Lecture 3: Software Process Models

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Lecture 3 Software Process Models

• Dr Valentina Plekhanova
• University of Sunderland, UK

http//www.cet.sunderland.ac.uk/cs0vpl/SE-Com185.
htm
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Week 8 24.04.2003-28.03.2003Project Control
Session

• Tutorial Time 10 minutes for each Team
• Students will present project file, particularly
  Schedule, plus any project documentation.
• Students will describe where they are in the
  project and any problems encountered.
• During the discussion reviewers will ask to see
  evidence of deliverables for any tasks that are
  complete to determine whether they have in fact
  been done.

3
Week 8 24.04.2003-28.03.2003Project Control
Session

• Tutorial Time 10 minutes for each Team
• Students will present project file, particularly
  Schedule, plus any project documentation.
• Students will describe where they are in the
  project and any problems encountered.
• During the discussion reviewers will ask to see
  evidence of deliverables for any tasks that are
  complete to determine whether they have in fact
  been done.

4
Week 8 24.04.2003-28.03.2003Project Control
Session

• Tutorial Time 10 minutes for each Team
• Students will present project file, particularly
  Schedule, plus any project documentation.
• Students will describe where they are in the
  project and any problems encountered.
• During the discussion reviewers will ask to see
  evidence of deliverables for any tasks that are
  complete to determine whether they have in fact
  been done.

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Basic Definitions Process Model

• A representation of a process.
• Some of the key components of a process model are
  the activities that must be performed, the agents
  that perform the activities, the products that
  are produced, and the resources that are needed
  for an activity Pankaj K. Garg, Mehdi Jazayeri,
  1996. .

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Traditional Definition Boehm, 1988

• The main function of a software development
  process model is to establish the order in which
  major tasks are performed within a project, and
  to establish the transition criteria for
  proceeding from one task to the next.

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Goal

• A process model presents desired phases or
  activities in a project
• ????

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Process Element

• Any component of a process.
• Process elements range from individual process
  steps to very large parts of process.

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Process Step

• An atomic action of a process that has no
  externally visible substructure.

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Agent

• An actor (human or machine) who performs a
  process elements.

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Role

• A coherent set of process elements to be assigned
  to an agent as a unit of functional
  responsibility.
• A single agent can perform multiple roles and a
  single role may be performed by multiple agents.

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Artifact

• A product created or modified by the enactment of
  a process element.

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Software Process Models Introduction

•  What are the common process models for
• developing software?

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Software Process Models

• A process model is a development strategy that
  encompasses the process, methods and tools layers
  and the generic phases (see Lecture 1).
• The process model for a project is selected to
  support the nature of the project, the
  application domain, tools available, and the
  controls and deliverables required.

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Software Process Models

• The process model for a project is selected to
  support the nature of the project, the
  application domain, tools available, and the
  controls and deliverables required.
• No agreement on a best process model.

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Software Process Models

• The life cycle history of each software product
  is different.
• Some products will spend years in the conceptual
  stage, other products will be quickly designed
  and implemented and some products must be
  developed from scratch.
• Various models have been designed to undertake
  the process.

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Software Process Models

• It is recognised that Process Model
• Describes development strategy encompassing
  processes, methods, tools.
• Brings order into chaotic activities.
• Assists in controlling and co-ordination of
  software projects.

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Software Process Models Benefits

• There are several benefits that have been
  suggested for utilising process modelling
  Kellner and Hansen, 1989 White, 1992. The main
  ones are (but not limited to)
• facilitating reasoning and communication about
  the process
• analysing, studying, controlling and managing the
  process
• determining ways in which the process may be
  improved.

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Some Models or Some Paradigms of Software
Development

• Waterfall
• Document-driven
• Time-Milestone Driven Model
• Evolutionary Prototyping
• Risk-Based Spiral Model
• "Buy Commercial-Off-The-Shelf" Model
• Evolutionary Development
• Risk Reduction/Waterfall
• Incremental Development

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The Linear Sequential Model

• Also called the waterfall model
• or classic life-cycle.
• The waterfall model presents a documentation-drive
  n approach Royce, 1970 Boehm, 1981. This
  model was developed to provide support for the
  developers to produce the necessary documentation.

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Sequential set of phases

• Software Requirements Analysis
• System and Software Design
• Writing the Programs (or coding, program
  implementation, code generation)
• Testing (unit testing, integration testing,
  system testing, acceptance testing)
• Maintenance

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The Waterfall Model
Requirements Capture
Requirements Documents
System Analysis
System Specification
System Design Models
System Design
Working Alpha System
System Implementation
Final Release System
System Testing
System Maintenance
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The Waterfall Model Advantages

• The waterfall model allows correction of the
  failed tasks. This is represented by the
  feedback loops for task performances.
• Simple to implement and manage.
• Well used.

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The Waterfall Model Disadvantages

• Real projects are not sequential.
• Can not state requirements completely early.
• Customer does not see system until after testing.
• Development often results in parts of project
  being blocked.
• Emphasis is on ease of project management.
• Method does not scale up to large projects well.
• Can not ensure that the delivered product
  satisfies the customers requirements and the
  project proceeds in an essentially fixed sequence
  of phases.

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The Prototyping Model

• The prototyping model is often used when some
  aspects of the system are not well defined.
• Prototyping is focused on a target product
  Lantz, 1985 Connell and Shafer, 1989 Gane,
  1989.
• This model helps to ensure (in comparison with
  the waterfall model) the development of delivered
  products that satisfy the customers real needs.

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The Prototyping Model

• The general idea is to construct a series of
  prototypes to allow users to examine these and
  then to indicate what changes they want in the
  next one.
• It should be pointed out that the prototypes are
  usually built using a waterfall model process.

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The Prototyping Model Basic Steps - Requirements
Gathering

• Obtain requirements from customer.
• Identify areas of uncertainty.
• Quick Design and Build
• Quick design of aspects of system visible to
  customer.
• Quick development of a prototype implementation.

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The Prototyping Model Basic Steps - Customer
Evaluation of Prototype

• Prototype used to refine the requirements of the
  system.
• These basic steps are repeated until requirements
  are well defined.
• Should discard prototype and build the real
  system.

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The Prototyping Model Advantages

• Customer can provide input to system early during
  the development of the prototype, and will
  usually get a system which does what they
  require, and which they want.
• Reduces problems in requirements which are the
  most expensive to fix.

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The Prototyping ModelDisadvantages

• Customers see a working version early, and unless
  they have a basic knowledge of the process, can
  not understand why it should be thrown away.
• The customer sees the prototype built quickly -
  may not understand why the real version takes
  so long.
• Developer may make implementation decisions to
  get the prototype working quickly, and these may
  be carried over to the real system.

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The Spiral Model

• The Spiral model is a risk-driven approach
  Boehm, 1988.
• The primarily goal of this model is to determine
  the risks involved in the process of product
  development and then to attempt to minimise those
  risks.
• Spiral model is divided into a number of
  framework activities (or task regions).

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The Spiral Model

• The specific character of the spiral model is
  that it provides risk analysis before initiating
  each life-cycle phase.
• This model represents a significant advantage by
  the incorporation of risk analysis, multiple
  paradigms, managerial and planning issues.

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The Spiral Model

• The spiral model views the development process in
  polar coordinates. (!!!)
• The plane is divided into four quadrants that
  represent different kinds of activities, as
  follows
• Determination of objectives, alternatives, and
  constraints.
• Evaluation of alternatives identification and
  resolution of risks.
• Development activities.
• Review and planning for future cycles.

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The Spiral Model (figure shows a single cycle of
the spiral)
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The Spiral Model

• Specific activities may overlap multiple spirals.
  
• Also, concurrent spirals may be required to
  address varying areas of risk.
• The commitment line may involve a decision to
  terminate the project or change direction based
  on the review results.
• Some cycles of the spiral may require months to
  complete, while others require only days.

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The Spiral Model

• Each cycle of the model addresses all the
  activities between review and commitment events.
• Early in the process, cycles may be short as
  alternatives in the decision space of the project
  are explored.
• As risks are resolved, cycles may stretch, with
  the development quadrant subsuming several steps
  in the waterfall.

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The Spiral Model

• The spiral may be terminated with product
  deliveryin which case modification or
  maintenance activities are new spiralsor
  continue until the product is retired.

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The Spiral Model Advantages

• Matches reality in that requirements change over
  the life of a project.
• Has advantages of prototyping - customer has
  early input into system.
• Many adaptations possible.
• Requires consideration of risks at all stages of
  the development

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The Spiral Model Disadvantages

• Customers may be concerned about control of the
  process in the spiral model. Can lead to
  difficulties in contract negotiations.
• Requires considerable expertise in risk
  assessment.
• Has not been as widely used as linear sequential
  model (seen as new).

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Summary

• Current software process models differ from each
  other by the goals and the different ordering of
  project tasks.
• Most current software models present only one
  aspect that bears upon process development -
  namely, to present a description of process
  activities for software creation and evolution.

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Summary

• However, software is developed by people within a
  complex environment which can be defined by
  organisational, social, technical, psychological
  and other aspects.
• Hence, one of the major reasons for the lack of
  process modelling success in practical
  applications is that the existing process models
  do not represent the human factors and
  environment settings in which projects exist.

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Week 4 24.02.03- 28.02.03

• Project Control Session
• Tutorial Time 10 minutes for each Team
• Project Team will present project file Schedule,
  any project documentation.
• Students will describe where they are in the
  project and any problems encountered.
• During the discussion reviewers will ask to see
  evidence of deliverables for any tasks that are
  complete to determine whether they have in fact
  been done.