HP WBEM Services Software Developer's Kit for HP-UX Provider and Client Developer's Guide A.01

Schema Design and Implementation
Schema Design
Chapter 336
be confident that clients which have implemented using the standard
CIM schema will be able to find desired management data in expected
places in the schema hierarchy.
During this process of refining the draft schema through examination of
the DMTF CIM schema, it may also be interesting to identify properties
and methods for some of the classes as a means to better understand the
objects being modeled. A high-level look at the DMTF classes can be seen
through their Visio or PDF UML diagrams. Further detail is available in
the Managed Object Format (MOF) file class, property, and method
descriptions. (Note: MOFs are text files describing CIM schema. This is
similar to the way MIBs describe SNMP and MIFs describe DMI.)
Note that classes may not only be those specifically defined by the DMTF,
but also may be DMTF-derived classes. This class extension is facilitated
through the objected oriented concept of inheritance, wherein a class may
be specialized to add more specific properties and methods. Thus, if the
DMTF schema does not have the richness required for the domain being
modeled, a provider can add additional properties and methods through
the use of subclassing.
While a provider can (and is encouraged to) instrument a DMTF CIM
class, there are several good reasons to subclass. Reasons for extending
the DMTF CIM schema include:
• Needing to add properties/methods to the subclass.
• Needing to override properties/methods to add/change qualifiers.
• Allowing clients to easily enumerate (list) a subset of the instances
as represented by a particular subclass.
• Allowing multiple providers to implement instances of the class. (For
example, if a given provider isn't sure that it owns all the instances
and another not-yet identified provider may have additional
instances.)
• Needing to derive a concrete class from an abstract one defined by
the DMTF. (For example, needing to derive a concrete type of service
from the abstract CIM_Service class).
In some cases, there may appear to be no significant coverage of the
modeled domain within the DMTF CIM schema. In this case, one should
focus on the aspects of the Core Schema which most closely resemble the
domain. Generally it is best to subclass from some DTMF class rather
than making up an entirely new schema from scratch.