Software Process Models: SDLC Overview

Software Process Models Overview

• Process model or life-cycle model defines the steps a product progresses through
• Different process models include:
  • Build-and-fix model
  • Waterfall model
  • Incremental model
  • Evolutionary process models
    • Rapid prototyping model
    • Spiral model
  • Agile process models
    • Extreme programming
  • Object-oriented life-cycle models
    • Unified Process
• Criteria exist for determining which model to use

Software Process Model Stages

• Model includes stages such as:
  • Requirements phase
  • Specification phase
  • Design phase
  • Implementation phase
  • Integration phase
  • Maintenance phase
  • Retirement

"Build-and-Fix" Model

• Prone to problems such as no specifications or design
• Ultimately deemed unsatisfactory
• Leads to high costs and difficult maintenance

Waterfall Model

• Sequential design process where progress flows steadily downwards through SDLC phases
• An example of a sequential model where software development is split into phases each with different tasks and objectives
• The pioneer of SDLC processes
• Characterized by feedback loops and documentation being key

Waterfall Model: Advantages

• Enforces a disciplined approach
• Documentation exists for each phase
• Products checked by SQA group at each phase
• Results in easier maintenance with every change reflected in documentation

Waterfall Model: Disadvantages

• Fully working software version unavailable until late in the project timeframe
• Specifications are lengthy, highly detailed, and in client-unfamiliar style
• "Blocking states" that cause delays when one team needs another to complete dependent tasks

Rapid Prototyping Model

• Develops functional product or system replication for engineering
• Provides a small-scale replica of the end product as a means to obtain customer feedback
• Prototypes are used in the requirements phase
• Prototypes are evaluated by the customer/user
• Prototypes are discarded rather than turned into the final product

Rapid Prototyping Model: Caveats

• Not a proven model with its own shortcomings
• Potential solution is to use rapid prototyping for requirement definition
• A waterfall model implementation can be used for the remainder of the life cycle

Incremental Model

• Software development process where requirements are broken down into standalone modules
• Each iteration goes through: requirements, design, coding, and testing
• Typical product takes between 5 and 25 iterations
• Models are delivered in portions

Incremental Model: Advantages

• Software is generated quickly during the life cycle
• Changes to requirements and scope are less costly
• Changes can be done throughout development
• Less costly compared to other models
• Customers can respond to each building increment
• Errors are easy to identify

Incremental Model: Disadvantages

• Requires good planning and designing
• System architecture problems may occur since not all requirements are collected up front
• Iteration phases are rigid and do not overlap
• Correcting a problem in one unit requires correction in all units

Incremental Model: Use Cases

• Requirements of the system are distinctly understood
• Demand exists for an early software release
• Software engineering teams are made up of less-skilled individuals
• High-risk features and goals are involved
• Commonly used for web applications and product-based companies

Spiral Model

• Risk-driven software development process model
• Teams adopt elements of incremental, waterfall of evolutionary prototyping depending on risk patterns
• Risk analysis done as risk mitigation strategy is planned and finalized
• Phases are preceded by alternative and risk analyses
• Phases are followed by evaluation and planning of next phase

Spiral Model: Use Cases

• Projects are large
• Frequent releases are required
• Prototype creation is applicable
• Risk/cost evaluation is important
• Medium to high-risk projects
• Requirements are unclear and complex
• Changes may be required at any time
• Long-term commitments are not feasible

Spiral Model: Advantages

• Functionality/changes can be done at a later stage
• Cost estimation is done in small fragments making it easier
• Repeated development helps in risk management
• Streamlined development and systematic feature additions
• Open to customer feedback

Spiral Model: Disadvantages

• Risk of not meeting the schedule or budget exists
• Only works for larger projects and requires risk assessment expertise
• Protocols must be followed strictly for smooth operation
• Robust documentation is required due to intermediate phases
• Not advisable for smaller projects

Agile Process Models

• Philosophy combines with development guidelines
• Philosophy:
  • Promotes customer satisfaction through early, incremental software delivery
  • Small, highly motivated development teams
  • Informal methods
  • Minimal software engineering work products
  • Overall development simplicity
• Development guidelines:
  • Prioritization of delivery over analysis and design
  • Active and continuous communication between developers and customers
• Top Agile Process Models:
  • Scrum
  • Crystal Methodologies
  • DSDM (Dynamic Software Development Method)
  • Feature driven development (FDD)
  • Lean software development
  • Extreme Programming (XP)

Agile vs. Waterfall

• Agile proposes incremental iterative approaches to software design vs. waterfall sequentially flows from start to end
• Agile processes break down into individual models while waterfall does not design individual models
• Agile offers early and frequent opportunities to look at the product, waterfall is at end of project
• Agile considers less structured than waterfall, which is plan oriented
• Agile can be implemented quickly for small projects, while all sorts of projects can be completed in Waterfall
• Agile errors can be fixed in middle of project vs waterfall at the end of project
• Agile development iterative in short iterations (2-4 weeks) vs phased and larger iterations in waterfall
• Agile less priority on documentation vs waterfall which prioritizes documentation

Agile Model: Advantages

• Customer satisfaction through rapid, continuous software delivery
• Emphasis on individuals and interactions
• Frequent software delivery (weeks rather than months)
• Face-to-face communication
• Close, daily cooperation between business people and developers
• Regular adaptation to changing circumstances
• Welcomes late requirement changes

Agile Model: Disadvantages

• Difficult to assess needed effort at the beginning of life cycle
• Emphasizes the necessary designing and documentation
• The project can be taken off track if the customer is unclear
• Only experienced software engineers can make the decisions

Agile Model: Use Cases

• New changes need to be implemented
• Freedom to change is very important
• Developers can roll back and implement new feature in hours
• Stakeholders get greater freedom than rigid approaches
• Avoids project standstill

Extreme Programming (XP)

• New approach involving incremental model variance
• Determines client's desired features
• Estimate duration and cost per each feature
• Client then selects stories (features) for each successive build
• Tasks are divided into builds
• Test cases drawn up for each task
• Pair programming done with partner on one screen
• Continuous integration

Extreme Programing (XP): Features

• The computers will all be put in a room lined with cubicles
• The client is working with the team
• Individuals don't work more than two successive week over time
• No division of labor. Specification, design, code and testing can all be done be the team members.
• There is no design before builds – Refactoring

Unified Process

• Framework for OO (object-oriented) software engineering using UML (Unified Modeling Language)
• An attempt to use conventional software process models with agile principles
• Characteristics include:
• Iterative and incremental development framework
• Architecture-centric
• Risk-focused
• Use-case and UML model driven

Unified Process: Phases

• Inception: customer communication and planning activities, rough architecture, plan, and preliminary use-cases
• Elaboration: customer communication and modeling, refines/expands use-cases, expands architectural representation, and carefully reviewed/modified plan
• Contstruction: reflect the software increment, integrate activities
• Transition: end-users get software for beta testing

SDLC Method Selection

• Understand that models are suited for different projects/environments, and requirements
• Waterfall best for simple, straightforward projects with fixed requirements
• Iterative/spiral better for large-scale, multicomponent projects with several segments
• No one SDLC model is best on its own
• SDLC methodology should be selected by assessing stakeholder needs and fitting them to the tool
• Factors for deciding include:
  • Product Complexity
  • Product Size
  • Magnitude of Changes
  • Frequency of Changes
  • Dev Team skills
  • Time Constraints
  • User Access