Traditional Process Models

Outline
• Generic process framework
• Waterfall model
• Incremental model
• Prototyping model
• Spiral model
• Summary

3
Generic Process Framework
• Communication
– Involves communication among the customer and
other stake holders; encompasses requirements
gathering
• Planning
– Establishes a plan for software engineering work;
addresses technical tasks, resources, work products,
and work schedule
• Modelling (Analyse, Design)
– Encompasses the creation of models to better under
the requirements and the design

• Construction (Code, Test)
– Combines code generation and testing to uncover
errors
• Deployment
– Involves delivery of software to the customer for
evaluation and feedback

5
Modelling: Software
Requirements Analysis
• Helps software engineers to better understand the
problem they will work to solve
• Encompasses the set of tasks that lead to an
understanding of what the business impact of the
software will be, what the customer wants, and how
end-users will interact with the software
• Uses a combination of text and diagrams to depict
requirements for data, function, and behavior
– Provides a relatively easy way to understand and review
requirements for correctness, completeness and
consistency

6
Modelling: Software Design
• Brings together customer requirements, business needs,
and technical considerations to form the “blueprint” for
a product
• Creates a model that provides detail about software
data structures, software architecture, interfaces, and
components that are necessary to implement the
system
• Architectural design
– Represents the structure of data and program components
that are required to build the software
– Considers the architectural style, the structure and properties
of components that constitute the system, and
interrelationships that occur among all architectural
components

• User Interface Design
– Creates an effective communication medium
between a human and a computer
– Identifies interface objects and actions and then
creates a screen layout that forms the basis for a
user interface prototype
• Component-level Design
– Defines the data structures, algorithms, interface
characteristics, and communication mechanisms
allocated to each software component

9
Process Model
• Defines a distinct set of activities, actions,
tasks, milestones, and work products that
are required to engineer high-quality
software
• The activities may be linear, incremental, or
evolutionary

10
Waterfall Model
(Diagram)
Communication
Project initiation
Requirements
gathering
Planning
Estimating
Scheduling
Tracking Modeling
Analysis
Design Construction
Code
Test Deployment
Delivery
Support
Feedback

11
Waterfall Model
(Description)
• Oldest software lifecycle model
• Used when requirements are well understood and risk
is low
• Work flow is in a linear (i.e., sequential) fashion
• Used often with well-defined adaptations or
enhancements to current software

12
Waterfall Model
(Problems)
• Doesn't support iteration, so changes can cause
confusion
• Difficult for customers to state all requirements
explicitly and up front
• Requires customer patience because a working
version of the program doesn't occur until the final
phase
• Problems can be somewhat alleviated in the model
through the addition of feedback loops (see the next
slide)

13
Waterfall Model with Feedback
(Diagram)
Communication
Project initiation
Requirements
gathering
Planning
Estimating
Scheduling
Tracking Modeling
Analysis
Design Construction
Code
Test Deployment
Delivery
Support
Feedback

14
Incremental Model
(Diagram)
Increment #1
Communication
Increment #2
Increment #3
Planning
Modeling
Construction
Deployment
Communication
Planning
Modeling
Construction
Deployment
Communication
Planning
Modeling
Construction
Deploy………

15
Incremental Model
(Description)
• Used when requirements are well understood
• Multiple independent deliveries are identified
• Work flow is in a linear (i.e., sequential) fashion within
an increment and is staggered between increments
• Iterative in nature; focuses on an operational product
with each increment
• Provides a needed set of functionality sooner while
delivering optional components later
• Useful also when staffing is too short for a full-scale
development

16
Prototyping Model
(Diagram)
Communication
Quick
Planning
Modeling
Quick Design
Construction
Of Prototype
Deployment,
Delivery,
and Feedback
Start

17
Prototyping Model
(Description)
• Follows an evolutionary and iterative approach
• Used when requirements are not well understood
• Serves as a mechanism for identifying software
requirements
• Focuses on those aspects of the software that are
visible to the customer/user
• Feedback is used to refine the prototype

18
Prototyping Model
(Potential Problems)
• The customer sees a "working version" of the
software, wants to stop all development and then buy
the prototype after a "few fixes" are made
• Developers often make implementation compromises
to get the software running quickly (e.g., language
choice, user interface, operating system choice,
inefficient algorithms)
• Lesson learned
– Define the rules up front on the final disposition of the
prototype before it is built
– In most circumstances, plan to discard the prototype and
engineer the actual production software with a goal toward
quality

19
Spiral Model
(Diagram)
Communication
Start
Start
Planning
Modeling
Deployment Construction

20
Spiral Model
(Description)
• Follows an evolutionary approach
• Used when requirements are not well understood and
risks are high
• Inner spirals focus on identifying software requirements
and project risks; may also incorporate prototyping
• Outer spirals take on a classical waterfall approach after
requirements have been defined, but permit iterative
growth of the software

• Operates as a risk-driven model…a go/no-go
decision occurs after each complete spiral in
order to react to risk determinations
• Requires considerable expertise in risk
assessment
• Serves as a realistic model for large-scale
software development

22
General Weaknesses of
Evolutionary Process Models
1) Prototyping poses a problem to project planning
because of the uncertain number of iterations
required to construct the product
2) Evolutionary software processes do not establish
the maximum speed of the evolution
• If too fast, the process will fall into chaos
• If too slow, productivity could be affected

3) Software processes should focus first on flexibility
and extensibility, and second on high quality
• We should prioritize the speed of the development over
zero defects
• Extending the development in order to reach higher
quality could result in late delivery

Traditional Process Models

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