CSC/ECE 517 Fall 2009/wiki2 12 PatternClassification

Originality, Clarity, Coverage, Definitions, Examples, Links, Organization.

Definition

A pattern is a solution to a problem in its context. A context is specific to the domain in which the problem is being solved. For example, In computer science software engineers often are surrounded by problems in similar contexts such as web and database infrastructure. Within these contexts, there are repeatable solutions. Those solutions when described abstractly are are what we call patterns.

A more abstract concept than a pattern is a schema. A schema does not necessarily have any context but it does have structure and attributes. Many times schemas and patterns are mistakenly interchanged. The key for a pattern is that its context be described so that others can identify its usages.

The goal of patterns is to recognize and make use of a library of knowledge that can be used to solve problems. Often times, the same problems are 're-solved' without learning from the past experiences. As long as we similar contexts we will have patterns or knowledge that develop from those solved problems.

Problem

The problem with patterns is classification and recognition or usage. For example, a pattern that is not classified correctly will not be remembered and used. Several attempts to classify patterns has been made. The existing software classification of patterns uses the categories of:

1. Structural
2. Behavioral
3. Creational

While these are good starting points for patterns, it is very hard to make use of this information in different contexts. For example, when building a webapp, it is common knowledge to use the Model-View-Controller pattern to organize access and control to and from the domain data. This pattern is structural. However, if a novice developer was to start building a webapp from scratch, would they turn to the literature on MVC to start their webapp architecture?

I propose that they would not because it would be too difficult to understand 'how' MVC applies to their context, webapps. This is a failure of patterns and the existing pattern classification scheme. We propose that for software patterns to be more recognizeable and useful for the average developer, the specific context should be explicitly described. For example, the context of MVC in webapps is very different than the context in GUI applications. Although the same pattern can be used in both, the problems are solved differently. Thus we actually say MVC is a schema and not a pattern. This may be controversial so we shall provide evidence for why view is worthy of acceptance.

Although patterns always existed, they were first described in a book by Christopher Alexander called "A Pattern Language - Towns, Buildings, Construction"