Working with the relation service - JMX in Action - page 256

Relation type
The relation type is an object that acts as a template for relations. It contains a
list of role information objects that describe the members of a relation. The rela-
tion service uses relation types to validate relations by examining the relation
type role information and comparing it to the role values in the relation. The
relation type is defined by the class javax.management.relation.RelationType .
Role information
As we mentioned earlier, role information objects are contained within Relation-
Type objects. The role information distinctly defines a member of a relation. An
instance of the RoleInfo class contains a unique name of a role as well as the mul-
tiplicity of the MBeans for that role. The role name cannot be null and must be
unique for a given relation type. The multiplicity defines the number of MBeans
that can be referenced by the role.
For example, a FamilyCar relation type might have a RoleInfo object wheels
with a multiplicity of four Wheel MBeans, whereas a Tricycle relation type would
have a multiplicity of three Wheel MBeans. In addition, a CookieJar relation type
might have a multiplicity of 0 to 100 Cookie MBeans, depending on how hungry
you were.
A RoleInfo object also specifies two more constraints for a relation type: the
class name of the MBeans that can act as a role in a relation of the type that con-
tains the RoleInfo object, and the access mode of the role. The access mode
determines the read and write permissions of the role.
Relation
Relations are represented by the Relation class. Every relation is an object rep-
resenting an instance of a particular relation type (not an actual instance of the
RelationType class, but the Relation class). The relation is an object, registered
in the relation service, that defines an association between existing MBeans in
an agent. A relation can reference several different classes of MBeans and
present them in such a way that the user can clearly understand the association
between them.
MBeans participating in a relation are said to be a role of a relation. A role is
validated against the RoleInfo object of the Relation 's relation type. As we men-
tioned earlier, a relation is either internal or external . An external relation
exposes methods that can operate of any number of its member MBeans; this
functionality provides a powerful way to manage your enterprise's collection of
existing MBeans.