Process models

Process ModelsProcess Models

(Unit 2)(Unit 2)

Preeti MishraCourse Incharge

The Details

Process Model

• A software process model is a standardised format for:

• planning• organising, and• Running

developing a project• Definition: A (software/system) process model

is a description of the sequence of activities carried out in an SE project, and the relative order of these activities.

Project Plan

• project plan =process model + project parameters

• Project specific parameters will include:• Size, (person-years)• Budget,• Duration

Hence..

• A process model covers the entire lifetime of a product

• From birth of a commercial idea to final de-installation of last release

• The three main phases:– design,– build,– maintain. (50% of IT activity goes here!)

Process Models

• Several models exist to streamline the development process.

• Each one has its pros and cons• It is up to the development team to adopt

the most appropriate one for the project. • Sometimes a combination of the models may

be more suitable.

Code and Fix

Code and Fix

code firstversion

retirement

operations mode

modify untilclient is satisfied

Advantages

• No planning whatsoever; little management overhead

• Applicable for very small projects and short-lived prototypes

• Signs of progress early• Low expertise- anyone can use it

Disadvantages

• Dangerous!• No visibility/control• No resource Planning• No Deadline• Mistakes hard to detect and correct

• Code becomes expensive to fix (bugs are not found until late in the process)

• Code didn't match user's needs (no requirements!)

• Code was not planned for modification, not flexible

The Waterfall Model

• The waterfall model is sequential design process, used in software development processes, in which progress is seen as flowing steadily downwards (like a waterfall) through the phases of

• Conception• Initiation•  Analysis•  Design• Construction•  Testing• Production/Implementation•  Maintenance

Advantages

• This model is simple and easy to understand and use.

• It is easy to manage• In this model phases are processed and

completed one at a time. Phases do not overlap.• Waterfall model works well for smaller

projects where requirements are very well understood.

Disadvantages

• Once an application is in the testing stage, it is very difficult to go back and change something that was not well-thought out in the concept stage.

• No working software is produced until late during the life cycle.

• High amounts of risk and uncertainty.

Disadvantages..

• Not a good model for complex and object-oriented projects.

• Poor model for long and ongoing projects.• Not suitable for the projects where

requirements are at a moderate to high risk of changing.

When to use Waterfall Model

• This model is used only when the requirements are very well known, clear and fixed.

• Product definition is stable.• Technology is understood.• There are no ambiguous requirements• Ample resources with required expertise are

available freely• The project is short.

Incremental and Iterative Development models

Approach for developing a system

• Incremental Development Model

• Iterative Development Model

Incremental Development-staging and scheduling strategy

• Incrementally add software a time. • Each increment adds more software.• Its like adding bricks to a wall. After

lots of increments, you've got a big wall.

Incremental Development

•  Various parts of the system are developed at different times or rates, and integrated as they are completed

•  Develop the entire system with a “big bang” integration.” 

Iterative Development-rework scheduling strategy

• We build something, then evaluate whether it'll work for us, then we make changes to it.

• We are expecting to change it.•  We never expected it to be right. If it was,

it's a happy accident• Iterative” development helps you improve

your product. Each time around the process you get to change and improve the product itself 

Remember..

• Incremental fundamentally means add onto. – Incremental development helps you improve

your process. •

Iterative fundamentally means re-do. – Iterative development helps you improve

your product.

Spiral ModelSpiral Model

•This model was first described This model was first described by Barry Boehm in his 1986by Barry Boehm in his 1986

Spiral Model

• The spiral is simply a sequence of waterfall increments;

• All project activities follow a single spiral sequence; and

• Every activity in the diagram must be performed

Phases of Spiral Development

• Planning Phase• Risk Analysis Phase• Engineering Phase• Coding and Implementation Phase• Evaluation Phase

Perform 4 basic activities in every cycle

– Consider the win conditions of all success-critical stakeholders.

– Identify and evaluate alternative approaches for satisfying the win conditions.

– Identify and resolve risks that stem from the selected approach(es).

– Obtain approval from all success-critical stakeholders, plus commitment to pursue the next cycle.

Hence

• Thus, the incremental, waterfall, prototyping, and other process models are special cases of the spiral model that fit the risk patterns of certain projects.

Advantages

• High amount of risk analysis hence, avoidance of Risk is enhanced.

• Good for large and mission-critical projects.• Strong approval and documentation control.• Additional Functionality can be added at a later

date.• Software is produced early in the software life

cycle.• .

Disadvantages

• Management is more complex.• Can be a costly model to use.• Risk analysis requires highly specific expertise.• Project’s success is highly dependent on the risk

analysis phase.• Doesn’t work well for smaller projects.

When to use Spiral Model

• When costs and risk evaluation is important• For medium to high-risk projects• Long-term project commitment unwise because of potential

changes to economic priorities• Users are unsure of their needs• Requirements are complex• New product line• Significant changes are expected (research and exploration)

Rapid Application Development (RAD) Model

– Starting with the ideas of Barry Boehm and others, James Martin developed the rapid application development approach during the 1980s at IBM and finally formalized it by publishing a book in 1991

Rapid Application Development (RAD) Model

• RAD approaches to software development put less emphasis on planning tasks and more emphasis on development.

• knowledge gained from the development process itself can feed back to the requirements and design of the solution

Phases

– The James Martin approach to RAD divides the process into four distinct phases:• Requirements Planning phase • User design phase • Construction phase • Cutover phase

Requirements Planning Phase

– combines elements of the system planning and systems analysis phases

– Users, managers, and IT staff members discuss and agree on:• business needs, • project scope, • constraints, • system requirements.

– It ends when the team agrees on the key issues and obtains management authorization to continue.

User Design Phase

– users interact with systems analysts and develop models and prototypes that represent all system processes

– User Design is a continuous interactive process that allows users to understand, modify, and eventually approve a working model of the system that meets their needs.

Construction Phase

–  focuses on program and application development– users continue to participate and can still

suggest changes or improvements as actual screens or reports are developed.

– Its tasks are programming and application development, coding, unit-integration and system testing.

Cutover Phase

–  testing, changeover to the new system, and user training.

– the entire process is compressed. As a result, the new system is built, delivered, and placed in operation much sooner.

Advantages

• Risk reduction. A prototype could test some of the most difficult potential parts of the system early on in the life-cycle. 

• users give much more useful feedback when they can experience a prototype of the running system rather than abstractly define what that system should be.

•  users could get useful business functionality much earlier in the process.

• More projects completed on time and within budget

Limitations

• The risk of a new approach- developers not willing to accept this

• Requires time of scarce resources. – Less control- due to flexibility of design

• Poor design- as we are incorporating changes many times

• Very large systems could not be handled

Where to use RAD

• RAD should be used only when a system can be modularized to be delivered in incremental manner.

• It should be used if there's high availability of designers for modeling.

• It should be used only if the budget permits use of automated code generating tools.

• Should be used where the requirements change during the course of the project and working prototypes are to be presented to customer in small iterations of 2-3 months.

Concurrent Development Model

– The activity – modeling- may be in any one of the states noted at any given time.

– All activities exist concurrently but reside in different states.

– The concurrent process model defines a series of events that will trigger transitions from state to state for each of the Software engineering activities, actions, or tasks.

Under review

Baselined

Done

Under

revision

Await ing

changes

Under

development

none

Modeling act ivit y

represents the stateof a sof tware engineeringact ivity or task

• It defines s/w engg as network of activities each in different state, each dependent on one another and each going on concurrently

• Awaiting changes state- after completion of the 1st iteration • None state- while initial communication is carried out • Underdevelopment state- after communication is completed• Awaiting changes state- if any changes in the requirements

must be made

Concurrency is achieved in two ways

• system and component activities occur simultaneously and can be modeled using the state-oriented approach described previously;

• a typical client/server application is implemented with many components, each of which can be designed and realized concurrently.

Disadvantages

– Movement of a process among states inside activity is confusing sometimes.

– Besides it requires more Resource (intellectual).