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The remaining attribute type is variable. Alias and variable types are free-floating, in that they are never
assigned to entity types or relationship types, but you can assign values for them to any entity or
relationship. An alias attribute contains an alias value for an entity or relationship. An attribute of type
variable, also referred to as a variable-length attribute, has no maximum length, and normally stores
descriptions, edit masks, default values, long names, and other variable-length values. Since not every
entity or relationship needs alias and variable-length attributes, System Dictionary sets aside storage
space for them only if you explicitly assign them. When you delete an alias or variable-length attribute,
the storage space is released.
A relationship class is a name, like contains or uses that describes that action or connection of a
relationship. In System Dictionary, a relationship type belongs to a relationship class. The relationship
types RECORD contains ELEMENT and FORM contains ELEMENT, for example, belong to the contains
relationship class. In some cases, a relationship class serves as a qualifier for a relationship type. For
example, System Dictionary has three relationship types that involve element pairs:
ELEMENT contains ELEMENT
ELEMENT redefines ELEMENT
ELEMENT references ELEMENT
The first type creates relationships between parent and child entities. The second type indicates that two
elements share common storage in a program. The third type documents an element that references
another element, as in a Pascal type reference. These ELEMENT ELEMENT relationship types are
qualified by a relationship class.
Extensibility
System Dictionary provides a built-in set of structures called the core set. The core set includes entity
types, relationship types, relationship classes, attributes, and type-attribute associations. You can extend
your dictionaries by creating new entity types, relationship types, relationship classes, attributes, or type-
attribute associations. You are also allowed to make limited changes to the core set, such as changing the
external names of structures. The extensibility feature allows you to customize the dictionary. It also
allows Hewlett-Packard to localize System Dictionary to your native language, and to update the core set
as new subsystems are added to MPE.
Domains and Versions
In System Dictionary, you can divide a single physical dictionary into multiple, logically separate domains.
A domain is a name space that contains entities and relationships. You typically use domains to keep the
entity and relationship definitions of one application group separate from others. You can also use
domains to avoid naming conflicts. Every System Dictionary has a built-in domain called the common
domain. The common domain is always present and is always public. You can add your own domains,
called local domains, and assign them a sensitivity level of public or private.
You can further subdivide domains into named partitions called versions. Versions allow you to designate
one set of entities and relationships in a domain as the current production version, and other sets as test or
archival versions. Unlike domains, which are considered separate name spaces, you should think of
versions as complete copies of the entities and relationships in a domain. In a version of test status, you
can add or delete entities and relationships, or change their attribute values. You cannot modify an
archival version, and you usually keep this version for historical purposes. Also, you cannot modify a
production version. System Dictionary allows only one production version in a given domain. The version
feature allows you to maintain a stable production version while you experiment with other versions. It
also allows you to reactivate an archival version if it becomes necessary to return to a previously used set
of definitions.
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