Introduction to Software Engineering ppt

Challenge in large projects• Developing large/complex software application is very challenging - Effort intensive - High cost - Long development time - Changing needs for users - High ri

Introduction to Software

Engineering

• Characteristics of a good process

• Waterfall model for development

• Other models

• Project planning

Software systems

• Ubiquitous, used in variety of applications

- Business, engineering, scientific

applications

• Simple to complex, internal to public,

single function to enterprise-wide, one

location to distributed, batch or real time, informational to mission-critical,…

Challenge in large projects

• Developing large/complex software application is very challenging

- Effort intensive

- High cost

- Long development time

- Changing needs for users

- High risk of failure, user acceptance,

performance, maintainability

• Quite different from one-time programs where

author and user are same!

Successful software system

• Software development projects have not always been successful

• When do we consider a software application

Reasons for failure

• Schedule Slippage

• Cost over-runs

• Does not solve user’s problem

• Poor quality of software

• Poor maintainability

Reasons for failure…

• Ad hoc software development results in such problems

- No planning of development work (e.g no

milestones defined)

- Deliverables to user not identified

- Poor understanding of user requirements

- No control or review

- Technical incompetence of developers

- Poor understanding of cost and effort by both developer and user

Engineering: other disciplines

• Large projects common and successfully

- Building bridges, dams

effectively and ensuring quality

- Using scientific principles

• Requires well-defined approach: repeatable,

predictable

• Large projects requires managing the project itself

- Manage people, money (cost), equipment,

schedule

- Scale makes big difference: compare building a hut, 2-storeyed house, or 50-storeyed apartment building

• Quality extremely important: relates to failures,

efficiency, usability,…

- People willing to pay for quality!

Large Projects

• Involve different types of people

- Large building: architect, civil engineer, electrical engineer, workers (masons,

Large projects…

• Many deliverables: architecture plan,

model, structure diagrams, electrical

Software projects

• Software is different from other products

- Cost of production concentrated in

development

- Maintenance consists of making

corrections and enhancing or adding

functions

- Progress in development is difficult to measure

Apply Engineering Approach

• Hence planning and control even more important in software development -> engineering approach

- Attempt to estimate cost/effort

- Plan and schedule work

- Involve users in defining requirements

- Identify stages in development

- Define clear milestones so that progress can be

Job of Software developer is difficult

• Dealing with users

- Ill-defined requirements

- Concern with ease-of-use and response

time

• Dealing with technical people

- Concerned with coding, databases, file

structures, etc

• Dealing with management

- Concerned with return on their investment

- Cost-benefit analysis

- Schedule

For success in large software

development, it is important to follow an

engineering approach, consisting of a defined process

• Consists of different types of process

• Process for software development:

produces software as end-result

- multiple such processes may exist

- a project follows a particular process

Process types

• Process for managing the project

- defines project planning and control

- effort estimations made and schedule prepared

- resources are provided

- feedback taken for quality assurance

- monitoring done

Process types…

• Process for change and configuration

management.

- Resolving requests for changes

- Defining versions, their compositions

- Release control

• Process for managing the above processes

themselves

- Improving the processes based on new

techniques, tools, etc.

- Standardizations and certifications (ISO, CMM)

Multiple processes

• A large software development company may

have multiple development processes

A For client-server based conventional

applications (sales processing, payroll)

B For enterprise-level (ERP) projects based on packages and customization

C For web-based e-commerce type

D For data-warehousing/decision-support type

• The company may have many projects in each category

Step in a process

• Each step has a well-defined objective

• Requires people with specific skills

• Takes specific inputs and produces defined outputs

well-• Step defines when it may begin (entry criteria) and when it ends (exit criteria)

• Uses specific techniques, tools,

guidelines, conventions

- E.g adding more resources

• A steps ends in a review (V&V)

- Verification: check consistency of outputs with inputs (of the step)

- Validation: check consistency with user needs

Process step

Project control info

Info for management

(exit criteria)

Review

V &V Inputs

Actions to

be carried

out (entry criteria)

outputs

Characteristics of a Good Process

• Should be precisely defined – no ambiguity

about what is to be done, when, how, etc.

• It must be predictable – can be repeated in other projects with confidence about its outcome

- Predictable with respect to effort, cost:

-> Project A: Web-based library application done

by 3 persons in 4 months

-> another project B (guest house bookings),

similar in complexity should also take about 12 persons in a month.

A Good Process…

• Predictable for quality, with respect to number and type of defects, performance

• Predictable process is said to be “under

statistical control’, where actual values are close

to expected values

• It supports testing and maintainability

- Maintenance by third party

- Follow standards, provide necessary

documentation

- This characteristic differentiates between

prototype and product

A Good Process…

• Facilitates early detection of and removal of defects

- Defects add to project cost

- Late detection/correction is costly

• It should facilitate monitoring and improvement

- Based on feedback

- Permit use of new tools, technologies

- Permit measurements

Summary of lecture 1

• Challenges in large software development

• Benefits of engineering approach

• Types of software processes

• A typical step

• Characteristics of a good process

Waterfall Model for Development

• Here, steps (phases) are arranged in

linear order

- A step takes inputs from previous step, gives outputs to next step (if any)

- Exit criteria of a step must match with

entry criteria of the succeeding step

• It follows ‘specify, design, build’ sequence that is intuitively obvious and appears

natural

Waterfall Model…

• Produces many intermediate deliverables, usually documents

- Standard formats defined

- Act as “baseline” used as reference (for contractual obligations, for maintenance)

- Important for quality assurance, project management, etc

Waterfall model

Deliverables in Waterfall Model

• Project plan and feasibility report

• Requirements document (SRS: Software

Requirement Specifications)

• System design document

• Detailed design document

• Test plans and test reports

• Source code

• Software manuals (user manual, installation

manual)

• Review reports

Shortcomings of Waterfall model

• Requirements may not be clearly known, especially for applications not having

existing (manual) counterpart

- Railway reservation: manual system

existed, so SRS can be defined

- Knowledge management for central bank -new

• Requirements change with time during

project life cycle itself

- Users may find solutions of little use

- Better to develop in parts in smaller

increments; this is common for packages, system software

• Considered documentation-heavy: so

much documentation may not be required for all types of projects

Prototyping Model

• When customer or developer is not sure

- Of requirements (inputs, outputs)

- Of algorithms, efficiency,

• Quick design focuses on aspects visible to user; features clearly understood need not

be implemented

• Prototype is tuned to satisfy customer needs

- Many interactions may be required to

incorporate changes and new requirements

• Final product follows usual define-design

build-test life cycle

Limitations of Prototyping

• Customer may want prototype itself!

• Developer may continue with

implementation choices made during

Interactive Development

• Useful for product development where

developers define scope, features to serve many customers

• Early version with limited feature important

to establish market and get customer

feedback

• Initial version may follow any method

• A list of features for future versions

maintained

3 Develop the software

4 Plan next step

- Do prototype for user interface risks

- Use basic waterfall model when user interface and performance issues are understood but only development risk remains

• Risk driven: allows us mix of

specification-oriented, prototype-specification-oriented, simulation based or any other approach

Project Management Process

• Runs in parallel to development process

• Project planning, monitoring and control are the basic goals

• Project plan indicates how project will be

executed successfully

- Without plan, there is no management

- Without measurement, not much planning

possible

- Plan allows progress to be measured

- Plan produced before development begins and constantly updated

Project Planning

• Prepare cost/effort estimation

- Based on project complexity, scope, productivity levels, other historical data

- Many models available

• Select development process, define milestones and

prepare schedule

- Know how effort is distributed over phases from

historical data

• Decide project staffing

• Make quality control plans: define reviews, inspections, testing strategies to detect/remove defects

Project Monitoring and Control

• Plan defines various activities: many ways

to represent: Gantt chart, time bar chart, PERT/CPM project activity network

- Show activities, expected durations,

resources allocated

- Critical paths can be identified

Project monitoring

• Each development step gives information for tracking progress for identifying delays, bottlenecks, etc

- Allows corrective action; add more

resources, reduce scope, renegotiate cost/schedule, etc

• What is engineering, and importance of

applying engineering approach in software development

• Types of software processes

• Stepwise process definition

• Waterfall, prototype, iterative, spiral

models

• Project management Process

• Project Planning

1 Lecture Series on Software Engineering

by Prof.N.L Sarda, Prof Umesh

Bellur,Prof.R.K.Joshi and Prof.Shashi

Kelkar, Department of Computer Science

& Engineering ,IIT Bombay

2 P Jalote A Concise Introduction to

Software Engineering Springer, 2008