Chapter 1
Introduction to Software Engineering
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Limitations of Non-engineered Software
Requirements
Software
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Objectives of the Class
Appreciate Software Engineering:
Build complex software systems in the context of frequent change
Understand how to
produce a high quality software system within time
while dealing with complexity and change
Acquire technical knowledge (main emphasis)
Acquire managerial knowledge
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Example: Space Shuttle Software
Cost: $10 Billion, millions of dollars more than planned
Time: 3 years late
Quality: First launch of Columbia was cancelled because of a
synchronization problem with the Shuttle's 5 onboard
computers.
Error was traced back to a change made 2 years earlier when a
programmer changed a delay factor in an interrupt handler from
50 to 80 milliseconds.
The likelihood of the error was small enough, that the error caused
no harm during thousands of hours of testing.
Substantial errors still exist.
Astronauts are supplied with a book of known software problems
"Program Notes and Waivers".
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Software Engineering Perspectives
Software Engineering is a:
Modeling activity: software engineers deals with complexity through
modeling
Problem solving activity: models are used to search for solutions
within constraints (time and budget)
Knowledge a acquisition activity: data collection and formalization
from models and solutions
Rationale activity
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Software Engineering: A Problem Solving Activity
Analysis: Understand the nature of the problem and break the
problem into pieces
Synthesis: Put the pieces together into a large structure
For problem solving we use
Techniques (methods):
Formal procedures for producing results using some well-defined
notation
Methodologies:
Collection of techniques applied across software development
Tools:
Automated systems to accomplish a technique
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Software Engineering: Definition
Software Engineering is a collection of techniques,
methodologies and tools that help with the production of
a high quality software system
with a given budget
before a given deadline
while change occurs.
Software engineering is a problem solving activity to develop
quality software for a complex problem within a limited time
while things are changing.
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Factors affecting the quality of a software system
Complexity:
The system is so complex that no single programmer can understand it
anymore
The introduction of one bug fix causes another bug
Change:
As time goes on, the cost to implement a change will be too high, and
the system will then be unable to support its intended task. This is true
of all systems, independent of their application domain or technological
base.
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Why are software systems so complex?
The problem domain is difficult
The development process is very difficult to manage
Software offers extreme flexibility
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Modeling
Modeling consists of building an abstraction of reality.
Abstractions are simplifications because:
They ignore irrelevant details and
They only represent the relevant details.
Models allow us to visualize and understand systems
What is relevant or irrelevant depends on the purpose of the
model.
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Why model software?
Why model software?
Software is getting increasingly more complex
Windows XP > 40 mio lines of code
A single programmer cannot manage this amount of code in its
entirety.
Code is not easily understandable by developers who did not
write it
We need simpler representations for complex systems
Modeling is a mean for dealing with complexity
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System Models
Object Model: What is the structure of the system? What are
the objects and how are they related?
Functional model: What are the functions of the system? How
is data flowing through the system?
Dynamic model: How does the system react to external events?
How is the event flow in the system ?
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Software Lifecycle Activities ...and their models
Requirements System Object Implemen-
Analysis Testing
Elicitation Design Design tation
Implemented
By
Expressed in Structured By Realized By Verified
Terms Of
By
class...
class...
class... ?
class....?
Application Solution
Use Case Subsystems
Domain Source Test
Model Domain
Objects Code Cases
Objects
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Software Lifecycle Definition
Software lifecycle:
Set of activities and their relationships to each other to support the
development of a software system
Typical Lifecycle questions:
Which activities should I select for the software project?
What are the dependencies between activities?
How should I schedule the activities?
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Reusability
A good software design solves a specific problem but is general
enough to address future problems (for example, changing
requirements)
Experts do not solve every problem from first principles
They reuse solutions that have worked for them in the past
Goal for the software engineer:
Design the software to be reusable across application domains and
designs
How?
Use design patterns and frameworks whenever possible
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Design Patterns and Frameworks
Design Pattern:
A small set of classes that provide a template solution to a recurring
design problem
Reusable design knowledge on a higher level than datastructures
(link lists, binary trees, etc)
Framework:
A set of classes that collaborate to carry out a set of responsibilities
in an application domain.
⧫ Examples: User Interface Builder
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Software Engineering Concepts
Project
*
Activity
is produced by * consumes
* *
WorkProduct Task Resources
System Participant
Model Time
Document Equipment
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Project is composed of number of activities (ex: analysis phase
is an activity)
Each activity is a set of tasks (ex: determine functional
requirements)
A task consume recourses and produce a workproduct
A workproduct (artifact) is a system, model or document (ex:
source code, instruction manuals, specification document..).
Resources are participants, time, or equipment
Participants: all people involved in the project, each participant
has a role (ex: manager, client, user, developer).
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