For architectural patterns, the authors look at the Layers pattern, used in operating systems such as Windows NT and virtual machines. They also consider Pipes and Filters, which process streams of data. (This pattern, the authors point out, is a lynchpin of Unix.) Their Blackboard pattern shows how a complex problem, such as image or speech recognition can be broken up into smaller, specialized subsystems that work together to solve a problem. (For recognizing words from a raw waveform input, a Blackboard approach might have separate processes to find phonemes, then words, then sentences.)

This book also looks at today's distributed systems in considering the Broker pattern, which is used on the Internet and in Microsoft's OLE technology. This section also presents several powerful patterns for building effective graphical user interfaces, such as Model-View-Controller.

The authors define several well-known design patterns, such as the Proxy and Command patterns, and also basic, far-reaching patterns, such as Whole-Part and Master-Slave, which are widely used throughout computing. Their survey ends with a discussion on the way objects can communicate (using such patterns as Forwarder-Receiver, Client-Dispatcher-Server, and Publisher-Subscriber), which many developers will recognize as familiar patterns, but are codified here as "official" patterns. The book then discusses some idioms in C++ and a more far-reaching role for patterns in software design and architecture. By fitting patterns into traditional software engineering practices, the authors of Pattern-Oriented Software Architecture successfully argue that the role for patterns will only continue to diversify and enrich tomorrow's software engineering tools and methodologies. --Richard Dragan

Only chapter 2 really addresses patterns for the strategic, architectural level of a software system. It does a very adequate job, using a variety of notations, examples, and analysis steps. This book is from 1996, so time has changed our view of some patterns. "Reflection," for example, has become pervasive in applications based on plugins and software components. It is also a fundamental API in the major langauges (Java and C#) released since this book was published - perhaps reflection should be downgraded to an "idiom". That's just nitpicking, though, since reflection is even more important now than when the book was written.

For contrast, the authors present additional design patterns (including some from Gamma's book) for use at tactical design levels. They also discuss idioms patterns that typically involve just a few lines of code within on function. The contrast between the three different levels of implementation and design gives a useful discussion. The authors also present a weak chapter on "systems" or "langauges" of patterns The discussion is OK as far as it goes. The weakness is in what it omits. After reading this brief chapter, the programmer has very little practical information about choosing patterns from some library for some task. The poor programmer has no information at all about how to link patterns together, and that's a real stumbling block for beginning pattern users.

The final section of the book is really sociology. It's about the pattern community, what that community is for, and how to be a working member. I find the discussion un-helpful, but I expect opinions to differ.

Even today, this is a good second book (after Gamma's 'Design Patterns') on patterns and pattern usage. It lack the depth and precision of Gamma's book, and tends to add words without adding meaning. On the positive side, it's broader than Gamma's, and addresses a wider range of implementation levels.