DATA FLOW DIAGRAM

DATA FLOW DIAGRAM:

"a network representation of a system. The system may be automated, manual, or mixed. The DFD portrays the system in terms of its component pieces, with all interfaces among the components indicated."

- Tom DeMarcohence DFDs:

focus on the movement of data between external entities and processes, and between processes and data stores

WHERE DO THEY FIT IN?

Analysis (What do we do?) Fact finding

investigate business process and the current system

PlanningAnalysisDesign ImplementationMaintenance

WHERE DO THEY FIT IN?

Design Describes how the system will

fulfill the user requirements

PlanningAnalysisDesign ImplementationMaintenance

OVERVIEW

The Data flow diagram is a form of

This is a technique for organising and documenting the structure and flow of data through a system’s processes, and the procedures to be implemented by a system’s processes.

DFD help to represent and communicate Information and data flows Processes that handle or change data Stored data Sources and recipients of information and data

WHY CONDUCT PROCESS MODELING?

The reason we do process modeling is to improve the operation of a system, not just to change its physical form. Start with understanding (Understand components

of current logical or physical system for purpose of rebuilding in a different physical form/technology, possibly with some changed functionality)

Then analyze for improvements or where to add new functionality (Find inefficiencies in current system)

Then specify re-engineered system

COMPONENTS

• A DFD is made up of four components: processes, external entities, data flows and data stores.• Processes• External entities• Data flows• Data stores

COMPONENTS

1. Entities

These are the places which provide the organisation with data, or have data sent to them by the organisation (e.g. customers, partners, government bodies).

Form the boundaries of the system. The system and external entities exchange data in the

form of data flows. Must be named, titles preferred to names of individuals -

use a noun

ExternalEntity

COMPONENTS

2. Process Jobs that are done with the data.

Process box

COMPONENTS

3. Data Flows

Connects the processes, external entities and data stores

Arrows show how data flows from one place to another

Label the arrows with the name of the data that moves through ite.g. Customers logs into systems

Data Flow

Logs in

COMPONENTS

4. Data Stores Represents holding areas for collection of

data, processes add or retrieve data from these stores

Only processes are connected to data stores They can be manual, digital or temporary.

D1 Data Store

M1 Data Store

T1 Data Store

Manual Data Store = M1

Digital Data Store = D1

Temporary Data Store = T1

NOTATION

Process box

D1 Data Store

ExternalEntity

Data Flow• Data Flow

• Process

• External Entity

• Data Store

BENEFITS OF DFD

Provide a pictorial, non-technical representations

Quick to produce and easy to amend

Concise system descriptions

LOGICAL VERSUS PHYSICAL MODELS What versus How

Shows what a system is or does Focuses on how the business operates Is implementation-independent

Shows how a system operates Is implementation-dependent because it shows how a

system is physically and technically implemented Systems analysts use logical models to depict

business requirements, and physical models to depict technical designs

System analysis activities tend to focus on the logical models

CONTEXT LEVEL DFD

The Context Level DFD attempt to show the workings of the entire system at a glance.

It is the and the first to be drawn

This level consists of The process description usually outlines in

general terms what the system is or does, for example, ‘Order Processing System’, ‘Video Rental Systems’.

The single process represents the entire system Depicts the interaction between the

system and external entities. This level

CONTEXT LEVEL DATA FLOW DIAGRAM

a. Entity

b. Entity

Process

CONTEXT DATA FLOW DIAGRAM EXAMPLE

a.Personnel Department

b. Management

c.Employee

Payroll System

New Staff Details

Hours Worked

Pay Cheque

Name/Address Change

Cash List

Salary Changes

RULES FOR DRAWING DFDS...

1. I. External entities cannot flow directly to each otherII. A data flow cannot link a data store to an external

entityIII. Data cannot move between data stores without first

being processeda.

Entity

b. Entity

a. Entity

Communicates

D1 Data Store

Communicates

x x

RULES FOR DRAWING DFDS...

1. All data flows must flow to or from a process continued..

RULES FOR DRAWING DFDS...

2. A process must have at least one input flow and one output flow

I. A arises when a process has input flows but no output flows

II. A is a situation which occurs when a process has outputs but no input flows.

Process Process

RULES FOR DRAWING DFDS...

3. The inputs to a process must be sufficient to produce output flows.

I. If a process has outputs that are greater than the sum of its inputs, a is said to occur

4. The inputs to a process must be sufficient to produce output flows.

I. When naming data flows, adjectives should be used which show how processing has changed the data flow.

DFD – COMMON ERRORS

Miracle

Black HoleGray Hole

RULES FOR DRAWING DFDS...

5. Data flows cannot cross each other To overcome this problem, data stores and

entities can be . However, processes must never be duplicated Data flows must be unidirectional

Additional rules• Entities must be

• Title = “Context Level DFD” and System or Company Name

RULES FOR CONNECTING PROCESSES, DATA STORES, AND ENTITIES IN A DFD

STEPS IN CONSTRUCTING STEPS IN CONSTRUCTING CONTEXT LEVEL DFDCONTEXT LEVEL DFD

You will be supplied with a piece of narrative text describing a particular system. Read and re-read this text.

Extract and state a set of assumptions from the narrative text.

Identify the entities from the text – these will be depicted by nouns.

Highlight the process for the context level DFD Identify data flows from the text – data flows

are usually nouns or noun phrases. Construct the Context Level DFD using all the

preceding information. Name and label the entities, data flows and name the main process.

CREATING A SET OF DFDS Draw a Context Diagram

1. Put a title on the page = Context Level DFD for ‘name of company’.

2. Draw the context diagram so it fits on one page

3.

4. Use unique names within each set of symbols

5. 6. Provide a unique name and reference

number for process

LAB WORK: PAYROLL SYSTEM CONTEXT LEVEL DFD XYZ LTd. is a nationwide retail chain. Management is considering conducting

Systems Analysis and Design with a view to computerising the payroll system. Hours worked per employee per month are forwarded by the Personnel

Department. This is used to create an employee time record for each employee and this is held in the Employee Time File. This is used to calculate Gross Pay, and Gross Pay is used to determine net pay. Pay slips are issued on the last Friday of every month.

The employee Master File must be updated on a regular basis. One instance which necessitates updating the system is when a new employee joins the firm. The Personnel Department gives the payroll system all new staff details. When an Employee gets married or moves house, details of the name/address change are given to the payroll system. Pay cheques are then prepared.

Management is in charge of salary changes – these are input to the Employee Master File when the time arises. Every month, management is sent a cash list with details of salary payments for each employee for the month previous.

Required: Draw up a reasonable set of assumptions, based on the narrative above. You are now required to construct a Context Level DFD based on those

assumptions, to help the company in its efforts computerise the payroll system.

ASSUMPTIONS

Assumptions 1(a): It is assumed that when employee details

change, such as when he/she gets married or moves house, it is the responsibility of the individual employee to notify the payroll system as to name/address changes.

CONTEXT LEVEL

Identify Process Payroll System

Identify entitiesa. Personnel Deptb. Managementc. Employee

Identify data flows

• Hours Worked• Salary Changes• Cash List• Pay Cheque• Name/Address

Change• New Staff Details

LAB WORK: PAYROLL SYSTEM CONTEXT LEVEL DFD XYZ LTd. is a nationwide retail chain. Management is considering conducting

Systems Analysis and Design with a view to computerising the payroll system. Hours worked per employee per month are forwarded by the Personnel

Department. This is used to create an employee time record for each employee and this is held in the Employee Time File. This is used to calculate Gross Pay, and Gross Pay is used to determine net pay. Pay slips are issued on the last Friday of every month.

The employee Master File must be updated on a regular basis. One instance which necessitates updating the system is when a new employee joins the firm. The Personnel Department gives the payroll system all new staff details. When an Employee gets married or moves house, details of the name/address change are given to the payroll system. Pay cheques are then prepared.

Management is in charge of salary changes – these are input to the Employee Master File when the time arises. Every month, management is sent a cash list with details of salary payments for each employee for the month previous.

Required: Draw up a reasonable set of assumptions, based on the narrative above. You are now required to construct a Context Level DFD based on those

assumptions, to help the company in its efforts computerise the payroll system.

PAYROLL SYSTEM (A): CONTEXT LEVEL DATA FLOW DIAGRAM

a.Personnel Department

b. Management

c.Employee

Payroll System

New Staff Details

Hours Worked

Pay Cheque

Name/Address Change

Cash List

Salary Changes

ASSUMPTIONS

Assumptions 1(b): It is assumed that when employee details

change, such as when he/she gets married or moves house, the personnel department is responsible for informing the payroll system. The personnel department updates the payroll system directly.

The Employee is notified when changes are made by the Personnel Dept.

The Personnel Dept receives an acknowledgement that states Employee file is updated.

PAYROLL SYSTEM (B): CONTEXT LEVEL DATA FLOW DIAGRAM

a.Personnel Department

b. Management

c.Employee

Payroll System

New Staff Details

Hours Worked

Pay Cheque

Cash List

Salary Changes

Name/Address Change

Confirmation of Detail Changes

a.Personnel Department

Acknowledgement of changes

VALIDATING THE DFD

Syntax errors – diagram follows the rules Assure correct DFD structure

For each DFD:Check each process for: •A unique name: action verb phrase; number; description•At least one input data flow•At least one output data flow•Output data flow names usually different than input data flow names•Between 3 and 7 processes per level 1+ DFD

VALIDATING THE DFD

For each DFD: Check each data flow for:

A unique name: noun; descriptionConnects to at least one processShown in only one direction (no two-headed arrows)

Check each external entity for:A unique name: noun; descriptionAt least one input or output data flow

Check each data store for:A unique name: noun; descriptionAt least one input data flowAt least one output data flow

EXERCISE

Precision Tools sells a line of high-quality woodworking tools. When customers place orders on the company’s Web site, the system checks to see if the items are in stock, issues a status message to the customer, and generates a shipping order to the warehouse, which fills the order. When the order is shipped, the customer is billed. The system also produces various reports, such as inventory reports for Accounting. Draw a context diagram for the order systemDocument any assumptions you may have made about the system.