusion, there is a lot more to the “CIM versus SML” question than a direct replacement. Those who just look at ways to do syntactical translation between the two approaches will repeat the same errors that created (and still create) so many problems for those who saw Web services as just another way to do object to object RPC. The usage model (some would say the architecture) is what matters, not the syntax.

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