Defining Relations

The relation service defines and maintains logical relations between registered MBeans. Users define the relation type and establish the relation instance which that associates any number of MBeans. The relation service provides query mechanisms to retrieve MBeans that are related to one another.

In the JMX architecture a relation type is defined by the class and cardinality of MBeans that it associates in named roles. For example, if Books and Owner are roles, Books represents any number of owned books of a given MBean class, and Owner is a single book owner of another MBean class. You could define a relation type containing these two roles and call it Personal Library. This represents the concept of book ownership.

Figure 2–5 compares this sample relation type to the unified modeling language (UML) model of its corresponding association.

Figure 2–5 The Relation Model Defined by the JMX Specification

Through the relation service, users can create relation types and then create, access, and delete instances of a relation. In the preceding example, a management application can add Book MBeans to a Personal Library relation, or it can replace the MBean in the Owner role with another MBean of the same class. All MBeans are referenced by their object name, so that a relation can be accessed from a remote application. An MBean does not need to know what relations it participates in. New kinds of relations can be added to an agent without having to modify the code of the MBeans they link.

The relation service is notified when MBeans in a relation are unregistered, and it verifies that any relation involving that MBean still has the required cardinality. For example, if an Owner MBean were unregistered, the relation service would remove any Personal Library relations where that MBean was the designated owner.

The relation service can represent a relation instance either internally or externally. If the user defines a relation instance through the API of the relation service, the relation is represented by internal structures that are not accessible to the user. This is the simplest way to define relations, because the relation service handles all coherence issues through its internal structures.

A relation instance can also be a separate MBean object that fulfills certain requirements. The user instantiates and registers these MBeans, ensures that they represent a coherent relationship, and places these MBeans under the control of the relation service. This process places the responsibility of maintaining coherency on the user, but external relations have certain advantages: they can implement operations on a relation instance.

For example, a Personal Library relation could be implemented by an MBean with an operation called Loan. This operation would search the list of book MBeans for a title and implement some mechanism to mark that book as being on loan. And because external relations are MBeans, these extended operations are available to remote management applications.