IT-SC book: Advanced CORBA® Programming with C++

Chapter 2. An Overview of CORBA

2.1Introduction

2.2The Object Management Group

2.3Concepts and Terminology

2.4CORBA Features

2.5Request Invocation

2.6General CORBA Application Development

2.7Summary

2.1Introduction

Computer networks typically are heterogeneous. For example, the internal network of a small software company might be made up of multiple computing platforms. There might be a mainframe that handles transactional database access for order entry, UNIX workstations that supply hardware simulation environments and a software development backbone, personal computers that run Windows and provide desktop office automation tools, and other specialized systems such as network computers, telephony systems, routers, and measurement equipment. Small sections of a given network may be homogeneous, but the larger a network is, the more varied and diverse its composition is likely to be.

There are several reasons for this heterogeneity. One obvious reason is that technology changes over time. Because networks tend to evolve rather than being built all at once, the best technologies from different time periods end up coexisting on the network. In this context, "best" may refer to qualities such as the lowest cost, the highest performance, the least expensive mass storage, the most transactions per minute, the tightest security, the flashiest graphics, or other qualities deemed important at the time of purchase. Another reason for network heterogeneity is that one size does not fit all. Any given combination of computer, operating system, and networking platform will work best for only a subset of the computing activities performed within a network. Still another reason is that diversity within a network can make it more resilient because any problems in a given machine type, operating system, or application are unlikely to affect other networked systems running different operating systems and applications.

The factors that lead to heterogeneous computer networks are largely inevitable; thus, developers of practical distributed systems, whether they like it or not, must cope with heterogeneity. Whereas developing software for any distributed system is difficult, developing software for a heterogeneous distributed system sometimes borders on the impossible. Such software must deal with all the problems normally encountered in distributed systems programming, such as the failure of some of the systems in the network, partitioning of the network, problems associated with resource contention and sharing, and security-related risks. If you add heterogeneity to the picture, some of these problems become more acute, and new ones crop up.

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IT-SC book: Advanced CORBA® Programming with C++

For example, problems you encounter while porting a networked application for use on a new platform in the network may result in two or more versions of the same application. If you make any changes to any version of the application, you must go back and modify all the other versions appropriately and then test them individually and in their various combinations to make sure they all work properly. The degree of difficulty presented by this situation increases dramatically as the number of different platforms in the network rises.

Keep in mind that heterogeneity in this context does not refer only to computing hardware and operating systems. Writing a robust distributed application from top to bottom—for example, from a custom graphical user interface all the way down to the network transports and protocols—is tremendously difficult for almost any real-world application because of the overwhelming complexity and the number of details involved. As a result, developers of distributed applications tend to make heavy use of tools and libraries. This means that distributed applications are themselves heterogeneous, often glued together from a number of layered applications and libraries. Unfortunately, in many cases, as the distributed system grows, the chance decreases dramatically that all the applications and libraries that compose it were actually designed to work together.

At a very general level, you can tackle the problem of developing applications for heterogeneous distributed systems by following two key rules.

Find platform-independent models and abstractions that you can apply to help solve a wide variety of problems.

Hide as much low-level complexity as possible without sacrificing too much performance. These rules are general enough to be used to develop any portable application whether or not it is distributed. However, the additional complexities introduced by distribution make each rule carry more weight. Using the right abstractions and models can essentially provide a new homogeneous application development layer over the top of all the distributed heterogeneous complexity. This layer hides low-level details and allows application developers to solve their immediate problems without having to first solve the low-level networking details for all the diverse computing platforms used by their applications.

The CORBA specification, written and maintained by the OMG, supplies a balanced set of flexible abstractions and concrete services needed to realize practical solutions for the problems associated with distributed heterogeneous computing. After describing the OMG and CORBA, the remainder of this chapter provides a high-level overview of the computing model, the components, and the important concepts of CORBA.

2.2 The Object Management Group

In 1989, the Object Management Group was formed to address the problems of developing portable distributed applications for heterogeneous systems. The OMG has received a tremendous amount of industry backing since then and is now the world's

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IT-SC book: Advanced CORBA® Programming with C++

largest software consortium, with more than 800 members. This is due in no small part to the skills that OMG participants have for specifying reasonable high-level abstractions that hide low-level details. In particular, the first key specifications produced by the OMG—the Object Management Architecture (OMA) and its core, the CORBA specification—provide a complete architectural framework that is both rich enough and flexible enough to accommodate a wide variety of distributed systems.

The OMA uses two related models to describe how distributed objects and the interactions between them can be specified in platform-independent ways. The Object Model defines how the interfaces of objects distributed across a heterogeneous environment are described, and the Reference Model characterizes interactions between such objects.

The Object Model defines an object as an encapsulated entity with an immutable distinct identity whose services are accessed only through well-defined interfaces. Clients use an object's services by issuing requests to the object. The implementation details of the object and its location are kept hidden from clients.

The Reference Model provides interface categories that are general groupings for object interfaces. As Figure 2.1 shows, all interface categories are conceptually linked by an Object Request Broker (ORB). Generally, an ORB enables communication between clients and objects, transparently activating those objects that are not running when requests are delivered to them. The ORB also provides an interface that can be used directly by clients as well as objects.

Figure 2.1 OMA interface categories.

Figure 2.1 shows the interface categories that use the ORB's activation and communication facilities.

Object Services are domain-independent, or horizontally oriented, interfaces used by many distributed object applications. For example, all applications must obtain references to the objects they intend to use. Both the OMG Naming Service and the OMG Trading Service [21] are object services that allow applications to look up and discover object

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IT-SC book: Advanced CORBA® Programming with C++

references. Object services are normally considered part of the core distributed computing infrastructure.

Domain Interfaces play roles similar to those in the Object Services category except that domain interfaces are domain-specific, or vertically oriented. For example, there are domain interfaces used in health care applications that are unique to that industry, such as a Person Identification Service [28]. Other interfaces are specific to finance, manufacturing, telecommunications, and other domains. The multiple Domain Interface bubbles in Figure 2.1 indicate this multiplicity of domains.

Application Interfaces are developed specifically for a given application. They are not standardized by the OMG. However, if certain application interfaces begin to appear in many different applications, they become candidates for standardization in one of the other interface categories.

As the OMG gradually populates the interface categories, the bulk of its standardization efforts will shift upward from the ORB infrastructure and Object Services levels into domain-specific object frameworks. The object framework concept, illustrated in Figure 2.2, builds from the interface categories just described, recognizing and promoting the notion that CORBA-based programs are composed of multiobject components supporting one or more of the OMA interface categories. Figure 2.2 represents these components as circles, some with only one interface category and others with multiple categories. Unfortunately, the term framework is overused in general, but used in this context it follows the classic definition of a software framework: a partial solution to a set of similar problems that requires application customization for completeness. The OMG is likely to standardize specifications for object frameworks for use in industries represented by its Domain Task Forces.

Figure 2.2 OMA object frameworks.

These models may not seem very complicated or profound, but their apparent simplicity is misleading. Many pages of this book, as well as other books, articles, and

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