BACHELOR OF TECHNOLOGY YEAR 3

OBJECT-ORIENTED ANALYSIS AND DESIGN

WEEK 3 & 4

FUNCTIONAL MODELLING

Functional Modeling gives the process perspective of the object-oriented

analysis model and an overview of what the system is supposed to do. It

defines:-

− The function of the internal processes in the system is illustrated with

the aid of Data Flow Diagrams (DFDs)

− Functional derivation of the data values without indicating how they

are derived when they are computed, or why they need to be computed.

DATA FLOW DIAGRAMS

Functional Modeling is represented through a hierarchy of DFDs. A DFD

is a graphical representation of a system that shows the inputs to the system,

the processing upon the inputs, the outputs of the system as well as the

internal data stores. DFDs illustrate the series of transformations or

1|Page
computations performed on the objects or the system, and the external

controls and objects that affect the transformation.

Definition - “A data flow diagram is a graph which shows the flow of data

values from their sources in objects through processes that transform them

to their destinations on other objects.”

The four main parts of a DFD are:-

- Processes

- Data Flows

- Actors

- Data Stores

The other parts of a DFD are:-

- Constraints, and

- Control Flows.

2|Page
FEATURES OF A DFD

a) Processes

Processes are the computational activities that transform data values. A

whole system can be visualized as a high-level process. A process may be

further divided into smaller components. The lowest-level process may be a

simple function.

Representation in DFD: A process is represented as an ellipse with its

name written inside it and contains a fixed number of input and output data

values.

Example: The following figure shows a process of computing LCM

Compute_HCF_LCM that accepts two integers as inputs i.e. integer_a and

integer_b and outputs their HCF (highest common factor) and LCM (least

common multiple).

3|Page
 b) Data Flows

Data flow represents the flow of data between two processes. It could be

between an actor and a process, or between a data store and a process. A data

flow denotes the value of a data item at some point of the computation. This

value is not changed by the data flow.

Representation in a DFD: Data flow is represented by a directed arc or an

arrow, labeled with the name of the data item that it carries.

In the above figure, Integer_a and Integer_b represent the input data flows

to the process, while L.C.M. and H.C.F. are the output data flows.

A data flow may be forked in the following cases:-

4|Page
 - The output value is sent to several places as shown in the following

figure. Here, the output arrows are unlabeled as they denote the same

value.

- The data flow contains an aggregate value, and each of the components

is sent to different places as shown in the following figure. Here, each

of the forked components is labeled.

c) Actors

Actors are the active objects that interact with the system by either producing

data and inputting them to the system, or consuming data produced by the

system. In other words, actors serve as the sources and the sinks of data.

Representation in DFD: An actor is represented by a rectangle. Actors are

connected to the inputs and outputs and lie on the boundary of the DFD.

Example: The following figure shows the actors, namely, Customer and

Sales_Clerk in a sales system.

5|Page
 d) Data Stores

Data stores are the passive objects that act as a repository of data. Unlike

actors, they cannot perform any operations. They are used for data storage

and retrieval of the stored data. They represent a data structure, a disk file,

or a table in a database.

Representation in DFD: A data store is represented by two parallel lines

containing the name of the data store. Each data store is connected to at least

one process.

Input arrows contain information to modify the contents of the data store,

while output arrows contain information retrieved from the data store. When

a part of the information is to be retrieved, the output arrow is labeled. An

unlabeled arrow denotes full data retrieval. A two-way arrow implies

retrieval (read) and update (read).

6|Page
Example: The figure below shows:-

- A data store, Sales_Record that stores the details of all sales

- Input to the data store comprises of details of sales such as item, billing

amount, date, etc.

- To find the average sales, the process retrieves all the required sales

records and computes the average.

e) Constraints

Constraints specify the conditions or restrictions that need to be satisfied

over time. They allow adding new rules or modifying existing ones.

Constraints can appear in all the three models of object-oriented analysis.

- In Object Modeling, the constraints define the relationship between

objects. They may also define the relationship between the different

values that an object may take at different times.

7|Page
 - In Dynamic Modeling, the constraints define the relationship between

the states and events of different objects.

- In Functional Modeling, the constraints define the restrictions on the

transformations and computations.

Representation in a DFD: A constraint is rendered as a string within

braces.

Example: The following figure shows a portion of DFD for a payroll system

of a company that has decided to give incentives to all employees of the sales

department and increment the salary of all employees of the HR department.

It can be seen that the constraint {Dept:Sales} causes incentive to be

calculated only if the department is sales and the constraint {Dept:HR}

causes increment to be computed only if the department is HR.

8|Page
 f) Control Flows

A process may be associated with a certain Boolean value and is evaluated

only if the value is true, though it is not a direct input to the process. These

Boolean values are called the control flows.

Representation in DFD: Control flows are represented by a dotted arc or

line from the process producing the Boolean value to the process controlled

by them.

Example: The following figure represents a DFD for arithmetic division.

The Divisor is tested for non-zero. If it is not zero, the control flow OK has

a value True and subsequently the Divide process computes the Quotient and

the Remainder.

9|Page
DEVELOPING A DFD MODEL OF A SYSTEM

In order to develop a DFD model of a system, a hierarchy of DFDs is

constructed. The top-level DFD comprises of a single process and the

actors interacting with it.

At each successive lower level, further details are gradually included. A

process is decomposed into sub-processes, the data flows among the sub-

processes are identified, the control flows are determined, and the data

stores are defined.

Note:

While decomposing a process, the data flow into or out of the process should

match the data flow at the next level of DFD.

Example: Let us consider a Wholesaler Software System that automates the

transactions of a wholesale shop. The shop sells in bulk and has a clientele

comprising of merchants and retail shop owners. Each customer is asked to

register with his/her particulars and is given a unique customer code,

10 | P a g e
C_Code. Once a sale is done, the shop registers its details and sends the

goods for dispatch.

Each year, the shop distributes Christmas gifts to its customers, which

comprise of a silver coin or a gold coin depending upon the total sales and

the decision of the proprietor.

The functional model for the Wholesale Software below shows the top-level

DFD illustrating the software as a single process and the actors that interact

with it. This is also called a Context Diagram

The actors in the system are:-

- Customers

- Salesperson

- Proprietor

11 | P a g e
In the next level DFD, as shown in the following figure below, the major

processes of the system are identified, the data stores are defined and the

interaction of the processes with the actors and the data stores are

established.

In the system, three processes can be identified, which are:-

- Register Customers

- Process Sales

- Ascertain Gifts

The data stores that will be required are:-

- Customer Details

- Sales Details

- Gift Details

12 | P a g e
DECOMPOSITION OF THE PROCESSES ILLUSTRATION

a) Decomposition of the Register Customer Process

The following figure shows the details of the process Register Customer.

There are three processes in it, Verify Details, Generate C_Code, and Update

Customer Details. When the details of the customer are entered, they are

verified. If the data is correct, C_Code is generated and the data store

Customer Details is updated.

b) Decomposition of the Ascertain Gifts Process

The following figure shows the decomposition of the process Ascertain

Gifts. It has two processes in it, Find Total Sales and Decide Type of Gift

Coin. The Find Total Sales process computes the yearly total sales

corresponding to each customer and records the data. Taking this record and

the decision of the proprietor as inputs, the gift coins are allotted through

Decide Type of Gift Coin process.

13 | P a g e
 ADVANTAGES AND DISADVANTAGES OF DFD

Advantages Disadvantages

DFDs depict the boundaries of a DFDs take a long time to create,

system and hence are helpful in which may not be feasible for
portraying the relationship between practical purposes.
the external objects and the
processes within the system.
They help the users to have DFDs do not provide any
knowledge about the system. information about the time-
dependent behavior, i.e., they do
not specify when the
transformations are done.
The graphical representation serves They do not throw any light on the
as a blueprint for the programmers frequency of computations or the
to develop a system. reasons for computations.

14 | P a g e
DFDs provide detailed information The preparation of DFDs is a
about the System processes. complex process that needs
considerable expertise.
They are used as a part of the system Also, it is difficult for a non-
documentation. technical person to understand.
The method of preparation is
subjective and leaves ample scope
to be imprecise.

RELATIONSHIP BETWEEN OBJECT, DYNAMIC, AND FUNCTIONAL MODELS

The Object Model, the Dynamic Model, and the Functional Model

complement each other for a complete Object-Oriented Analysis.

- Object modeling develops the static structure of the software system

in terms of objects. Thus, it shows the system's “doers.”

- Dynamic Modelling develops the temporal behavior of the objects in

response to external events. It shows the sequences of operations

performed on the objects.

- The functional model gives an overview of what the system should

do.

15 | P a g e
 a) Functional Model and Object Model

The four main parts of a Functional Model in terms of object model are:

- Process: Processes imply the methods of the objects that need to be

implemented.

- Actors: Actors are the objects in the object model.

- Data Stores: These are either objects in the object model and contain

attributes of objects.

- Data Flows: Data flows to or from actors represent operations on or

by objects. Data flows to or from data stores represent queries or

updates.

b) Functional Model and Dynamic Model

The dynamic model,

- States when the operations are performed,

While the functional model,

- States how they are performed and which arguments are needed.

As actors are active objects, the dynamic model must specify when they act.

16 | P a g e
The data stores are passive objects and they only respond to updates and

queries; therefore the dynamic model need not specify when they act.

c) Object Model and Dynamic Model

The dynamic model

- Shows the status of the objects and the operations performed on the

occurrences of events and the subsequent changes in states

- The state of the object as a result of the changes as shown in the object

model

STRUCTURED ANALYSIS VS. OBJECT-ORIENTED ANALYSIS

The Structured Analysis/Structured Design (SASD) approach is the

traditional approach of software development is based upon the waterfall

model. The phases of development of a system using SASD are:

- Feasibility Study

- Requirement Analysis and Specification

- System Design

- Implementation

17 | P a g e
 - Post-implementation Review

Now, we will look at the relative advantages and disadvantages of

structured analysis approach and object-oriented analysis approach.

ADVANTAGES/DISADVANTAGES OF OBJECT-ORIENTED ANALYSIS

Advantages Disadvantages
Focuses on data rather than the Functionality is restricted within
procedures as in Structured objects. This may pose a problem for
Analysis systems which are intrinsically
procedural or computational in nature.
The principles of encapsulation and It cannot identify which objects would
data hiding help the developer to generate an optimal system design.
develop systems that cannot be
tampered with by other parts of the
system.
It allows effective management of The object-oriented models do not
software complexity by the virtue easily show the communications
of modularity. between the objects in the system.
It can be upgraded from small to All the interfaces between the objects
large systems at a greater ease than cannot be represented in a single
diagram.

18 | P a g e
in systems following structured
analysis.

ADVANTAGES/DISADVANTAGES OF STRUCTURED ANALYSIS

Advantages Disadvantages
Since it follows a top-down approach in In traditional structured analysis
contrast to the bottom-up approach of models, one phase should be
OOAD, it can be more easily completed before the next phase.
comprehended
It is based on functionality. The overall The initial cost of constructing the
purpose is identified and then functional system is high since the whole
decomposition is done system needs to be designed at
for developing the software. This gives a once leaving very little option to
better understanding of the system but add functionality later.
also generates more complete systems.
The specifications are written in simple It does not support the reusability
English language and hence can be of code thus time and cost of
more easily analyzed by non-technical development are inherently high.
personnel.

19 | P a g e