Page 24 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Principles of Software Engineering

Notes • System and software design: The systems design process partitions the requirements to
either hardware or software systems. It establishes overall system structural design.
Software design involves representing the software system functions in a form- that may
be transformed into one or more executable programs.

• Implementation and unit testing: during this stage, the software design is realized as a set
of programs or program units. Unit testing involves verifying that each unit meets its
specification.
• Integration and system testing: The individual program units or programs are integrated
and tested as a complete system to ensure that the software requirements have been met.
After testing, the software system is delivered to the customer.
• Operation and maintenance: Normally (although not necessarily) this is the longest life
cycle phase. The system is installed and put into practical use. Maintenance involves
correcting errors which were not discovered in earlier stages of the life cycle, improving the
implementation of system units and enhancing the system’s services as new requirements
are discovered.
Actually, these stages extend beyond and feed information to each other. During design,
problems with requirements are identified; during coding, design problems are found and so
on. The software process is not a simple linear model but involves a sequence of iterations of
the development activities.

During the final life cycle phase (operation and maintenance) the software is put into use. Errors
and omissions in the original software requirements are discovered. Program and design errors
emerge and the need for new functionality is identified. Modifications become necessary for
the software to remain useful.
(1) Evolutionary Development

This moves towards interleaves the performance of specification, growth and validation.
An initial system is quickly developed from very abstract specifications. This is then refined
with customer input to produce a system which satisfies the customer’s needs. The system
may then be delivered. Alternatively, it may be re-implemented using a more structured
approach to produce a more robust and maintainable system.

(2) Formal Transformation
This approach is based on producing a formal mathematical system specification and
transforming this specification, using mathematical methods, to a program. These
transformation are correctness preserving. This means that you can be sure that the
developed program meets its specification.

(3) System assembly From Reusable Components
This technique assumes that parts of the system already exist. The system development
process focuses on integrating these parts rather than developing them from scratch.
The first two of these approaches, namely the waterfall approach and evolutionary development,
are now widely used for practical systems development. Some systems have been built using
correctness-preserving transformations but this is still an experimental.

1.11 Role of Management in Software Development

The management of software development is heavily dependent on four factors: People, Product,
Process, and Project. Order of dependency is as shown in Figure. 1.11.

18 LOVELY PROFESSIONAL UNIVERSITY