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The Promise Of Patterns

The Promise Of Patterns

The concept of patterns has been around as long as we humans have been around. In fact, just about everything we do is centered on recognition, repeatable processes, and routine.

Patterns as a formal concept, however, are a recent development that relatively few professional disciplines have begun to leverage - the most active of which happens to be the software industry. Ever since the now famous building architect Christopher Alexander made an impression on us, numerous books, articles, and training courses have emerged to catalog patterns and educate us in their use.

An interesting thing to note is that although many software developers have heard of patterns and have seen the advertised benefits, most of them still don't fully understand what they are and how to effectively apply them in their daily work. Despite the exciting promise of patterns, they are still a somewhat elusive concept for many.

What I'll attempt to do in this article is introduce what patterns really are, clear up some common misconceptions, and begin to explore what the software industry needs to do to finally bring the benefits of patterns to the general body of software developers. You may find some of what I have to say a bit surprising.

What a Pattern Is
There are many interpretations of what a pattern is, but one of the better definitions I have seen comes from Alexander himself. The following definition was applied to building architecture but it is equally well suited to software.

Each pattern describes a problem which occurs over and over again in our environment and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice.

Patterns are becoming popular in software mostly because they encapsulate the experience (years of hard-won experience) of those who have gone before us. This experience provides proven solutions to common problems many of us face. Among other things, patterns tell us why a given solution works, what its consequences are, and when it is appropriate for use. The value of patterns cannot be underestimated in our craft (or any other) because they really form a collective body of knowledge that no software developer should be without.

An important quality of patterns is that they rarely work alone. You will often use more than one pattern to address different aspects of larger problems. Indeed, properly combining patterns together helps tame complexity and facilitates the communication of a design. Alexander introduced the concept of a pattern language to describe relationships among patterns.

What a Pattern Is Not
Patterns are not code snippets, components, or even a specific solution in a programming language like Java. Although the application of patterns may yield these types of products, patterns themselves describe classes of solutions to common problems. Developers new to patterns may look at a source code example or a UML diagram and treat it as a representation of the pattern. These examples and visual aids either help you understand the pattern or serve as a specific application of the pattern - not as the actual pattern. A pattern may have a number of slightly varying implementations, so as a developer, you must customize the pattern to fit your own particular situation.

Patterns are not a new methodology. They should not be used to drive your designs and architecture. Instead, they should be used to improve your designs and architectures where appropriate. You should not begin by wondering how you can make use of a certain set of patterns; rather, you should seek out patterns when you run into an awkward design problem, when you are faced with complexity, when your software is not as scalable or extensible as it needs to be, etc. In other words, use patterns and the experience they offer to guide (not control) you as you design and implement software.

Patterns are not invented or created; they are discovered. It may seem like I'm splitting hairs here, but treating patterns this way will give you a better understanding of what patterns are and how they should be approached. The Rule of Three is a rule of thumb introduced by Ralph Johnson, which basically states that a pattern is not a pattern unless at least three independent applications of its solution have been observed. In this way, patterns are mined from existing software (not invented or created).

Benefits of Patterns
I won't bore you with a top ten list here, but there are three main benefits of patterns that I would like to highlight.

  • A Common Vocabulary: Any pattern worth its salt will have a good name. This name captures the essence of the pattern, and with just a couple of words allows you to communicate what would otherwise take several sentences (perhaps even a diagram or two) and much hand waving. These names form a collective vocabulary that helps developers communicate better and faster.
  • A Teaching Aid: As I mentioned earlier, patterns encapsulate the experience of others. Less experienced developers become more productive, and work proceeds more quickly and efficiently with proven solutions to common problems. An important side effect of using a pattern is that you learn how the solution it describes resolves the forces behind the problem it addresses. Patterns don't just give you some code you can plug in; they guide you in applying a general solution to your specific problem.
  • A Higher Level of Reuse: As opposed to reusing a component or sections of source code, patterns serve to encapsulate designs and architectures that exist at a higher level and that remain generally independent of any particular programming language. This type of reuse is extremely valuable but difficult to achieve. Patterns provide a concrete means of capturing, layering, and communicating these reusable concepts.

    The Problem with Patterns
    Patterns today are generally misunderstood and misapplied. They have received great interest from software developers lately thanks mainly to their promised benefits. However, there are several barriers to injecting patterns into the mainstream.

    Where to Begin
    Most developers new to patterns aren't quite sure where to begin. The amount of information about patterns in software is rapidly increasing (just look at all of the books devoted to the topic), and a not-so-trivial amount of this information is suspect.

    From Books to Practice - Difficult
    After reading and even understanding a pattern (probably in a book), you still need to take the general solution it offers and specialize it for your particular problem. Remember, patterns provide general solutions (not specific ones) to common problems. I actually treat this as a benefit, but many inexperienced developers have trouble making the jump.

    Number of Patterns Increasing
    The number of patterns applicable to software is increasing day by day. Some are even redundant, poorly named, or poorly documented. When you add in the fact that patterns are scattered throughout many books, articles, and the Web, finding what you need can be a daunting task.

    No Support from Tools or Technology
    Oddly enough, many product companies are silent on the topic of patterns. Given some of the problems noted above, tools and technology are well suited to assisting software developers in harnessing the experience that patterns offer. They can help you navigate what is available, find what you need, guide you in applying what you have chosen, visualize the consequences, and even help you learn along the way.

    Some Recommendations
    We need central sources of high quality information on patterns. Places like Hillside.net and PatternsCentral are promising efforts, but we need to get more of the development community involved in supporting and improving them.

    We need centralized (perhaps even distributed) repositories that offer peer-reviewed, well-documented patterns and pattern languages. These repositories should be accessible and should provide a range of tools for navigating them (including visualization techniques).

    We need tools and technologies that help us make effective use of patterns. Although the demand for patterns has increased noticeably of late, the support for them in products has lagged considerably. Whether we are creating software from scratch or refactoring existing software, pattern-based tools and technology have the potential to revolutionize the way we create and maintain software.

    In Closing
    As a WebLogic (and J2EE) developer, you are fortunate to have some excellent resources on patterns. After studying pattern concepts as described by Alexander and some of the fundamental patterns and principles as presented by the Gang of Four and Shalloway, I would highly recommend taking a look at Alur and Marinescu for patterns specific to the J2EE platform. Do yourself a favor and don't miss out on what patterns have to offer.

    SideBar

    ObjectVenture (www.objectventure.com) creates J2EE technology that leverages patterns to deliver on the promise of reusable object technology. ObjectAssembler (OA), the company's flagship product, is a visual, two-way, pattern-based development environment that fully supports the design, development, assembly, and deployment of enterprise and Web-based applications. OA's advanced pattern support allows the creation, addition, enhancement, and enforcement of software patterns. It allows developers to design, construct, and modify applications using visual representations of J2EE patterns, components, and assemblies; thus automating many of the tedious and error-prone implementation tasks.

    OA enables organizations to increase development productivity by enforcing design frameworks and models validating component development, flagging errors on a real time basis and automating much of the process for creating and managing component artifacts and assemblies. It provides integrated deployment support for both WebLogic and JBoss, offers complete support for the J2EE component-based architecture, and comes integrated with both Sun's Java Center Core J2EE Pattern Catalog and Apache's Jakarta Struts Framework. OA operates as a stand-alone IDE seamlessly integrated with NetBeans or as a plug-in for either Borland's JBuilder or Sun ONE Studio. With ObjectAssembler, architects, designers, and developers can harness the power of patterns to simplify, accelerate, and enforce their enterprise and Web development efforts. ObjectVenture is the sponsor of the growing patterns community, www.patternscentral.com

    References

  • Alexander, Christopher: www.patternlanguage.com/leveltwo/ca.htm
  • Alexander, Ishikawa, and Silverstein (1977). A Pattern Language. Oxford University Press.
  • Johnson, Ralph: http://st-www.cs.uiuc.edu/users/johnson
  • Hillside.net: http://hillside.net/patterns
  • PatternsCentral: www.patternscentral.com
  • Gamma,Erich; Helm, Richard; Johnson,Ralph; Vlissides, John. (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley.
  • Shalloway, Alan; and Trott, James R. (2001). Design Patterns Explained: A New Perspective on Object-Oriented Design. Addison-Wesley.
  • Alur, Deepak; Crupi, John; Malks, Dan. (2003). Core J2EE Patterns, Best Practices and Design Strategies. Second Edition. Prentice Hall.
  • Marinescu, Floyd (2002). EJB Design Patterns: Advanced Patterns, Processes, and Idioms. John Wiley & Sons.
  • More Stories By Bill Willis

    Bill Willis is the director of engineering for ObjectVenture Inc., where he spends a great deal of time developing new pattern-based tools and technology. He is also the director of PatternsCentral, an online community and portal devoted to helping people make more effective use of patterns.

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