Patterns act as building blocks for constructing more complex designs, employing predefined artifacts explains Dr R Srinivasan

IT is very common and natural that in any branch f engineering, when the experienced people work on a particular problem, it is very unusual for them to tackle it by inventing a new solution, which is completely distinct from existing ones. Their practice will be to examine whether a similar problem was solved earlier and then reuse the important aspects of its solution to solve the new problem. This phenomenon and practice is what we described as "Patterns". The Gang of Four, Eric Gamma, et. al., were the first to talk about patterns in the design of software development and they illustrate the same point that, "one thing expert designers know "not" to do is solving every problem from first principles. Rather, they reuse solutions that have worked for them in the past. When they find a good solution, they employ it again and again".

In a nutshell, patterns help the designers to build on the collective experience of skilled software engineers in such a way that it deals with a specific, recurring problem in design or implementation of a software system. Instead of the knowledge remaining only in the heads of few experts, Patterns make it more generally available to the designers in future. By its characteristic, Patterns provide a common vocabulary and understanding in design. In an earlier article, I had stressed upon the importance of deciding the proper architecture before going to the design aspects in software development. Here also Patterns help as a means of documenting Software Architecture and illustrate the vision of the software architect. By adopting this method, it is very easy to extent and modify either the original architecture or the system's code. Another property is that Patterns support the construction of software with defined properties, viz., to provide a skeleton of functional behavior and then implement the functionality of the specific application. Moreover, Patterns will help in maintaining consistency between co-operating components and provide transparent peer-to-peer inter-processor communication. Patterns also address non-functional requirements like reliability, testability, reusability and changeability. By providing a predefined set of components, roles and relationships between them, Patterns specify particular aspects of concrete software architectures. GOF say that Patterns act as building blocks for constructing more complex designs and this method of employing predefined design artifacts helps in faster and reliable design. Inspite of all these attributes to Patterns-based-design, it should be borne in mind that Patterns provide only a method for generic solution rather than a prefabricated module - Patterns help in solving problems but do not provide complete solution.

Every Pattern is represented by a three part schema. They are context, the problem and the solution. Context provides the design situation leading to the design problem, which in turn describes a set of forces repeatedly arising under the context. The solution comes out with the configuration to balance the forces. Patterns vary in different ranges of scales and abstraction. Among the categories of Patterns, some of them help in structuring a software system and the constituent subsystems, some of them help in refining the subsystems defining the components therein and stating the relationships among them, and some help in implementing a particular design. Patterns can also be grouped as Architectural Patterns, Design Patterns and Idioms.

In his paper on "AntiPatterns, refactoring Software, Architectures, and Projects in Crisis", Hays McCormick says that the emergence of Design Patterns has provided the most effective form of software guidance. The whole patterns movement has gone a long way in codifying a concise terminology for conveying sophisticated computer science thinking. To describe in a generic way, Design Pattern is a general solution to a frequently occurring design problem, often increasing the flexibility of the architecture of The system. Design Patterns help in choosing design alternatives that make a system reusable and avoid alternatives that compromise reusability. Design Patterns provide a common sense perspective that allows software developers and project managers to make and justify correct decisions. The main reasons for motivation in adopting Design Patterns in Software Engineering are: availability of beneficial solutions reached in earlier contexts which may be general or application-dependent, characteristic of Design Pattern being the explicit higher-level structure which is not directly supported by a specific programming language and also the ability to use in documentation so that it will be useful for developers in future. Most large Object Oriented Systems use Design Patterns. Design Patterns show how to use primitive techniques such as objects, inheritance and polymorphism. They show how to parameterize a system with an algorithm, a behavior, a state, or the kind of objects they create (GOF). To conclude, Design Patterns illustrate a methodology to describe more of the "why" of a design and not just record the result of the decisions.

(To be continued)

(The author is Chief Technology Officer, Internet Component Management Group, Bangalore and can be contacted at: r.srinivasan@iCMGworld.com)