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Software Life Cycle Models/Processes. CMSC 345 Software Engineering Fall 05 Section 0301. Objectives. To introduce the general phases of the software development life cycle To introduce the software process model concept To describe different generic process models and their pros and cons.

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software life cycle models processes

Software Life Cycle Models/Processes

CMSC 345 Software EngineeringFall 05 Section 0301

  • To introduce the general phases of the software development life cycle
  • To introduce the software process model concept
  • To describe different generic process models and their pros and cons
the software process
The Software Process
  • A structured set of activities required to develop a software system. These activities include:
    • Requirements (Specification)
    • Design
    • Implementation (Coding)
    • Testing (Validation)
    • Maintenance (Evolution)
  • A software process model is an abstract representation of a process.

The process of establishing

  • what services are required of the system
  • the constraints on the system’s operation and development

Requirements are the what part

software design
Software Design

The process of converting the system specification (requirements) into a software structure that realizes that specification

Design is the How part

  • Translating a design into a program and removing errors from that program
  • Programming is a personal activity - there is no generic programming process.
  • Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process.
  • The activities of design and implementation are closely related and generally are interleaved.
  • Verification and validation is intended to show that a system conforms to its specification and meets the requirements of the system customer.
  • Involves checking and review processes and system testing
  • System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.
system maintenance
System Maintenance
  • Software is inherently flexible and can change (as opposed to hardware).
  • In the past, there has been a demarcation between development and evolution (maintenance). This is increasingly irrelevant as fewer and fewer systems are completely new.
  • Software engineering should be thought of as an evolutionary process where software is continually changed over its lifetime in response to customer needs.
software process models
Software Process Models
  • The Waterfall model
    • Separate, non-overlapping phases of specification and development
  • Evolutionary development
    • Specification and development are interleaved
  • Reuse-based development
    • The system is assembled from some (most likely) or all existing components
evolutionary development
Evolutionary Development

Two general types:

  • Exploratory development
    • Objective is to work with the customers to evolve a final system from an initial outline specification. Process starts with the well-understood requirements.
  • Prototyping (throw-away?)
    • Objective is to understand the system requirements. Process starts with the poorly understood requirements.

Should a prototype be evolved into the final system or thrown away?

reuse oriented development
Reuse-oriented Development
  • Based on systematic reuse where systems are integrated from existing components or COTS (commercial-off-the-shelf) systems
  • This approach is becoming more important, but there is still limited experience with it.
process iteration
Process Iteration
  • System requirements ALWAYS evolve in the course of a project. So, process iteration where earlier stages are reworked is always part of the process, especially for large systems.
  • Iteration can be applied to any of the generic process models.
  • Examples of two iterative approaches:
    • Incremental development
    • Spiral development
incremental development
Incremental Development
  • Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality.
  • User requirements are prioritized and the highest priority requirements are included in early increments.
  • Once the development of an increment is started, the requirements are frozen, though requirements for later increments can continue to evolve.
incremental development advantages
Incremental Development Advantages
  • Customers do not have to wait until the entire system is delivered until they can gain value from it.
  • Early increments act as a prototype to help elicit requirements for later increments.
  • Lower risk of overall project failure
  • The highest priority system services tend to receive the most testing.
spiral development
Spiral Development
  • Process is represented as a spiral rather than as a sequence of activities with backtracking
  • Each loop in the spiral represents a phase in the process.
  • No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required
  • Risks are explicitly assessed and resolved throughout the process.
spiral model sectors
Spiral Model Sectors
  • Objective setting
    • Specific objectives for the phase are identified
  • Risk assessment and reduction
    • Risks are assessed and activities put in place to reduce the key risks
  • Development and validation
    • A development model for the system is chosen which can be any of the generic models
  • Planning
    • The project is reviewed and the next phase of the spiral is planned
good agile development is does isn t doesn t

Lots of “just in time”

Adjust to circumstances

(Re)Plan regularly

Lots of person-to-person comm.

Adaptively cut fat in the process


Giant Energy Up Front (GEUF)

Only XP (XP is one alternate)


People sitting in isolation

rigid adherence

Good agile developmentIs / DoesIsn’t / Doesn’t

Agility *can*

Be heavier or lighter, depending on circumstances

Use various requirements techniques

(e.g., use cases, stories, features)

In agile development we value

following the principles over following specific practices !

lighter agile vs heavier agile light is good but has limits

(Never forget this!)

Lighter-agile vs. Heavier-agile : Light is good, but has limits.

# people needed



more people

Light methodology

fewer people

Problem Size

  • Agile methodologies
  • What is XP?
  • Risk: the basic problem and how XP address risks
  • Basic principles
  • Basic activities
  • Practices
agile alliance
Agile alliance
  • February 2001 – group of 17 people
  • Agile word
  • The manifesto
  • Principles and recommendations
  • Software development methodology – the set of conventions the team adopts
  • Methodology can be lightened
    • By getting running software out for examination sooner
    • By increasing the richness of the communication channels
  • Light delivers more quickly and with reduced cost
light methodologies
Light methodologies
  • XP
  • Adaptive Software Development
  • Scrum
  • Crystal
  • Feature Driven Development
  • DSDM (Dynamic System Development Method)
why extreme
Why “extreme”

XP takes commonsense principles and practices to extreme levels.

  • If code reviews are good, we'll review code all the time (pair programming).
  • If testing is good, everybody will test all the time (unit testing), even the customers (functional testing).
why extreme 2
Why “extreme”-2
  • If design is good, we'll make it part of everybody's daily business (refactoring).
  • If simplicity is good, we'll always leave the system with the simplest design that supports its current functionality (the simplest thing that could possibly work).
  • If architecture is important, everybody will work defining and refining the architecture all the time (metaphor).
why extreme 3
Why “extreme”-3
  • If integration testing is important, then we'll integrate and test several times a day (continuous integration).
  • If short iterations are good, we'll make the iterations really, really short—seconds and minutes and hours, not weeks and months and years (the Planning Game).
xp promises
XP promises
  • To reduce project risk,
  • To improve responsiveness to business changes,
  • To improve productivity throughout the life of a system,
  • To add fun to building software in teams

All at the same time.

what is xp
What Is XP?

XP is a

  • lightweight,
  • efficient,
  • low-risk,
  • flexible,
  • predictable,
  • scientific, and
  • fun

way to develop software.

innovation of xp
Innovation of XP
  • Putting all proven practices (for the implementation strategy and for the management strategy) under one umbrella
  • Making sure they are practiced as thoroughly as possible.
  • Making sure the practices support each other to the greatest possible degree.
risk the basic problem
Risk: The Basic Problem
  • Schedule slips
  • Project canceled
  • System goes sour
  • Defect rate
  • Business misunderstood
  • Business changes
  • False feature rich
  • Staff turnover
how does xp address risks
How does XP address risks?
  • Schedule slips
    • short Release cycles - a few months at most,
    • Iterations of customer requested features - one- to four-week.
    • Tasks - one- to three-day
    • implementing the highest priority features first
how does xp address risks 2
How does XP address risks?-2
  • Project canceled
    • XP asks the customer to choose the smallest release that makes the most business sense.
  • System goes sour
    • comprehensive suite of tests, which are run and re-run after every change.
    • XP always keeps the system in prime condition.
how does xp address risks 3
How does XP address risks?-3
  • Defect rate - XP tests from the perspective of
    • programmers - writing tests function-by function
    • customers - writing tests program-feature-by-program-feature.
  • Business misunderstood
    • customer to be an integral part of the team.
how does xp address risks 4
How does XP address risks?-4
  • Business changes
    • XP shortens the release cycle, so there is less change during the development of a single release.
  • False feature rich
    • only the highest priority tasks are addressed.
  • Staff turnover
    • programmers accept responsibility for estimating and completing their own work
    • human contact among the team
xp development cycle
XP development cycle
  • Pairs of programmers program together.
  • Development is driven by tests. You test first, then code.
  • Pairs don't just make test cases run. They also evolve the design of the system. Pairs add value to the analysis, design, implementation, and testing of the system.
  • Integration immediately follows development, including integration testing.
four variables
Four variables
  • Cost
  • Time
  • Quality
  • Scope
four values
Four Values
  • Communication
  • Simplicity
  • Feedback
  • Courage
basic principles
Basic principles
  • Rapid feedback
  • Assume simplicity
  • Incremental change
  • Embracing change
  • Quality work
less central principles
Less central principles
  • Teach learning
  • Small initial investment
  • Play to win
  • Open, honest communication
  • Accepted responsibility
  • Local adaptation
basic activities
Basic Activities
  • Coding
  • Testing
  • Listening
  • Designing
  • The Planning Game— Quickly determine the scope of the next release by combining business priorities and technical estimates. As reality overtakes the plan, update the plan.
  • Small releases— Put a simple system into production quickly, then release new versions on a very short cycle.
practices 2
Practices - 2
  • Metaphor— Guide all development with a simple shared story of how the whole system works.
  • Simple design— The system should be designed as simply as possible at any given moment. Extra complexity is removed as soon as it is discovered.
practices 3
Practices - 3
  • Testing — Programmers continually write unit tests, which must run flawlessly for development to continue. Customers write tests demonstrating that features are finished.
  • Refactoring — Programmers restructure the system without changing its behavior to remove duplication, improve communication, simplify, or add flexibility.
practices 4
Practices - 4
  • Pair programming — All production code is written with two programmers at one machine.
  • Collective ownership — Anyone can change any code anywhere in the system at any time.
  • Continuous integration — Integrate and build the system many times a day, every time a task is completed.
practices 5
Practices - 5
  • 40 hour week — Work no more than 40 hours a week as a rule. Never work overtime a second week in a row.
  • On-site customer — Include a real, live user on the team, available full-time to answer questions.
  • Coding standards — Programmers write all code in accordance with rules emphasizing communication through the code.
  • Software Engineering, by Ian Sommerville, 6th edition, Chapter 3
  • Extreme Programming Principles - Sylvia Ilieva
  • Use Cases for Agile and Traditional Development - Alistair Cockburn