Software Engineering

1. Formal Method
Lecture # 3
Umber Noureen Abbas
Lecturer
Department of Computer Science
10/27/2022
Computer Science Department,
University of Sahiwal
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2. Software
• Computer software, or simply software, is a
collection of data or computer instructions
that tell the computer how to work.
• It is used not only to provide applications on
our PC, but also to control many systems
around us.
• Often the software is integrated into a
mechanical or electronic system (called
embedded software).
10/27/2022
Computer Science Department,
University of Sahiwal
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3. High Integrity Software
• Software that has a higher expectation of
correctness called as high integrity software.
• This expectation of correctness is closely
linked to the risks inherent in software failure.
• Therefore, the development of high integrity
software demands greater resources than the
development of a ‘regular’ software product.
10/27/2022
Computer Science Department,
University of Sahiwal
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4. Critical Software
• The term critical software applies to software
that cause danger while failure.
• Critical software can further be categorized
depending upon the types of danger imposed
by failure.
– business critical software
– mission critical software
– safety critical software
10/27/2022
Computer Science Department,
University of Sahiwal
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5. Business Critical Software
• Failure could adversely affect the economic
success of an enterprise;
• Examples include
– Software used to control a bank’s ATM
transactions
– software aimed at providing security for sensitive
information.
10/27/2022
Computer Science Department,
University of Sahiwal
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6. Mission Critical Software
• Failure could impair the goal of the given
mission;
• Examples
– Satellite Operation
– Rocket Launch Systems.
10/27/2022
Computer Science Department,
University of Sahiwal
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7. Safety Critical Software
• It could result in harm to people, property or
the environment;
• Examples include
– Medical software
– Air traffic control software.
10/27/2022
Computer Science Department,
University of Sahiwal
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8. The Importance of the Specification
• When a software contains an ‘error’ we mean
it does not behave as expected.
• There could be two reasons for this;
– either the software does not conform to its
specification
– there are errors or omissions in the original
specification
10/27/2022
Computer Science Department,
University of Sahiwal
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9. Do you know?
• The crash of the European Ariane5 rocket,
because of a software failure
– Rocket took 10 years and 7 billion dollars to
develop
• Radiation overdose in medical equipment
– failed to detect the radiation overdose and causes
death of several patients
10/27/2022
Computer Science Department,
University of Sahiwal
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10. Testing
• is the process of locate these software errors.
• Testing involves running a program with a set
of inputs and comparing the actual outputs
from the program against the expected
outputs (as defined in the specification).
10/27/2022
Computer Science Department,
University of Sahiwal
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11. Testing limitation
• Testing cannot take place until some
implementation is available.
• Testing can only help to uncover errors – it
cannot guarantee the absence of them.
• Testing is always carried out with respect to
specification. If the specification is ambiguous,
test/software will fail always.
10/27/2022
Computer Science Department,
University of Sahiwal
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12. Mechanism for Formal methods
Ambiguities in the specification and the limitations of
testing can result in errors in the final application
10/27/2022
Computer Science Department,
University of Sahiwal
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13. Formal Specification
• There are two important aspects to an object:
• The information
– that it holds (referred to as its attributes)
• The things
– it can do (referred to as its methods or
operations).
10/27/2022
Computer Science Department,
University of Sahiwal
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14. Class Diagram
a typical UML class diagram
specifying a BankAccount class.
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Computer Science Department,
University of Sahiwal
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15. Method (withdraw)
• receives a requested amount to withdraw
from the bank account
– if there are sufficient funds in the account, meets
the request.
– Returns a boolean value indicating success or
failure of the attempt to withdraw money from
the account.
10/27/2022
Computer Science Department,
University of Sahiwal
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16. Formal Methods
• Constitute a branch of software engineering
that incorporates the use of mathematics for
software development.
• A formal method provides a formal language
in which to express the initial specification and
all future design steps towards the final
program.
10/27/2022
Computer Science Department,
University of Sahiwal
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17. Formal Approach
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Computer Science Department,
University of Sahiwal
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18. Cont.
• A formal method is more than just a
specification language for recording these
transformations
• It also includes a proof system
– for demonstrating that each transformation
preserves the formal meaning captured in the
previous step
– guaranteeing the correctness of a statement and
relies upon mathematical logic
10/27/2022
Computer Science Department,
University of Sahiwal
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19. Advantages of FM
• Formal specifications can help considerably in
generating suitable test cases.
• the ability to analyse a specification
• increasing confidence that the specification
accurately captures the real system requirements
• initial specification can be checked
mathematically
• Proofs can help uncover design errors as soon as
they are made, rather than having to wait for
testing of the final implementation
10/27/2022
Computer Science Department,
University of Sahiwal
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20. Method of Specification
• Many formal methods approaches have been
established but these two are principal
approaches
– Algebraic
– model-based
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Computer Science Department,
University of Sahiwal
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21. Classifying Leading FMs
Algebraic Model-based
Sequential
systems
• Larch • Vienna Development
Method (VDM)
• Z
• B
Concurrent
Systems
• Calculus of
Communicating
Systems (CCS)
• OBJ
• Prototype Verification
System (PVS)
• Communicating
Sequential Processes
(CSP)
10/27/2022
Computer Science Department,
University of Sahiwal
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22. VDM (Vienna development Method)
• VDM specification corresponds closely to the
notion of a class in an object-oriented
methodology
• a lightweight approach
– At first; informal specification of software using
the UML class notation
– formal specification for a UML class in the form of
a VDM specification
10/27/2022
Computer Science Department,
University of Sahiwal
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23. A Lightweight Approach
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Computer Science Department,
University of Sahiwal
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24. Cont.
• we will examine the data types
– natural numbers
– sets and sequences
• and demonstrate their use through example
specifications.
10/27/2022
Computer Science Department,
University of Sahiwal
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25. Questions
Any Question Please?
You can contact me at: umber@uosahiwal.edu.pk
Your Query will be answered within one working day.
10/27/2022
Computer Science Department,
University of Sahiwal
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26. Thanks
10/27/2022
Computer Science Department,
University of Sahiwal
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