Data flow diagram

How to :
Data Flow Diagrams
(DFDs)

Data Flow Diagrams (DFDs)
 Data flow diagram
(DFD) is a picture of
the movement of
data between
external entities and
the processes and
data stores within a
system

DFD Symbols (Gane & Sarson)
Process
Data Flow
Data Store
Source/Sink (External Entity)

Process
 Work or actions performed on data (inside the
system)
 Labels should be verb phrases
 Receives input data and produces output

Rule 1: Process
 Can have more than one outgoing data flow
or more than one incoming data flow

Rule 2: Process
 Can connect to any other symbol (including
another process symbol)

Data Flow
 Is a path for data to move from one part of the
IS to another
 Arrows depicting movement of data
 Can represent flow between process and data
store by two separate arrows

Data Store
 Is used in a DFD to represent data that the
system stores
 Labels should be noun phrases

Rule: Data Store
 Must have at least one incoming and one
outgoing data flow

Data Store: Correct/Incorrect?

Source/Sink (External Entity)
 External entity that is origin or destination of
data (outside the system)
 Is the singular form of a department, outside
organisation, other IS, or person
 Labels should be noun phrases
 Source – Entity that supplies data to the
system
 Sink – Entity that receives data from the
system

Rule: Source/Sink
 Must be connected to a process by a data flow

Source/Sink: Correct/Incorrect?

Rules for Using DFD Symbols
 Data Flow That Connects
YES NO
A process to another process
A process to an external entity
A process to a data store
An external entity to another external entity
An external entity to a data store
A data store to another data store

Context Diagram
 Top-level view of IS
 Shows the system boundaries, external entities that
interact with the system, and major information flows
between entities and the system.
 Example: Order system that a company uses to
enter orders and apply payments against a
customer’s balance

Context Diagram
of Order System

Level-0 DFD
 Shows the system’s major processes, data flows,
and data stores at a high level of abstraction
 When the Context Diagram is expanded into DFD
level-0, all the connections that flow into and out of
process 0 needs to be retained.

Lower-Level Diagrams
 Functional Decomposition
 An iterative process of breaking a system description
down into finer and finer detail
 Uses a series of increasingly detailed DFDs to
describe an IS
 Balancing
 The conservation of inputs and outputs to a data flow
process when that process is decomposed to a lower
level
 Ensures that the input and output data flows of the
parent DFD are maintained on the child DFD

Strategies for Developing DFDs
 Top-down strategy
Create the high-level diagrams (Context
Diagram), then low-level diagrams (Level-0
diagram), and so on
 Bottom-up strategy
Create the low-level diagrams, then higher-
level diagrams

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.
 Draw a context diagram for the order system
 Draw DFD diagram 0 for the order system

Identify Entities,Process,Data Stores & Data Flow
 Entities
 Customer
 Warehouse
 Accounting
 Processes
 1.0 Check Status
 2.0 Issue Status Messages
 3.0 Generate Shipping Order
 4.0 Manage Accounts
Receivable
 5.0 Produce Reports
 Data Stores
 D1 Pending Orders
 D2 Accounts Receivable
 Data Flows
 Order
 In-Stock Request
 Order Data
 Status Data
 Status Message
 Shipping Order
 Order Data
 Invoice
 Shipping Confirmation
 Payment
 Accounting Data
 Accounts Receivable Data
 Order Data
 Inventory Reports
1.0
2.0
3.0
4.0
5.0

Context
Diagram of
Order
System

Data flow diagram

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Data flow diagram