11.3.2 Major Classes and Associations

   Schema

   Dimension Cube

   CubeDimensionAssociation

   calcHierarchy

   Hierarchy

   CubeDeployment

   {ordered}

   Figure 11-1 OLAP Metamodel: Major Classes and Associations

   The major classes and associations of the OLAP metamodel are shown in Figure 11-1 .

   Schema is the logical container of all elements comprising an OLAP model. It is the root element of the model hierarchy and marks the entry point for navigating OLAP models.

   March 2003 OMG-CWM, v1.1: Organization of the OLAP Package

   A Schema contains Dimensions and Cubes. A Dimension is an ordinate within a multidimensional structure and consists of a list of unique values; that is, members that share a common semantic meaning within the domain being modeled. Each member designates a unique position along its ordinate.

   A Cube is a collection of analytic values; that is, measures that share the same dimensionality. This dimensionality is specified by a set of unique Dimensions from the Schema. Each unique combination of members in the Cartesian product of the Cube’s Dimensions identifies precisely one data cell within a multidimensional structure.

   CubeDimensionAssociation relates a Cube to its defining Dimensions. Features relevant to Cube-Dimension relationships (for example, calcHierarchy) are exposed by this class.

   A Dimension has zero or more Hierarchies. A Hierarchy is an organizational structure that describes a traversal pattern through a Dimension, based on parent/child relationships between members of a Dimension. Hierarchies are used to define both navigational and consolidation/computational paths through the Dimension; that is, a value associated with a child member is aggregated by one or more parents. For example, a Time Dimension with a base periodicity of days might have a Hierarchy specifying the consolidation of days into weeks, weeks into months, months into quarters, and quarters into years.

   A specific Hierarchy may be designated as the default Hierarchy for display purposes (for example, a user interface that displays the Dimension as a hierarchical tree of members). CubeDimensionAssociation can also identify a particular Hierarchy as the default Hierarchy for consolidation calculations performed on the Cube.

    Dimensions and Hierarchies are described further in Section 11.3.3, “Dimension and Hierarchy,? on page 11-5.

   MemberSelection models mechanisms capable of partitioning a Dimension’s collection of members. For example, consider a Geography Dimension with members representing cities, states, and regions. An OLAP client interested specifically in cities might define an instance of MemberSelection that extracts the city members.

   CubeRegion models a sub-unit of a Cube that is of the same dimensionality as the Cube itself. Each “dimension? of a CubeRegion is represented by a MemberSelection of the corresponding Dimension of the Cube. Each MemberSelection may define some subset of its Dimension’s members.

   CubeRegions are used to implement Cubes. A Cube may be realized by a set of CubeRegions that map portions of the logical Cube to physical data sources. The MemberSelections defining CubeRegions can also be grouped together via MemberSelectionGroups, enabling the definition of CubeRegions with specific semantics. For example, one can specify a CubeRegion containing only the “input level? data cells of a Cube.

   A CubeRegion may own any number of CubeDeployments. CubeDeployment is a metaclass that represents an implementation strategy for a multidimensional structure. The ordering of the CubeDeployment classes may optionally be given some implementation-specific meaning (for example, desired order of selection of several possible deployment strategies, based on optimization considerations).