Introduction to Patterns

1. Csci 490 / Engr 596Special Topics / Special ProjectsSoftware Design and Scala ProgrammingSpring Semester 2010Lecture Notes

2. Introduction to Patterns Created: 19 August 2004. Revised: 13 April 2010

3. Concept of Design Pattern • Arose originally in the field of (building) architecture in 1970’s – Christopher Alexander • Imported into software architecture and design in late 1980s and 1990s 1

4. Pattern • Describes a particular recurring design problem that arises in specific design contexts • Presents a well-proven generic scheme for its solution • The solution scheme is specified by describing its constituent components, their responsibilities and relationships, and the ways in which they collaborate 1

5. Characteristics of Patterns • Describe solutions to recurring problems that arise in specific design situations. • Distilled from practical experience • Describe a group of components (e.g., classes or objects), how the components interact, and the responsibilities of each component. • Higher level abstractions than classes or objects. • Provide vocabulary for communication among designers • Choice of name for pattern is very important. 2

6. Characteristics of Patterns (cont.) • Help document architectural vision of a design • If vision clearly understood, then less likely violated when system modified • Provide conceptual skeleton for solution, encourage the construction of software with well-defined properties • Are building blocks for construction of complex designs • Help designers manage complexity of software 3

7. Descriptions of Patterns • Context • Problem • Solution 4

8. Descriptions of PatternsContext • Describes situation in which design problem arises 5

9. Descriptions of PatternsProblem • Describes problem that arises repeatedly in the context • set offorces -- aspects that must be considered when attempting a solution • requirements the solution must satisfy • e.g., efficiency • constraints that must be considered • e.g., use of a certain algorithm or protocol • desirable properties of a solution • e.g., easy to modify 6

10. Descriptions of PatternsSolution • Describes a proven solution to the problem • Specifies a configuration of elements to balance the forces associated with the problem • Describes static structure of the configuration, identifying components and connectors • relationships among the components • Describes dynamic runtime behavior of the configuration, identifying control structure of components and connectors 7

11. Categories of Patterns • Architectural patterns • Design patterns • Idioms 8

12. Categories of PatternsArchitectural patterns “An architectural pattern expresses a fundamental structural organization schema for software systems. It provides a set of predefined subsystems, specifies their responsibilities, and includes rules and guidelines for organizing the relationships between them” • High-level abstraction • Fundamental design decision in development of a software system • Independent of the implementation language • Example: Pipes and Filters pattern 9

13. Categories of PatternsDesign patterns “A design pattern provides a scheme for refining the subsystems or components of a software system, or the relationships between them. It describes a commonly-recurring structure of communicating components that solves a general design problem within a particular context.” • Mid-level abstraction • Affects the structure of a subsystem • Independent of the implementation language • Examples: • Adapter (or Wrapper) pattern • Iterator pattern • Strategy (or Policy) pattern 10

14. Categories of PatternsIdioms “An idiom is a low-level pattern specific to a programming language. An idiom describes how to implement particular aspects of components or the relationships between them using the features of the given language” • Low-level abstraction • Language-specific • Examples: • Language-specific iterator defined to implement interfaceIterator in Java • Counted Pointer for storage management of shared objects in C++ 11

15. References • Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, and Michael Stal. Pattern-Oriented Software Architecture: A System of Patterns, Wiley, 1996. • Mary Shaw and David Garlan. Software Architecture: Perspectives on an Emerging Discipline, Prentice-Hall, 1996. 12

16. Acknowledgement This work was supported by a grant from Acxiom Corporation titled “The Acxiom Laboratory for Software Architecture and Component Engineering (ALSACE).” 13