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The Common Object Request Broker Architecture (CORBA) is structured to allow integration of a wide variety of object systems. The motivation for some of the features may not be apparent at first, but as we discuss the range of implementations, policies, optimizations, and usages we expect to encompass, the value of the flexibility becomes more clear.

FIG. 2 A Request Being Sent Through the Object Request Broker

FIG. 3 The Structure of Object Request Broker Interfaces

FIG. 3 on page 2-2 shows the structure of an individual Object Request Broker (ORB). The interfaces to the ORB are shown by striped boxes, and the arrows indicate whether the ORB is called or performs an up-call across the interface.

To make a request, the Client can use the Dynamic Invocation interface (the same interface independent of the target object's interface) or an OMG IDL stub (the specific stub depending on the interface of the target object). The Client can also directly interact with the ORB for some functions.

The Object Implementation receives a request as an up-call either through the OMG IDL generated skeleton or through a dynamic skeleton. The Object Implementation may call the Object Adapter and the ORB while processing a request or at other times.

Definitions of the interfaces to objects can be defined in two ways. Interfaces can be defined statically in an interface definition language, called the OMG Interface Definition Language (OMG IDL). This language defines the types of objects according to the operations that may be performed on them and the parameters to those operations. Alternatively, or in addition, interfaces can be added to an Interface Repository service; this service represents the components of an interface as objects, permitting runtime access to these components. In any ORB implementation, the Interface Definition Language (which may be extended beyond its definition in this document) and the Interface Repository have equivalent expressive power.

FIG. 4 A Client using the Stub or Dynamic Invocation Interface

The client performs a request by having access to an Object Reference for an object and knowing the type of the object and the desired operation to be performed. The client initiates the request by calling stub routines that are specific to the object or by constructing the request dynamically (see FIG. 4 on page 2-3).

The dynamic and stub interface for invoking a request satisfy the same request semantics, and the receiver of the message cannot tell how the request was invoked.

FIG. 5 An Object Implementation Receiving a Request

The ORB locates the appropriate implementation code, transmits parameters and transfers control to the Object Implementation through an IDL skeleton or a dynamic skeleton (see FIG. 5 on page 2-4). Skeletons are specific to the interface and the object adapter. In performing the request, the object implementation may obtain some services from the ORB through the Object Adapter. When the request is complete, control and output values are returned to the client.

The Object Implementation may choose which Object Adapter to use. This decision is based on what kind of services the Object Implementation requires.

FIG. 6 Interface and Implementation Repositories

FIG. 6 on page 2-5 shows how interface and implementation information is made available to clients and object implementations. The interface is defined in OMG IDL and/or in the Interface Repository; the definition is used to generate the client Stubs and the object implementation Skeletons.

The object implementation information is provided at installation time and is stored in the Implementation Repository for use during request delivery.

2.1.1 Object Request Broker

1. Operations that are the same for all ORB implementations

2. Operations that are specific to particular types of objects

3. Operations that are specific to particular styles of object implementations

Different ORBs may make quite different implementation choices, and, together with the IDL compilers, repositories, and various Object Adapters, provide a set of services to clients and implementations of objects that have different properties and qualities.

There may be multiple ORB implementations (also described as multiple ORBs) which have different representations for object references and different means of performing invocations. It may be possible for a client to simultaneously have access to two object references managed by different ORB implementations. When two ORBs are intended to work together, those ORBs must be able to distinguish their object references. It is not the responsibility of the client to do so.

The ORB Core is that part of the ORB that provides the basic representation of objects and communication of requests. CORBA is designed to support different object mechanisms, and it does so by structuring the ORB with components above the ORB Core, which provide interfaces that can mask the differences between ORB Cores.

2.1.2 Clients

Clients generally see objects and ORB interfaces through the perspective of a language mapping, bringing the ORB right up to the programmer's level. Clients are maximally portable and should be able to work without source changes on any ORB that supports the desired language mapping with any object instance that implements the desired interface. Clients have no knowledge of the implementation of the object, which object adapter is used by the implementation, or which ORB is used to access it.

2.1.3 Object Implementations

A variety of object implementations can be supported, including separate servers, libraries, a program per method, an encapsulated application, an object-oriented database, etc. Through the use of additional object adapters, it is possible to support virtually any style of object implementation.

Generally, object implementations do not depend on the ORB or how the client invokes the object. Object implementations may select interfaces to ORB-dependent services by the choice of Object Adapter.

2.1.4 Object References

The representation of an object reference handed to a client is only valid for the lifetime of that client.

All ORBs must provide the same language mapping to an object reference (usually referred to as an Object) for a particular programming language. This permits a program written in a particular language to access object references independent of the particular ORB. The language mapping may also provide additional ways to access object references in a typed way for the convenience of the programmer.

There is a distinguished object reference, guaranteed to be different from all object references, that denotes no object.

2.1.5 OMG Interface Definition Language

IDL is the means by which a particular object implementation tells its potential clients what operations are available and how they should be invoked. From the IDL definitions, it is possible to map CORBA objects into particular programming languages or object systems.

2.1.6 Mapping of OMG IDL to Programming Languages

A language mapping also defines the interaction between object invocations and the threads of control in the client or implementation. The most common mappings provide synchronous calls, in that the routine returns when the object operation completes. Additional mappings may be provided to allow a call to be initiated and control returned to the program. In such cases, additional language-specific routines must be provided to synchronize the program's threads of control with the object invocation.

2.1.7 Client Stubs

Object-oriented programming languages, such as C++ and Smalltalk, do not require stub interfaces.

2.1.8 Dynamic Invocation Interface

2.1.9 Implementation Skeleton

The existence of a skeleton does not imply the existence of a corresponding client stub (clients can also make requests via the dynamic invocation interface).

It is possible to write an object adapter that does not use skeletons to invoke implementation methods. For example, it may be possible to create implementations dynamically for languages such as Smalltalk.

2.1.10 Dynamic Skeleton Interface

The implementation code must provide descriptions of all the operation parameters to the ORB, and the ORB provides the values of any input parameters for use in performing the operation. The implementation code provides the values of any output parameters, or an exception, to the ORB after performing the operation. The nature of the dynamic skeleton interface may vary substantially from one programming language mapping or object adapter to another, but will typically be an up-call interface.

Dynamic skeletons may be invoked both through client stubs and through the dynamic invocation interface; either style of client request construction interface provides identical results.

2.1.11 Object Adapters

The wide range of object granularities, lifetimes, policies, implementation styles, and other properties make it difficult for the ORB Core to provide a single interface that is convenient and efficient for all objects. Thus, through Object Adapters, it is possible for the ORB to target particular groups of object implementations that have similar requirements with interfaces tailored to them.

2.1.12 ORB Interface

2.1.13 Interface Repository

In addition to its role in the functioning of the ORB, the Interface Repository is a common place to store additional information associated with interfaces to ORB objects. For example, debugging information, libraries of stubs or skeletons, routines that can format or browse particular kinds of objects, etc., might be associated with the Interface Repository.

2.1.14 Implementation Repository

In addition to its role in the functioning of the ORB, the Implementation Repository is a common place to store additional information associated with implementations of ORB objects. For example, debugging information, administrative control, resource allocation, security, etc., might be associated with the Implementation Repository.

2.2 Example ORBs

2.2.1 Client- and Implementation-resident ORB

2.2.2 Server-based ORB

2.2.3 System-based ORB

2.2.4 Library-based ORB

2.3 Structure of a Client

The ORB manages the control transfer and data transfer to the object implementation and back to the client. In the event that the ORB cannot complete the invocation, an exception response is provided. Ordinarily, a client calls a routine in its program that performs the invocation and returns when the operation is complete.

Clients access object-type-specific stubs as library routines in their program (see FIG. 7 on page 2-12). The client program thus sees routines callable in the normal way in its programming language. All implementations will provide a language-specific data type to use to refer to objects, often an opaque pointer. The client then passes that object reference to the stub routines to initiate an invocation. The stubs have access to the object reference representation and interact with the ORB to perform the invocation. (See Chapter 14 for additional, general information on language mapping of object references.)

FIG. 7 The Structure of a Typical Client

An alternative set of library code is available to perform invocations on objects, for example when the object was not defined at compile time. In that case, the client program provides additional information to name the type of the object and the method being invoked, and performs a sequence of calls to specify the parameters and initiate the invocation.

Clients most commonly obtain object references by receiving them as output parameters from invocations on other objects for which they have references. When a client is also an implementation, it receives object references as input parameters on invocations to objects it implements. An object reference can also be converted to a string that can be stored in files or preserved or communicated by different means and subsequently turned back into an object reference by the ORB that produced the string.

2.4 Structure of an Object Implementation

The object implementation (see FIG. 8 on page 2-13) interacts with the ORB in a variety of ways to establish its identity, to create new objects, and to obtain ORB-dependent services. It primarily does this via access to an Object Adapter, which provides an interface to ORB services that is convenient for a particular style of object implementation.

FIG. 8 The Structure of a Typical Object Implementation

Because of the range of possible object implementations, it is difficult to be definitive about how in general an object implementation is structured. See the Basic Object Adapter chapter for the structure of object implementations that use the Basic Object Adapter.

When an invocation occurs, the ORB Core, object adapter, and skeleton arrange that a call is made to the appropriate method of the implementation. A parameter to that method specifies the object being invoked, which the method can use to locate the data for the object. Additional parameters are supplied according to the skeleton definition. When the method is complete, it returns, causing output parameters or exception results to be transmitted back to the client.

When a new object is created, the ORB may be notified so that the it knows where to find the implementation for that object. Usually, the implementation also registers itself as implementing objects of a particular interface, and specifies how to start up the implementation if it is not already running.

Most object implementations provide their behavior using facilities in addition to the ORB and object adapter. For example, although the Basic Object Adapter provides some persistent data associated with an object, that relatively small amount of data is typically used as an identifier for the actual object data stored in a storage service of the object implementation's choosing. With this structure, it is not only possible for different object implementations to use the same storage service, it is also possible for objects to choose the service that is most appropriate for them.

2.5 Structure of an Object Adapter

Object adapters are responsible for the following functions:

• Generation and interpretation of object references
• Method invocation
• Security of interactions
• Object and implementation activation and deactivation
• Mapping object references to the corresponding object implementations
• registration of implementations These functions are performed using the ORB Core and any additional components necessary. Often, an object adapter will maintain its own state to accomplish its tasks. It may be possible for a particular object adapter to delegate one or more of its responsibilities to the Core upon which it is constructed.

  As shown in FIG. 9 on page 2-15, the Object Adapter is implicitly involved in invocation of the methods, although the direct interface is through the skeletons. For example, the Object Adapter may be involved in activating the implementation or authenticating the request.

  FIG. 9 The Structure of a Typical Object Adapter

  The Object Adapter defines most of the services from the ORB that the Object Implementation can depend on. Different ORBs will provide different levels of service and different operating environments may provide some properties implicitly and require others to be added by the Object Adapter. For example, it is common for Object Implementations to want to store certain values in the object reference for easy identification of the object on an invocation. If the Object Adapter allows the implementation to specify such values when a new object is created, it may be able to store them in the object reference for those ORBs that permit it. If the ORB Core does not provide this feature, the Object Adapter would record the value in its own storage and provide it to the implementation on an invocation. With Object Adapters, it is possible for an Object Implementation to have access to a service whether or not it is implemented in the ORB Core-if the ORB Core provides it, the adapter simply provides an interface to it; if not, the adapter must implement it on top of the ORB Core. Every instance of a particular adapter must provide the same interface and service for all the ORBs it is implemented on.

  It is also not necessary for all Object Adapters to provide the same interface or functionality. Some Object Implementations have special requirements, for example, an object-oriented database system may wish to implicitly register its many thousands of objects without doing individual calls to the Object Adapter. In such a case, it would be impractical and unnecessary for the object adapter to maintain any per-object state. By using an object adapter interface that is tuned towards such object implementations, it is possible to take advantage of particular ORB Core details to provide the most effective access to the ORB.

2.6.1 Basic Object Adapter

2.6.2 Library Object Adapter

2.6.3 Object-Oriented Database Adapter

2.7 The Integration of Foreign Object Systems

FIG. 10 Different Ways to Integrate Foreign Object Systems

For object systems that simply want to map their objects into ORB objects and receive invocations through the ORB, one approach is to have those object systems appear to be implementations of the corresponding ORB objects. The object system would register its objects with the ORB and handle incoming requests, and could act like a client and perform outgoing requests.

In some cases, it will be impractical for another object system to act like a BOA object implementation. An object adapter could be designed for objects that are created in conjunction with the ORB and that are primarily invoked through the ORB. Another object system may wish to create objects without consulting the ORB, and might expect most invocations to occur within itself rather than through the ORB. In such a case, a more appropriate object adapter might allow objects to be implicitly registered when they are passed through the ORB.