A software Engineering slides for BCS Students

1. Software Engineering
Department of Computer Science
Northern University Nowshera
Lecture # 02
Engr. Zia Ullah

2. Software
Process Models

3. Outline
• Generic process framework
• Waterfall model
• Incremental model
• Prototyping model
• Spiral model
• Summary
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4. 4
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
Software Engineering: By Engr. Zia Ullah

5. • Construction (Code, Test)
– Combines code generation and testing to uncover
errors
• Deployment
– Involves delivery of software to the customer for
evaluation and feedback
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6. 6
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 behaviour
– Provides a relatively easy way to understand and review
requirements for correctness, completeness and
consistency
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7. 7
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
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8. Continued…
• 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
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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
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10. The Software Process
• A structured set of activities required to develop a
software system.
• A software process model is an abstract representation
of a process. It presents a description of a process from
some particular perspective.
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11. Software Process Descriptions
• When we describe and discuss processes, we usually
talk about the activities in these processes such as
specifying a data model, designing a user interface, etc.
and the ordering of these activities.
• Process descriptions may also include:
– Products, which are the outcomes of a process activity;
– Roles, which reflect the responsibilities of the people involved in
the process;
– Pre- and post-conditions, which are statements that are true
before and after a process activity has been enacted or a
product produced.
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12. Traditional Process Models

13. Waterfall Model
(Diagram)
Communication
Project initiation
Requirements
gathering
Planning
Estimating
Scheduling
Tracking Modeling
Analysis
Design Construction
Code
Test Deployment
Delivery
Support
Feedback
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14. 14
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
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15. Waterfall Model Phases
 There are separate identified phases in the waterfall model:
– Requirements analysis and definition
– System and software design
– Implementation and unit testing
– Integration and system testing
– Operation and maintenance
• The main drawback of the waterfall model is the difficulty of
accommodating change after the process is underway. In
principle, a phase has to be complete before moving onto the
next phase.
• But many issues are true too, such as risk addressing.
• War Stories but true.
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17. 17
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)
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20. 20
Waterfall Model with Feedback
(Diagram)
Communication
Project initiation
Requirements
gathering
Planning
Estimating
Scheduling
Tracking Modeling
Analysis
Design Construction
Code
Test Deployment
Delivery
Support
Feedback
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21. 21
Incremental Model
(Diagram)
Increment #1
Increment #2
Increment #3
Communication
Planning
Modeling
Construction
Deployment
Communication
Planning
Modeling
Construction
Deployment
Communication
Planning
Modeling
Construction
Deploy……
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22. 22
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 Software Engineering: By Engr. Zia Ullah 22

28. 28
Prototyping Model
(Diagram)
Communication
Quick
Planning
Modeling
Quick Design
Construction
Of Prototype
Deployment,
Delivery,
and Feedback
Start
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29. 29
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
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30. 30
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
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34. Chapter 2 Software Processes
Boehm’s Spiral Model - heavyweight model
 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.
 This was the motivation behind developing the Spiral Model - Risk
11
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35. 35
Spiral Model
(Diagram)
Start
Start
Communication
Planning
Modeling
Construction
Deployment
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36. 36
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
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41. • 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
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42. 42
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
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43. Continued…
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
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