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
 A complex system that works is invariably found to have
evolved from a simple system that worked. To manage the
complexity of the system we need to apply the principles of
separation of concern, modularity, and abstraction. This leads
to designs that are easy to understand and hence easy to
maintain.
 Separation of concern allows us to deal with different
individual aspects of a problem by considering these aspects in
isolation and independent of each other.
 A complex system may be divided into smaller pieces of lesser
complexity called modules. This is the classic divide-and-
conquer philosophy.
Managing Complexity of a
Software System


Software Design
 Once the requirements of a software system have
been established, we proceed to design that system.
During the design phase, the focus shifts from what
to how. That is, at this stage we try to answer the
question of how to build the system.
 The design activity provides a roadmap to
progressively transform the requirements through a
number on stages into the final product by
describing the structure of the system to be
implemented.


 Software design is not a sequential process. Design of a
software system evolves through a number of iterations. The
design process usually involves developing a number of
different models, looking at the system from different angles
and describing the system at various levels of abstraction.
 A activities performed at this stage include design of the
software architecture by showing the division of system into
sub-systems or modules, the specification of the services
provided by these sub-systems and their interfaces with each
other, division of each sub-system into smaller components and
services and interfaces provided by each one of these
components.
Software Design Process


 Software design process revolves around
decomposing of the system into smaller and simpler
units and then systematically integrates these units
to achieve the desired results. Two fundamental
strategies have been used to that end. These are
 functional or structured design and
 object oriented design.
Software Design Strategies


 Functional or Structured Design
 The structure of the system revolves around functions. The
entire system is abstracted as a function that provides the
desired functionality. The structure of the system revolves
around functions. The entire system is abstracted as a
function that provides the desired functionality.
 Object-oriented Design
 In this case the system is decomposed into a set of objects
that cooperate and coordinate with each other to implement
the desired functionality. In this case the system state is
decentralized and each object is held responsible for
maintaining its own state.
Software Design Strategies


 A software design can be looked at from different angles
and different parameters can be used to measure and
analyze its quality. These parameters include efficiency,
compactness, reusability, and maintainability. A good
design from one angle may not seem to be suitable when
looked from a different perspective.
 For example, if we need to design an embedded system
for the control of a nuclear reactor or a cruise missile, we
would probably require a system that is very efficient and
maintainability would be of secondary concern.
Software Design Qualities


 Maintenance contributes towards a major share of
the overall software cost, the objective of the design
activity, in most cases, is to produce a system that is
easy to maintain. A maintainable design is the one in
which cost of system change is minimal and is
flexible enough so that it can be easily adapted to
modify exiting functionality and add new
functionality.
Maintainable Design


 Coupling is a measure of independence of a module or
component. Loose coupling means that different system
components have loose or less reliance upon each other.
Hence, changes in one component would have a limited
affect on other components.
 Strong cohesion implies that all parts of a component
should have a close logical relationship with each other.
That means, in the case some kind of change is required
in the software, all the related pieces are found at one
place. Hence, once again, the scope is limited to that
component itself.
Coupling and Cohesion

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Lecture 11

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Lecture 11