Intro&Overview.md

Software Pattern

Describes a solution to a common problem arising within a context/domain especially in object oriented programs although they are not limited to OOP.

Patterns help improve software quality & developer productivity by

• Naming recurring design structures

  e.g the Observer pattern defines a one to many dependency between objects so that when one object changes state, all dependents are notified & updated

• Specifying design structure explicitly by identifying key properties of classes & objects e.g,
  • Roles & Relationships
  • Dependencies
  • Interactions
  • Conventions
• Abstracting from concete design elements

  e.g problem domain, form factor, vendor, programing languages etc

• Distilling & codifying knowledge gleaned from the successful design experience of experts

Common Charateristics of Patterns

• Patterns describe both a thing & a process
  • The "thing" (the "what") typically means a particular high-level design outline or description of implementation details
  • The "process" (the "how") typically describes the steps to perform to create the "thing"
• They can be independent of programming languages & implementation techniques
• They document/define "micro-architectures" -
  • In other words small clusters of classes and objects that re-occur (recurring designs)
  • Certain properties maybe modified for particular contexts
• The aren't code or concrete designs, so they must be redefined & applied in particular languages
• They are not methods, but can be used as an adjunct to methods
  • Rational Unified Process
  • Agile
  • Others
• There are also patterns for conducting requirements analysis, organizing agile software development teams & dealing with other complex settings

Common Parts of a Pattern Description

• Name
  • Should be pithy & memorable

    Observer

• Intent
  • Goal behind the pattern & the reason(s) for using it

    Observer: Define a one to many dependency between objects so that when one object changes state, all dependents are notified and updated

• Problem
  • Motivation "forces" & situations in which pattern is applicable

    • An abstraction has at least 2 aspects, one depends on the other
    • A change to one object requires change untold others
    • An object should notify unknown other objects

    

• Solution provided by the pattern
  • Visual & textual descriptions of the static structure, participants, & collaboration dynamics of a pattern
• Example implementation guidance
  • e.g. source code snippets in one or more programming languages
• Consequences

  • Benefits & Liabilities of applying the pattern

    + Modularity: Subjects & Objservers may vary independently

    + Extensibility: Can define & add any number of observers'

    + Customizability: Different observers offer different views of subject

    - Unexpected updates: Observers don't know about each other

    - Update overhead: Might need hints or filtering

• Known Uses
  • Example of real pattern uses

    For observer

    • Smalltalk Model View Controller(MVC)
    • InterViews
    • Java Observer/Observable
    • Pub/Sub middleware (e.g. CORBA Notification Service, Data Distribution Service, Java Message Service)
    • Mailing List

• Related Patterns
  • Summarize relationships & tradeoffs between alternative patterns for similar problems

Pattern Relationships

• Pattern Compliments
  • One pattern provides missing ingredient needed by another

    Factory Method with Disposal Method

  • One pattern contrasts w/ anohter to provide alternative solution to a related design problem

    Iterator pattern vs Batch pattern

• Pattern Compound
  • Patterns that occur frequently enough together that it warrants thinking about them as a single solution to a common design problem

    Composite with Command

• Pattern Sequence
  • Join predecessor patterns with successor patterns to form part of thier context
• Pattern Language
  • Network of related patterns that define a process for the orderly resolution of key development problems in particular domains

    

SCV Analysis

Domain Scope Commonality & Variability (SCV) analysis as a means to achieve systematic sofware reuse