Object Request Broker run-time interfaces

In the previous sections, the SOMDObjectMgr and SOMDServer classes were introduced. These are classes defined by DSOM to provide basic support in managing objects in a distributed application. These classes are built upon Object Request Broker interfaces defined by CORBA for building and dispatching requests on objects. The ORB interfaces, SOMDObjectMgr and SOMDServer, together provide the support for implementing distributed applications in DSOM.

CORBA 1.1 defines the interfaces to the ORB components in IDL. In DSOM, the ORB components are implemented as SOM classes whose interfaces are expressed using the same CORBA 1.1 IDL. Thus, an application can make calls to the DSOM run time using the SOM language bindings of its choice.

Interfaces for the following ORB run-time components are defined in CORBA 1.1, and are implemented in DSOM. They are introduced briefly here, and discussed in more detail throughout this chapter. (See the SOMobjects Developer Toolkit: Programmers Reference Manual for the complete interface definitions.)

Object

The Object interface defines operations on an "object reference", which is the information needed to specify an object within the ORB.

In DSOM, the class SOMDObject implements the CORBA 1.1 Object interface. (The"SOMD" prefix was added to distinguish this class from SOMObject.) The subclass SOMDClientProxy extends SOMDObject with support for proxy objects.

ORB
(Object Request Broker) The ORB interface defines utility routines for building requests and saving references to distributed objects. The global variable SOMD_ORBObject is initialized by SOMD_Init and provides the reference to the ORB object.
ImplementationDef
An ImplementationDef object is used to describe an object's implementation. Typically, the ImplementationDef describes the program that implements an object's server, how the program is activated, and so on.

(CORBA 1.1 introduces ImplementationDef as the name of the interface, but leaves the remainder of the IDL specification to the particular ORB. DSOM defines an interface for ImplementationDef.)

ImplementationDef objects are stored in the Implementation Repository (defined in DSOM by the ImplRepository class).

InterfaceDef
An InterfaceDef object is used to describe an IDL interface in a manner that can be queried and manipulated at run time when building requests dynamically, for example.

InterfaceDef objects are stored in the Interface Repository (described fully in Chapter 7, "The Interface Repository Framework").

Request
A Request object represents a specific request on an object, constructed at run-time. The Request object contains the target object reference, operation (method) name, a list of input and output arguments. A Request can be invoked synchronously (wait for the response), asynchronously (initiate the call, and later, get the response), or as a "oneway" call (no response expected).
NVList
An NVList is a list of NamedValue structures, used primarily in building Request objects. A NamedValue structure consists of a name, typed value, and some flags indicating how to interpret the value, how to allocate/free the value's memory, and so on.
Context
A Context object contains a list of "properties" that represent information about an application process's environment. Each Context property consists of a <name,string_value> pair,and is used by application programs or methods much like the "environment variables" commonly found in operating systems like AIX and OS/2 and Windows. IDL method interfaces can explicitly list which properties are queried by a method, and the ORB will pass those property values to a remote target object when making a request.
Principal
A Principal object identifies the principal ("user") on whose behalf a request is being performed.

(CORBA 1.1 introduces the name of the interface, Principal, but leaves the remainder of the IDL specification to the particular ORB. DSOM defines an interface for Principal.)

BOA
(Basic Object Adapter) An Object Adapter provides the primary interface between an implementation and the ORB "core". An ORB may have a number of Object Adapters, with interfaces that are appropriate for specific kinds of objects.

The Basic Object Adapter is intended to be a general-purpose Object Adapter available on all CORBA-compliant Object Request Brokers. The BOA interface provides support for generation of object references, identification of the principal making a call, activation and deactivation of objects and implementations, and method invocation on objects.

In DSOM, BOA is defined as an abstract class. The SOMOA (SOM Object Adapter) class, derived from BOA, is DSOM's primary Object Adapter implementation. The SOMOA interface extends the BOA interface with several of its own methods that are not defined by CORBA 1.1.

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