Introduction to Software Design

1
Introduction to Software Design
2
Outline

• Problems in software architectural design
• Function, form, and fabrication (the Vitruvian
  triad)
• The scope of design
• The psychology and philosophy of design
• General methodology of design

3
Problems in Software Architectural Design

• An ad hoc approach to design does not scale with
  respect to the size and complexity of the
  application.
• As the number of functions and quality attributes
  grows, so does the need for more control of the
  application design.

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Obstacles to Achieving High-Quality Architectural
Design

• A lack of awareness of the importance of
  architectural design to software development.
• A lack of understanding of the role of the
  software architect.
• A widespread view than designing is an art form,
  not a technical activity.
• A lack of understanding of the design process.

5
Obstacles to Achieving High-Quality Architectural
Design (Contd)

• A lack of design experience in a development
  organization.
• Insufficient software architecture design methods
  and tools.
• A lack of understanding of how to evaluate
  designs.
• Poor communication among stakeholders.

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When Are the Architecture Design Activities Done?

• They can be practiced during any phase of the
  development process.
• However, to be of greatest benefit they should be
  done as early as possible.
• These activities can be viewed as a linear set of
  steps that can be repeated whenever missing
  information or hidden assumptions are identified.

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Solving These Problems Involves

• Evangelizing the importance of software
  architecture.
• Improving software architecture education.
• Using architectural methods and tools.

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Function, Form, and Fabrication The Vitruvian
Triad

• In hisTen Books on Architecture Vitruvius wrote
  that the result of architecting should be an
  artifact exhibiting the principles of firmitas,
  utilitas, and venustas.
• These three principles are know as the Vitruvian
  Triad.

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The Vitruvian Triad

• Firmitas will be observed if, whatever the
  building materials may be, they have been chosen
  with care but not with excessive frugality.
  (fabrication/durability)
• Utilitas will be observed if the design allows
  faultless, unimpeded use through the disposition
  of the spaces and the allocation of each type of
  space is properly oriented, appropriate, and
  comfortable. (function/usefulness)
• Venustas will be upheld when the appearance of
  the work is pleasing and elegant, and the
  proportions of its elements have properly
  developed principles of symmetry. (form/beauty)

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The Vitruvian Triad (Contd)

• Architectural design is what brings function and
  form together.
• Function in the architectural sense means need,
  purpose, utility, or intended use of the system.
• An application system is constructed based on the
  specification of its form, which satisfies its
  function (i.e., need, purpose, etc.).
• The form includes the specification of distinct
  quality attributes that, through a realization of
  components in a system, satisfies the function.

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Function and Product Planning

• An applications architecture must address
  end-user needs.
• The architecture must be (partially) perceivable
  to the users of the application.
• Architecting begins with a specification of an
  application or system in terms of the functions,
  capabilities, and other qualities that it must
  possess.

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Form and Interaction Design

• According to Alan Cooper design can be divided
  into interaction design and program design.
• Interaction design is that which directly affects
  the ultimate end user of the application.
• All other design is called program design.
• Poor interface design contributes to cognitive
  friction or the resistance encountered by a
  human intellect when it engages with a complex
  system of rules that change as the problem
  permutes.
• The interaction design is the users conceptual
  model or virtuality.

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Interaction Design Human/Computer Interaction
(HCI)

• Four threads of research
• Software engineering methodologies of prototyping
  and iterative development.
• Software psychology and human factors of
  computing systems.
• Computer graphics and software user interfaces.
• Models, theories, and frameworks of cognitive
  science.

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The Application Domain

• The application domain is that part of the world
  in which an applications effects will be felt,
  evaluated, and approved by users.
• The virtuality of a software application must be
  congruent with the application domain.
• Use cases are an example of an application domain
  modeling technique.

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Fabrication

• Vitruviuss firmitas is the principle of quality
  (soundness, durability).
• The quality of a system is based on both the
  design of the system and the selection of
  technologies used to build it.
• The system must be realizable it must be
  possible to build the system with available
  resources of time, staff, budget, existing
  components, etc.

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The Scope of Design

• An design activity can be seen from many points
  of view
• Psychological design is a creative process that
  requires knowledge in software engineering,
  computer science, logic, cognitive science,
  linguistics, programming languages, and software
  design methodologies as well as application
  domain-specific knowledge.
• Systematic design is an architecting or
  engineering activity that involves finding
  optimized solutions while balancing obstacles.
• Organizational design is a set of steps or
  activities of the system life cycle, beginning
  with needs analysis and ending with the produce
  end of life.

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Tasks and Activities of Design

• Origin of the task
• Organization
• Novelty
• Production
• Technology
• Horizontal domain
• Quality attributes

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Origin of the Task

• Possible origins for a design task
• Product planning (especially for commercial
  software)
• In-house software (custom systems)
• Systems integration
• Production and field testing

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Organization

• The design process is usually organized around
  the structure of the development organization.
• Two common types of organization are
• Product-oriented product development and
  production responsibilities are divided among
  different divisions based on product type.
• Problem-oriented work is organized according to
  a division of labor along domain boundaries such
  as database administration, user interface
  design, etc.
• Each organizational type affects the choice of
  design activities and how they are performed.

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Novelty

• Some design task require much more invention and
  creative problem solving than others. The
  novelty of the problem can be categorized as
  follows
• Original design
• Adaptive design
• Variant design

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Original Design

• Starts with a clean slate.
• May be created through the synthesis of known
  solution principles and existing technology.
• May require inventing something new.

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Adaptive Design

• Starts with known or established solution
  principles.
• Adapts the principles to fit the current problem.
• May involve some original design as new
  components are added to meet new requirements.

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Variant Design

• Starts with an existing design.
• Modifies the design with respect to some
  nonfunctional quality attributes.
• For example, changes to improve performance of
  certain operations, or be able to handle greater
  user loads.

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Production

• Software may be created for either
• A One-off custom system
• Commercial software for sale
• For one-off systems functionality,
  maintainability, performance, reliability and/or
  availability are typically the most important
  qualities.
• For Commercial systems functionality,
  performance, extensibility, adaptability, and
  usability are commonly the paramount qualities.

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Technology

• Different technologies require different design
  methods. Some technology areas that affect
  design methods are
• Information representation
• Data storage
• Data transformation
• Business logic
• User interface design
• Vendor platforms

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Horizontal Domain

• Systems can be characterized by their relative
  complexity.
• Systems can be divided horizontally into layers
  or horizontal domains where each layer has its
  own complexities.
• The demarcation of a horizontal domain is based
  on the types of concerns addressed by design and
  the patterns, methods, and tools specific to a
  domain.

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Example of Horizontal Domains

• Integrated enterprises (B2B)
• Enterprise integration systems of applications
  (integrated applications)
• Enterprise applications (point or vertical
  solutions)
• Software libraries, databases, application
  servers, operating systems (platforms)

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Quality Attributes

• One of the largest factors that affect the design
  task is the set of required quality attributes of
  the system.
• The architect must find a balance among the
  competing quality attributes.
• Many passes through the design process may be
  needed as design tradeoffs are considered and
  proposed designs are evaluated.

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Common Quality Attributes

• Functionality
• Buildability
• Cost and time to market
• Performance
• Usability
• Security
• Availability and reliability
• Modifiability

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Architecture versus Engineering Design

• The difference between software architecture
  design and software engineering design can be
  characterized based on scope and complexity.
• Engineering design concentrates on implementing
  specific quality attributes using technologies.
• Architectural design concentrates on formulating
  the quality attributes and system characteristics
  and selecting the working principles that balance
  many competing quality attributes.

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The Psychology and Philosophy of Design

• Design is a creative problem-solving activity
  that involves intuitive and discursive thought.
• Intuitive thinking, mostly subconscious, is
  characterized by insight and inspiration usually
  triggered by some association of ideas.
• Discursive thinking is conscious and deliberate,
  where facts and relationships are analyzed and
  combined in various ways, evaluated, and then
  disposed of.

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Problems, Obstacles, and Solutions
Problems, Obstacles, and Solutions
Problems, Obstacles, and Solutions

• Design is the activity of finding or creating
  solutions to problems given a set of obstacles to
  overcome.
• Systematic approaches to design can add a degree
  of predictability with respect to the time
  required for design and its resulting quality.
• Systematic approaches dont hinder creativity,
  but rather serve as a catalyst for creative
  thought.

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Problems, Obstacles, and Solutions
Problems, Obstacles, and Solutions
Problems, Obstacles, and Solutions (Contd)

• There is no one best design method.
• There are both good and poor choices for which
  method to employ in a given set of circumstances.
• Object-oriented design is a popular choice but is
  not always the best, especially when you are not
  at the level of objects and classes.

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Aristotelian Reasoning

• In software design reason can be used as a
  meta-tool for both understanding and discoursing.
• The three rational operations used in the
  formation of knowledge definition, predication,
  and inference, can be used in architectural
  design.
• Definition and predication can be used for
  understanding and inference can be used for
  discoursing.

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Aristotelian Reasoning (Contd)

• Definition and predication are the basis of
  abstract thinking.
• Inference is used in the assessment of
  architectural designs.
• After using the operations of definition and
  predication to understand the problem it still
  may not be represented in a way that facilitates
  the discovery of existing solutions.

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Aristotelian Reasoning (Contd)

• We use divide and conquer to refine our
  understanding and look for solutions to our new
  set of problems.
• In order to master the art of software
  architecture, the architect needs to practice
  these techniques until they are part of his or
  her subconscious.

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General Methodology of Design

• Elements of Design
• Purposeful thinking
• Analysis
• Abstraction
• Synthesis
• General heuristics

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Purposeful Thinking

• Design requires systematic thinking.
• This does not preclude creativity or intuition.
• But a purely intuitive approach has
  disadvantages
• The right solution rarely comes at the right
  time.
• The results depend on the skill of the architect.
• The solution might be negatively influenced by
  preconceived ideas.

39
Purposeful Thinking (Contd)

• The architect should analyze the design for
  errors or weak points in the early stages of
  development.
• A discursive design approach helps reduce errors
  by enforcing systematic testing of design ideas
  and assumptions early.

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Purposeful Thinking (Contd)

• Creativity is inhibited or encouraged by
  different influences.
• Some techniques for encouraging creativity are
• Interrupt the activity to create incubation
  periods (but be careful too many interruptions
  can be disruptive).
• Apply different solution-finding methods.
• Move from abstract to concrete ideas.

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Purposeful Thinking (Contd)

• Some techniques for encouraging creativity
  (Contd)
• Find and collect information form design
  catalogues (such as design patterns).
• Divide work among architecture team members.
• Make realistic plans Realistic planning is found
  to encourage motivation and creativity, while
  unrealistic planning is inhibiting.

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Analysis

• Analysis is the decomposition of complex systems
  into elements and their relationships.
• Activities include identification, definition,
  and structuring for the purpose of acquiring
  information about a subject that can be
  transformed into knowledge.
• Analytic methods are use at all stages of
  software design.

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Analysis (Contd)

• Structured analysis in software emphasizes a
  functional hierarchy.
• Object-oriented analysis emphasizes an object
  hierarchy.
• Weak-spot analysis of a design can be done by
  evaluating it with respect to metamodels.
• Most software design problems are the result of a
  lack of understanding of the problem being solved.

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Abstraction

• Abstraction reduces the complexity of a problem
  while emphasizing the essential characteristics
  of it.
• Abstractions aid in the discovery of solutions.
• Abstractions are sometimes invented but often
  discovered.
• A given problem may have many abstractions.

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Synthesis

• Synthesis is the combining of individual elements
  or parts to produce a new effect.
• With respect to design it is the integration of
  subproblem solutions and the evaluation of the
  resulting system.

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General Heuristics

• Heuristics are techniques that are characterized
  as the searching for suitable solutions. Some
  heuristics are
• Persistent questions
• Negation
• Forward steps
• Backward steps
• Factorization
• Systematic variation
• Division of labor and collaboration

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The Method of Persistent Questions

• Requirements are almost never complete.
• The architect must engage in persistent
  questioning to stimulate ideas and remove
  preconceived notions.
• Architects may keep a database of questions for
  various purposes.

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The Method of Negation

• The method starts with a known solution that is
  divided into (truthful) statements about its
  parts.
• These statements are negated, one by one, to
  stimulate the creation of alternate solutions.

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The Method of Forward Steps

• This method starts with a first solution attempt
  and tries to follow as many solution paths as
  possible, by making decisions that lead to
  implementation.
• This is a good brainstorming technique.

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The Method of Backward Steps

• In this method the architect starts with a goal
  in mind rather than the initial problem.
• All possible paths that could have led up to the
  goal are retraced.
• This method is useful for organizing and
  engineering department around the products
  architecture and for preparing an engineering
  process.

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The Method of Factorization

• The method involves breaking a complex system
  into less complex elements or factors.
• This technique is used to find the essential
  problems being solved.
• This may involve taking what appears to be an
  indivisible component and finding a way to divide
  it into two components and a interface.

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The Method of Systematic Variation

• The method starts with a generalized
  classification structure that represents the
  various problem characteristics and possible
  solutions.
• By systematically varying a single
  characteristic, the architect may discover more
  optimized solutions.

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Summary

• Design is a creative problem-solving activity
  that involves finding or creating solutions to
  problems given a set of obstacles.
• The three Aristotelian operations of reason can
  be used in formulating problems and rationalizing
  about their solutions.
• The operations of definition and predication are
  used to gain understanding of the problem.

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Division of Labor and Collaboration

• Implementing large and complex tasks requires a
  division of labor.
• Software development tasks are commonly based not
  only on functional areas but also on the
  technology used.
• This division causes an information exchange
  problem.
• Systematic methods and the creation of models can
  help overcome this problem.

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Summary (Contd)

• Inference is used to find new solutions and
  evaluate existing solutions.
• There are three principles of architecture
  utilitas (function), firmitas (fabrication,
  quality), and venustas (form).
• Function is expressed as problems and, through
  analysis, is divided into subproblems.
• Analysis involves the rational operators of
  definition and predication.

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Summary (Contd)

• Product planning, the first phase of design,
  transforms the subproblems into a functional
  specification.
• During conceptual design the functional
  specification is used to elaborate the functional
  design and the structural design.
• The functional design corresponds to the
  externally facing or interaction design of the
  system (aimed at reducing cognitive friction).

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Summary (Contd)

• The structural design is concerned with the
  internal physical design of the software itself.
• Heuristic design methods are applied to produce
  several candidate structural architectural
  designs.
• The architecture design candidates are evaluated,
  disposed of, or further refined during embodiment
  design.

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Summary (Contd)

• The same rational operators and design methods
  are applied, but now the focus is on elaboration.
• Detail design involves implementation and testing.