2. SW Design
Software design is an iterative process
through which requirements are translated
into a “blueprint” for constructing the
software.
Initially, the blueprint depicts a holistic view of
software.
3. Process of Design Engineering
During the design process the software
specifications are transformed into design models
Models describe the details of the data structures,
system architecture, interface, and components.
Each design product is reviewed for quality before
moving to the next phase of software development.
At the end of the design process a design model
and specification document is produced.
This document is composed of the design models
that describe the data, architecture, interfaces and
components.
5. Design Specification Models
Data design – created by transforming the analysis information
model (data dictionary and ERD) into data structures required to
implement the software. Part of the data design may occur in
conjunction with the design of software architecture. More
detailed data design occurs as each software component is
designed.
Architectural design - defines the relationships among the
major structural elements of the software, the “design patterns”
than can be used to achieve the requirements that have been
defined for the system, and the constraints that affect the way in
which the architectural patterns can be applied. It is derived from
the system specification, the analysis model, and the subsystem
interactions defined in the analysis model (DFD).
6. Design Specification Models
Interface design - describes how the software
elements communicate with each other, with other
systems, and with human users; the data flow and
control flow diagrams provide much of the
necessary information required.
Procedural / Component-level design - created by
transforming the structural elements defined by the
software architecture into procedural descriptions of
software components using information obtained
from the process specification (PSPEC), control
specification (CSPEC), and state transition diagram
(STD).
9. Architecture Design
“The overall structure of the software and the ways in which that
structure provides conceptual integrity for a system.”
Design can be represented as
Structural Models
Defines the components of a system (e.g., modules, objects, filters) and
How the components are packaged and interact with one another.
Framework Models
Increase level of abstraction
Dynamic Models and Process Models
Predicts behavioral and reliability aspects
Functional Models
Depicts functional Hierarchy.
10. Patterns
a pattern is “a common solution to a common
problem in a given context.” While architectural
styles can be viewed as patterns describing the
high-level organization of software (their
macroarchitecture), other design patterns can be
used to describe details at a lower, more local level
(their microarchitecture).
Creational patterns (example: builder, factory,
prototype, and singleton)
Structural patterns (example: adapter, bridge,
composite, decorator, façade, flyweight, and proxy)
Behavioral patterns (example: command,
interpreter, iterator, mediator, memento, observer,
state, strategy, template, visitor)
11. Design Pattern
Design Pattern enables a designer to
determine whether the pattern :
is applicable to the current work
can be reused
can serve as a guide for developing a similar, but
functionally or structurally different pattern.
14. Information Hiding
Principle of information hiding says that a good split
of modules is when modules communicate with one
another with only the information necessary to
achieve the s/w function.
So information hiding enforces access constraints to
both
procedural detail with a module, and
local data structure used by that module.
Data hiding is a CRITERION for modular design.
How to know what modules to create.
16. 16
Information Hiding (Benefits)
reduces the likelihood of “side effects”
limits the global impact of local design decisions
emphasizes communication through controlled
interfaces
discourages the use of global data
leads to encapsulation—an attribute of high
quality design
results in higher quality software
17. 17
Functional Independence
COHESION - the degree to which a
module performs one and only one
function.
COUPLING - the degree to which a
module is "connected" to other
modules in the system.
18. Cohesion
Internal glue with which component is constructed
All elements of component are directed toward and
essential for performing the same task
19. Range of Cohesion
High Cohesion
Low
Functional
Sequential
Communicational
Procedural
Temporal
Logical
Coincidental
20. Examples of Cohesion-1
Function A
Function
B
Function
D
Function
C
Function
E
Coincidental
Parts unrelated
Function A
Function A’
Function A’’
logic
Logical
Similar functions
Time t0
Time t0 + X
Time t0 + 2X
Temporal
Related by time
Function A
Function B
Function C
Procedural
Related by order of functions
21. Examples of Cohesion-2
Function A part 1
Function A part 2
Function A part 3
Functional
Sequential with complete, related functions
Function A
Function B
Function C
Communicational
Access same data
Function A
Function B
Function C
Sequential
Output of one is input to another
22. Coupling
Degree of dependence among components.
No dependencies Loosely coupled-some dependencies
Highly couples-many dependencies
23. Ways components can be dependent
References made from one to another
Component A invokes B
A depends on B for completion of its function or process
Amount of data passed from one to another
Component A passes to B: a parameter, contents of an array, block of
data
Amount of control one has over the other
Component passes a control flag to B
Value of flag tells B the state of some resource or subsystem, process
to invoke, or whether to invoke a process
Degree of complexity in the interface between components
Components C and D exchange values before D can complete
execution
24. Range of Coupling
High Coupling
Loose
Low
Content
Common
External
Control
Stamp
Data
Uncoupled
25. Content Coupling : (worst) When a module
uses/alters data in another module
Common Coupling : 2 modules communicating
via global data
External Coupling :Modules are tied to an
environment external to the software
Control Coupling : 2 modules communicating
with a control flag
26. Stamp Coupling : Communicating via a
data structure passed as a parameter. The
data structure holds more information than
the recipient needs.
Data Coupling : (best) Communicating
via parameter passing. The parameters
passed are only those that the recipient
needs.
No data coupling : independent modules.
27. Summary
The measure of strength
of the association of
elements within a module
The measure of
interdependence of one
module to another
It is the degree to which
the responsibility of a
single component form a
meaningful unit
It describes the
relationship between
software components
It is a property or
characteristic of an
individual module
It is a property of a
collection of modules
COHESION COUPLING
28. Refinement
Refinement is a process of elaboration
It is a top-down design strategy
A program is developed by successfully refining levels of procedural
details
29. Stepwise Refinement
Open Door
walk to door;
reach for knob;
open door;
walk through;
close door.
repeat until door opens
turn knob clockwise;
if knob doesn't turn, then
take key out;
find correct key;
insert in lock;
endif
pull/push door
move out of way;
end repeat
30. Refactoring
Fowler [FOW99] defines refactoring in the
following manner:
"Refactoring is the process of changing a software system in
such a way that it does not alter the external behavior of the
code [design] yet improves its internal structure.”
When software is refactored, the existing
design is examined for
redundancy
unused design elements
inefficient or unnecessary algorithms
poorly constructed or inappropriate data structures
or any other design failure that can be corrected to yield a
better design.
