About This Book

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Over the past decade , concurrent object-oriented network programming has emerged as an effective paradigm for developing software applications whose collaborating objects can either be

1. Collocated within one process or computer or

2. Distributed across a set of computers connected by a network, such as an embedded system interconnect, a local area network (LAN), an enterprise intranet, or the Internet.

When objects are distributed, the various entities that constitute these objects must communicate and coordinate with each other effectively. Moreover, they must continue to do so as applications change over their lifetimes. The placement of objects, the available networking infrastructure, and platform concurrency options allow for a level of freedom that's powerful, yet challenging.

When designed properly, concurrent object-oriented network programming capabilities can add a great deal of flexibility to your application options. For instance, in accordance with the requirements and resources available to your projects, you can use

• Real-time, embedded, or handheld systems

• Personal or laptop computers

• An assortment of various- sized UNIX or Linux systems

• "Big iron" mainframes and even supercomputers

You'll likely encounter complex challenges, however, when developing and porting networked applications on multiple operating system (OS) platforms. These complexities appear in the form of incompatible networking protocols or component libraries that have different APIs and semantics on different hardware and software platforms, as well as accidental complexities introduced by limitations with the native OS interprocess communication (IPC) and concurrency mechanisms themselves . To alleviate these problems, the ADAPTIVE Communication Environment (ACE) provides an object-oriented toolkit that runs portably on dozens of hardware and OS platforms, including most versions of Win32 and UNIX, as well as many real-time and embedded operating systems.

Some would have you believe that de facto or de jure OS standards, such as POSIX, UNIX98, or Win32, are all programmers need to shield their applications from portability challenges. Unfortunately, the adage that "the nice thing about standards is that there are so many to choose from" [Tan96] is even more applicable today than it was a decade ago. There are now dozens of different OS platforms used in commercial, academic, and governmental projects, and the number of permutations grows with each new version and variant.

We've developed many multiplatform , concurrent, and networked systems for the past two decades. We can therefore assure you that OS vendors often choose to implement different standards at different times. Moreover, standards change and evolve . It's likely that you'll work on multiple platforms that implement different standards in different ways at different times. Programming directly to OS APIs therefore yields the following two problems:

1. It's error-prone since native OS APIs written in C often lack type-safe, portable, reentrant, and extensible system function interfaces and function libraries. For example, endpoints of communication in the widely used Sockets API (discussed in Chapter 2) are identified via weakly typed integer or pointer I/O handles, which increase the likelihood of subtle programming errors at run-time.

2. It encourages inadequate design techniques since many networked applications written using OS APIs are based upon algorithmic design, rather than object-oriented design. Algorithmic design decomposes the structure of an application according to specific functional requirements, which are volatile and likely to evolve over time. This design paradigm therefore yields nonextensible software architectures that can't be customized rapidly to meet changing application requirements [Boo94].

In this age of economic upheaval , deregulation , and stiff global competition, it's becoming prohibitively expensive and time consuming to develop applications entirely from scratch using native OS APIs and algorithmic design techniques.

If you've been developing networked software systems for many years , you may have learned to accept some of these problems as a fact of life. There is a better way, however. In this book, we show how C++ and ACE provide object-oriented capabilities that allow you to avoid many traps and pitfalls, while still leveraging standards ”and even certain platform-specific features ”whenever possible. Object-oriented designs exhibit greater stability over time than algorithmic designs, which makes them the preferred basis for developing many types of networked applications.

Not surprisingly, there's a price for all this flexibility: you may need to learn some new concepts, methods , patterns, tools, and development techniques. Depending on your background, this learning curve may be trivial or it may initially seem steep. The bottom line, however, is that the object-oriented paradigm can offer you a mature set of techniques that alleviates many challenges of networked application development. This book presents a series of concrete examples to illustrate the object-oriented techniques used to develop and apply the classes in the ACE toolkit. You can use the same techniques and ACE classes to simplify your own applications.

I l @ ve RuBoard