Data Flow Diagram

• Purpose – visually depict how data moves and changes through a top-down, logical model

• Logical model – requirements and the relationship among the requirements

• DFD is not program logic or processing logic

Data Flow Diagram

Data Flow Diagram

• Leveling– Top-down approach– General to detailed

DFD Process Symbol

• Processes change the data from one form to another form• Data is input for a process• Processes transform data to produce output

DFD Process Symbol Rules

• Place the name of the process inside the circle in ALL CAPS

• The name identifies its function

• The name consists of an active verb followed by a descriptive, single noun

APPLY DISCOUNT VERIFY ORDER

DFD Process Symbol Rules

• Process details are not shown

• A process is a black box, as for as details are concerned

• Levels of DFD (context / diagram 0)

DFD Data Flow Symbol

• A line with an arrowhead showing the direction of the data flow.

• Purpose is to depict the pathway that data moves from one part of an information system to another part

• Data Flow name identifies the data it represents

DFD Data Flow Symbol

• The name consists of adjectives and a singular noun

• The line can be straight or curved

• Each data flow equals a specific piece of data or a set of data (student name or class roster)

BANK DEPOSIT STUDENT GRADE

DFD Data Flow Symbol

• At least one data flow must enter each process symbol

• At least one data flow must leave each process symbol

• Often more than one

DFD Data Store Symbol

• The purpose is to depict a data repository

• The plural name consists of adjectives and a noun

• Place the data store name between the lines

STUDENTS PURCHASE ORDERS

DFD Data Store Symbol

• Data store is connected to a data flow with a proceses at the other end of the data flow

• Detailed contents not shown in the DFD (find this information in the data dictionary)

DFD External Entity Symbol

• The purpose is to depict the terminators or boundaries• A person, department, outside organization, or

other information system that provides data (orgin) to the system or recieves data (sink) or information from the system

• Entity is always connected by a data flow to a process

DFD External Entity Symbol

• The entity name is in the singular form of the name of the department, outside organization, other information system or person

CUSTOMER STUDENT

PAYROLL SYSTEM

Context Diagrams

Context Diagrams• Top-level view of information system (IS)• First DFD is general overview of the IS• Shows the boundaries of the IS• One process only (it represents the entire IS)• Add all external entities and connect to process

ORDER PROCESSING (process)

CUSTOMER (entity)

WAREHOUSE (entity)

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Context Diagrams

• The requirements gathering of the systems analysis phase will define all the external entities

• No data stores at this level of the DFD• Context diagram must fit on one page• Process name of the context diagram is the name

of the IS• Avoid crossing data flow symbols

Diagram 0

Diagram 0 for the context diagram DFD for a grading system.

Diagram 0

• First detailed DFD

• The purpose is depict the major processes, data flows, and data stores for an IS

• Repeat the external entities and data flows from the context diagram

• Add major processes and data stores

Lower Level Diagrams

• Expand all processes into functional primitives

• Functional primitives are processes that consists of a single function ( can not expand further)

                                                                                                                   

              

Data Dictionary

• The purpose is a central storehouse of data about an IS’s data and the data’s transformations

• Dictionary is used to document the detailed facts about an IS

Contents of data flows

Contents of data stores

External entities details

Data Dictionary

• Also defines and describes all data elements and meaningful combinations of data elements

• A data element is the smallest piece of data that has meaning within an IS

Salary student name

Social security number birth date

Data Elements

• DFD data flow

• DFD data store (data elements logically combined)

• DFD process (functional primitive)

• External entities (recommended)