500 DISTRIBUTED OBJECT-BASED SYSTEMS CHAP. 9
The licensing service allows developers of objects to attach a license to their object and enforce a specific licensing policy. A license expresses the rights a client has with respect to using an object. For example, a license attached to an object may enforce that the object can be used by only a single client at a time. Another license may ensure that an object is automatically disabled after a certain expiration time.
CORBA offers a separate naming service by which objects can be given a human-readable name that maps to the object’ s identifier. The basic facilities for describing objects is provided by a separate property service. This service allows clients to associate( attribute, value)-pairs with objects. Note that these attributes are not part of the object’ s state, but instead are used to describe the object. In other words, they provide information about the object instead of being part of it. Related to these two services is a trading service that allows objects to advertise what they have to offer( by means of their interfaces), and to allow clients to find services using a special language that supports the description of constraints.
A separate persistence service offers the facilities for storing information on disk in the form of storage objects. An important issue here is that persistence transparency is provided; a client need not explicitly transfer the data in a storage object between a disk and available main memory.
None of the services so far offer the facilities to explicitly relate two or more objects. These facilities are provided by a relationship service, which essentially provides support for organizing objects according to a conceptual schema like the ones used in databases.
Security is provided in a security service. The implementation of this service is comparable to security systems such as SESAME and Kerberos. The CORBA security service provides facilities for authentication, authorization, auditing, secure communication, nonrepudiation, and administration. We return to security in detail below.
Finally, CORBA offers a time service that returns the current time within specified error ranges.
As explained by Pope( 1998), the CORBA services have been designed with CORBA’ s object model as their basis. This means that all services are specified in CORBA IDL, and that a separation between interface specification and implementation is made. Another important design principle is that services should be minimal and simple. In the following sections we discuss a number of these services in more detail. From those descriptions, it can be argued to what extent this last principle has been successfully applied.
9.1.2 Communication
Originally, CORBA had a simple model of communication: a client invokes a method on an object and waits for an answer. This model was thought to be too