The document discusses software design and the design process. It describes software design as a representation of a software product that can be traced to requirements and assessed for quality. Several viewpoints and representations are discussed for designing different aspects of software, such as the structural, functional, behavioral, and data modeling viewpoints. Common notations used in software design like data flow diagrams, statecharts, and entity relationship diagrams are also explained. The design process involves creating abstractions using viewpoints to describe a system from different perspectives in a manageable way.

1. Dr. Javed Ali Khan
Software Design Process
CSE-302 Software Design & Architecture

2. Outline
 Software Design
 Viewpoints/Representations
 Design process
 Levels of design
 Software design principles
 Software design methods

3. Software Design
 A software design is a meaningful engineering representation of
some software product that is to be built.
The process of applying various techniques and principles for the purpose of defining a device,
a process or a system in sufficient detail to permit its physical realization” [TAY59]
 A design can be traced to the customer's requirements and can
be assessed for quality against predefined criteria.
 It lays down the groundwork for achieving non-functional
requirements (performance, maintainability, reusability, etc.)
 It takes target technology into account (e.g., kind of middleware,
database design, etc.)

4. Questions to Answer…
 What are the main functional elements of your
architecture?
 How will these elements interact with one another and
with the outside world?
 What information will be managed, stored, and
presented?
 What physical hardware and software elements will be
required to support these functional and information
elements?
 What operational features and capabilities will be
provided?
 What development, test, support, and training
environments will be provided?

5. Possible Solutions
 Try to answer all of these questions by means of a single,
heavily overloaded, all-encompassing model
 Use a mixture of formal and informal notations to describe
a number of aspects of the system on one huge sheet of
paper
 The functional structure, software layering, concurrency,
intercomponent communication, physical deployment environment,
and so on
 This sort of AD is really the worst of all worlds?
 Isn’t possible to describe a software architecture by using
a single model (Reported in many studies)
 Hard to understand and is unlikely to clearly identify the
architecture’s most important features
 Because of its complexity, a monolithic AD is often
incomplete, incorrect, or out-of-date?

6. Software Design …
 Manageable and comprehensible by a range
of business and technical stakeholders
 An approach is to attack the problem from
different directions simultaneously.
 From abstractions to systems
 abstractions allow us to ignore implementation
details of procedures and data structures
 for large systems, we need to abstract away even
more detail
 we need to represent higher level abstractions

7. Design Representations
 Help us to see the big picture
 allow us to communicate our designs with others
 customers, managers, other developers,
 people with varying technical expertise
 These representations may not be correct/complete
 allow us to measure various quality attributes
 completeness, consistency, complexity
 Help in documenting the design for future
reference
 Must be complete and correct

8. Viewpoints/Representations
 Viewpoints help in creating abstractions
 A view is a representation of one or more structural aspects of an
architecture/design that illustrates how the architecture/design
addresses one or more concerns held by one or more of its
stakeholders.
 A viewpoint is a collection of patterns, templates, and
conventions for constructing one type of view. It defines the
stakeholders whose concerns are reflected in the viewpoint and
the guidelines, principles, and template models for constructing
its views.
 A viewpoint tells you which details you can ignore when
forming an abstraction
 It defines which details are relevant and which are not
 Viewpoints can overlap
 Some aspects of a design are common to several viewpoints

9. Viewpoints/Representations..
 Example: Building a house
 Useful viewpoints:
 The architect’s viewpoint (plan views, elevations,
etc)
 The plumber’s viewpoint (routing diagrams for
pipework, fittings layouts, etc)
 The electrician’s viewpoint (wiring diagrams, etc)
 The buyer’s viewpoint (artist’s impression, floorplans,
etc)

10. Viewpoints
 Context viewpoint: addresses overall relationships and dependencies
between the components of the project and the way the system interacts
with its environment
 Functional viewpoint: describes the system’s runtime functional elements,
their responsibilities, interfaces, and interactions between them
 Information viewpoint: is intended to address the ways data is collected,
stored, and managed to make the system work
 Concurrency viewpoint: includes aspects of how concurrent units of the
program are handled and what parts specifically can execute concurrently
 Development viewpoint: is made for use of technical staff, which build and
test this software. It includes design specifications of software and is used
as a guideline when preparing the product
 Deployment viewpoint: describes where and how the system will be
deployed and includes technical details for dependencies and other
requirements
 Operational viewpoint: handles the part that comes in a production
environment, which includes how the software will be operated,
administered, and supported

11. Software Design Representations/Viewpoints
 Structural
 Domain: Static properties (structure) of the software
 Representation: Dependency graphs, structure charts, etc
 Functional
 Domain: Tasks performed by the software, information flow
 Representation: Dataflow diagrams, procedure abstraction, etc.
 Behavioural
 Domain: Cause and effect within the program, system behaviour
 Representation: state transition diagrams, statecharts, petri nets,
etc.
 Data Modeling
 Domain: Data objects and relationships between them
 Representation: Entity relationship diagrams

12. Notational Forms
➜Text
 often hard to see the big picture
 natural language is ambiguous
 best used in small chunks (e.g. for executive summaries)
➜Diagrams
 good for showing relationships and structure…
 if they’re kept simple:
➢small number of symbols (e.g. 2 types of box, 2 types of arrow)
➢Must represent an abstraction (e.g flow chart contain nearly all the
details of code, so is not an abstraction)
➢should be easy to sketch informally!
➜Mathematical Expressions (formal
specifications)
 very precise, very concise
 but require much training
 cannot (yet?) express all viewpoints (e.g. timing is difficult to express)

13. Example notations
Structure charts
hierarchical decomposition
of program
Dependency graphs
show the (static) control
flow
Structural notations
Objects modeled
usually program components
compilation units,
modules,
procedures
…
Relationships modeled
structural relationships between
components
static relationships only
“calls/controls”
“uses”
…
Note: structural notations
deal with structure of
the program, not
structure of the data.

14. The
Dependency
Graph
A dependency graph is a data structure formed by a directed graph
that describes the dependency of an entity in the system on the
other entities of the same system. The underlying structure of a
dependency graph is a directed graph where each node points to
the node on which it depends.
Notes:
all edges must be directed
all nodes must be labeled with the name of the procedure
only one edge between any two nodes (no matter how many times
the procedure is called)
recursive procedures (& data abstractions) use themselves
Useful for:
debugging, integration, measuring coupling
p
q r
e
d
Key
procedure
data abstraction
‘uses’
‘weakly uses’
(refers to but
does not use)
p
e

15. Example notations
Dataflow diagrams
show processes that transform
data
Procedural abstractions
(although these combine structural
viewpoint info too!)
Pseudo-code
Functional notations
Objects modeled
Program components
modules,
procedures,
Processes
these do not necessarily
correspond to
components of the
program
Relationships modeled
information flow
inputs and outputs
“communicates with”.

16. The Dataflow Diagram (DFD)
Notes:
every process, flow, and datastore must be labeled
representation is hierarchical
each process will be represented separately as a lower level DFD
processes are normally numbered for cross reference
processes transform data
can’t have the same data flowing out of a process as flows into it
Key
process
dataflow (no
control implied)
data store
external entity
system boundary
1.
determine
form of
travel
2.
check
schedule
3.
reserve
seats
4.
issue
tickets
Timetables
Fare tables
customer
booking
system
booking
system
customer
travel
request
customer
query
schedule
proposed
itinerary
proposed
itinerary
booked
itinerary
fares
tickets
booking
confirmation
booking
request
See also: van Vliet 1999, pp322-325

17. Statecharts
describes different states of a
component in a system, the states
are specific to a component
Behavioral means how things are executed rather
than the why of things are the way they are
Petri nets
for modeling process synchronization
It consist of places, transition, and
arcs
When arc is into transition called
input and vice-versa
Behavioral notations
Objects modeled
Dynamic nature of a system
properties events, states,
actions, conditions
To model lifetime of an object from
creation to termination
Relationships modeled
cause and effect
sequencing / parallelism
Example notations
State Transition Diagrams
model the program as a finite
state machine

18. Statecharts
Notes:
all states and transitions must be labeled
transitions may be conditional

19. Data modelling notations
Objects modeled
any kind of data
data types,
objects,
attributes of objects,
classes,
Relationships modeled
compositional
“part of”
“consists of”
classification
“is a kind of”
Example notations
Entity Relationship Diagrams
used in requirements modeling
Class diagrams
shows data abstraction
hierarchy
Note: in OOD, is used as a
structural notation for the
program!!!

20. Entity Relationship Diagram
Notes:
relationships relate entities, not their attributes
there is no standard way to show the cardinality of
Key
entity
attribute
relationship
1-to-1
1-to-many
many-to-many
star
film
cast
producer
director title
year
name
age
nationality
cast
film
age

21. Software Design in SDLC
 In SDLC (Software Development Life Cycle),
Design phase is one of the most important
phases.
 In the software engineering context, design
focuses on four major areas of concern: data,
architecture, interfaces and
procedures/components.

22. Software Design in SDLC
 In SDLC (Software Development Life Cycle),
Design phase is one of the most important
phases.
 In the software engineering context, design
focuses on four major areas of concern: data,
architecture, interfaces and
procedures/components.

23. Design Process
 During the design process, the software
specifications are transformed into design
models that describe the details of the data
structures, system architecture, interface, and
components.
 The emphasis in design phase/process is on
quality.
 This phase provides us with representation of
software that can be assessed for quality.

24. Analysis to Software design

25. Design Process Activities
 Architectural design
 Interface design
 Component design
 Data structure design
 Algorithm design

26. Levels of Software Design
 Architectural design (high-level design)
 architecture - the overall structure, main
modules and their connections
 addresses the main non-functional
requirements (e.g., reliability, performance)
 hard to change
 Detailed design (low-level design)
 the inner structure of the main modules
 detailed enough to be implemented in the
programming language

27. Software Design should be:
 Simple
 Correct & Complete
 Software design should be correct as per requirement.
 The design should have all components like data structures, modules,
and external interfaces, etc.
 Loosely coupled
 Loose coupling is an approach to interconnecting the components in a system
or network so that those components, also called elements, depend on each
other to the least extent practicable
 Understandable
 Flexibility: Able to modify on changing needs.
 Maintainable
 The design should be so simple so that it can be easily maintainable
by other designers.

28. Software Design Methods
 Structured Methods
 Process functions are identified
 Object-Oriented
 develop an object model of a system
 Data-Oriented
 Entities are determined for each sub-system, then
entity inter-relationships are examined to develop the
additional entities needed to support the relationships.
 Component-based
 Divide the system into components
 Formal Methods
 Requirements and programs are translated into
mathematical notation

29. Which method to choose?
 Data oriented design is useful for systems that process lots of
data, e.g. database and banking applications
 Structured design is useful for process intensive systems that
will be programmed using a procedural language such as
FORTRAN.
 OO methods are useful for any system that will be programmed
using an object oriented language such as C++.
 Component-based Methods are used for the large systems that
can be modularized.
 Formal methods are considered to be an alternative to OO and
classical design methods, but their use is expensive and claims
of reduced errors remain unproven. However, the ability to
formally validate the correctness of a software artifact is
appealing and research on formal methods is ongoing.

30. Software Design Strategies
 Divide-and-conquer/stepwise refinement
 Top-down vs. bottom-up
 Data abstraction and information hiding
 Use of heuristics
 Use of patterns and pattern languages
 Iterative and incremental approach

31. Reading for Next Week
 Chapter 5 of “Software Engineering” 9th Ed,
by Sommerville
 Rest of the reading material for Week 2 will
be available from
 http://learn.mcs.edu.pk
 Look for Software Design & Architecture course in
Fall 2012 category of MS Software Engineering
category.
 Login as guest.