While the WMI Object Model defines how programs work with WMI, the WMI schemas define the actual implementation of WMI objects. Consider an analogy of a driving manual versus a map. A driving manual explains the techniques of driving a car, whereas a map illustrates where the destinations are and how to get to them. The driving manual is analogous to the object model, while maps are analogous to schemas. Understanding WMI schemas allows you to understand the relationships among the objects that WMI manages.
Part of a WMI schema is illustrated in Figure B.3. In this case, specific types of network adapters are defined by extending a general definition of network adapters (CIM_NetworkAdapter).
Figure B.3 Part of a WMI schema
WMI objects are described by classes, providing definitions of their properties, attributes, and other information. These classes are organized into an inheritance hierarchy supporting object associations and grouped by areas of interest, such as networking, applications, and systems. Each area of interest represents a schema, which is a subset of the information that is available about the managed environment.
The Desktop Management Task Force defines a standard schema for WBEM called the CIM schema. This schema is implemented as the Cimv2 namespace in WMI. The CIM schema, in the form of the core and common models, provides a conceptual architecture for a managed environment. It is a framework of organizing principles that can be used by schema designers to understand and analyze the information requirements of management applications. The common model is represented by a set of abstract and concrete classes that define the basic characteristics of systems, networks, applications, and various groupings of statistical and other computer management-related data.
Some important concepts to understand about WMI schemas are:
Namespace Contains classes and instances. Namespaces are not physical locations; they are more like logical databases. Namespaces can be nested. Within a namespace, there can be other namespaces that define subsets of objects.
Class A definition of objects. Classes define the properties, their data types, methods, associations, and qualifiers for both the properties and the class as a whole.
Instance A particular manifestation of a class. Instances are more commonly thought of as data. Because instances are objects, the two terms are often used interchangeably. However, instances are usually thought of in the context of a particular class, whereas objects can be of any class..
MOF Managed Object Format.
MOF file A definition of namespaces, classes, instances, or providers; but in a text file. For more information, see the "Using MOF Files" section later in this appendix.
MOF compiling Parsing a MOF file and storing the details in the WMI repository.
CIM Common Information Model. For more information, see the Desktop Management Task Force Web site at http://www.dmtf.org/standards/standard_cim.php.
Association A WMI-managed relationship between two or more WMI objects.
You can extend a schema by adding new classes and properties that are not currently provided by the schema. For information about extending the WMI schema, see the WMI Tutorial at http://www.microsoft.com/downloads/release.asp?ReleaseID=12570.
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