Lecture 3 software process model

Introduction to Software Engineering
Software Process Model
Muhammad Nasir
m.nasir@iiu.edu.pk

Outline
 About software process model
 Build and Fix Model
 Why Models are needed?
 Process as a "black box“ & Problem
 Process as a “white box“ & Advantage
 Prescriptive Model
 Waterfall Model or Linear Sequential
 Incremental Process Models

Software Process Model
 Process models prescribe a distinct set of activities, actions,
tasks, milestones, and work products required to engineer
high quality software.
 Process models are not perfect, but provide roadmap for
software engineering work.
 Software models provide stability, control, and organization to
a project that if not managed can easily get out of control
 Software process models are adapted to meet the needs of
software engineers and managers for a specific project.

Build and Fix Model
The earlier approach
 Product is constructed without specification or any attempt
at design.
 Developers simply build a product that is reworked as many
times as necessary to satisfy the client.
 Model may work for small projects but is totally
unsatisfactory for products of any reasonable size.
 Maintenance is high.
 Source of difficulties and deficiencies
 impossible to Predict
 impossible to Manage

Why Models are needed?
 Symptoms of inadequacy: The
Software Crisis
 Scheduled Time and Cost
Exceeded
 User Expectations Not Met
 Poor Quality

Process as a "black box"
Product
Process
Informal
Requirements
Quality?
Uncertain /
Incomplete requirement
In the beginning

Problems
 The assumption is that requirements can
be fully understood prior to development
 Interaction with the customer occurs only at
the beginning (requirements) and end (after
delivery)
 Unfortunately the assumption almost never
holds true

Process as a "white box"
Product
Process
Informal
Requirements
feedback

Advantages
 Reduce risks by improving visibility
 Allow project changes as the project progresses
 based on feedback from the customer

Prescriptive Model
 Prescriptive process models advocate an orderly
approach to software engineering
 Organize framework activities in a certain order
 Process framework activity with set of software
engineering actions.
 Each action in terms of a task set that identifies the
work to be accomplished to meet the goals.
 The resultant process model should be adapted to
accommodate the nature of the specific project,
people doing the work, and the work environment.

Prescriptive Model
 Software engineer choose process
framework that includes activities like;
 Communication
 Planning
 Modeling
 Construction
 Deployment

Prescriptive Model
 Calling this model as “Prescribe”
because it recommend a set of
process elements, activities, action
task, work product & quality.
 Each elements is inter-related to one
another (called workflow).

Waterfall Model or Classic Life Cycle

Limitations of the waterfall model
 The model implies that you should attempt to
complete a given stage before moving on to
the next stage
 Does not account for the fact that
requirements constantly change.
 It also means that customers can not use
anything until the entire system is complete.
15

Limitations of the Waterfall Model
 The model implies that once the product is
finished, everything else is maintenance.
 Some teams sit ideal for other teams to
finish
 Therefore, this model is only appropriate
when the requirements are well-understood
and changes will be fairly
limited during the design process.

Waterfall Model - Problems
 Problems:
1. Real projects rarely follow the
sequential model.
2. Difficult for the customer to state all
the requirement explicitly.
3. Assumes patience from customer -
working version of program will not
available until product is complete.

V-Model
 V-model depicts the relationship of quality
assurance actions to the Frame Work Activities.
 In reality, there is no fundamental difference
between the classic life cycle and the V-model.
 The V-model provides a way of visualizing how
verification and validation actions are applied to
earlier engineering work.

Incremental Process Model
Delivers software in small but usable pieces, each piece builds
on pieces already delivered

The Incremental Model
 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.
 First Increment is often core product
 For Example: Word Processor System
 1st increment: Basic File Management, Editing, Production
 2nd increment: Spell-Checker & Grammar
 3rd increment: Advance Page Layout Properties
 4th increment: Document Printing

 It is particularly useful when enough
staffing is not available for the whole
project
 Increment can be planned to manage
technical risks.
 Incremental model focus more on
delivery of operational product with
each increment.

 User requirements are prioritised 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.
 Customer value can be delivered with each
increment so system functionality is available
earlier.

 Early increments act as a prototype
to help elicit requirements for later
increments.
 Lower risk of overall project failure.
 For example, a major system might
require the availability of new
hardware that is under development
and whose delivery date is uncertain.

The End
 Thanks for Listening
 Questions would be appriciated

Lecture 3 software process model

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