Csci 490 / Engr 596
Special Topics / Special Projects
Software Design and Scala Programming
Spring Semester 2010
Lecture Notes
Introduction to Patterns
Created: 19 August 2004. Revised: 13 April 2010
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
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
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
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 welldefined properties
Are building blocks for construction of complex
designs
Help designers manage complexity of software
3
Descriptions of Patterns
Context
Problem
Solution
4
Descriptions of Patterns
Context
Describes situation in which design problem
arises
5
Descriptions of Patterns
Problem
Describes problem that arises repeatedly in the
context
set of forces -- 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
Descriptions of Patterns
Solution
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
Categories of Patterns
Architectural patterns
Design patterns
Idioms
8
Categories of Patterns
Architectural 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
Categories of Patterns
Design 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 commonlyrecurring 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
Categories of Patterns
Idioms
“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 interface Iterator in
Java
 Counted Pointer for storage management of shared objects in C++
11
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
Acknowledgement
This work was supported by a grant from
Acxiom Corporation titled “The Acxiom
Laboratory for Software Architecture and
Component Engineering (ALSACE).”
13