Data Flow Modelling
Concepts
 Data Flow Diagrams
 External Entities, Data Stores, Processes
and Data Flows
 Elementary Process Descriptions
 Levelling
 Document Flow Diagrams
Key Definitions
 A process model is a formal way of representing
how a business operates
 Data flow diagramming shows business
processes and the data that flows between them
Sample 1
Sample 2
Key Definitions
 Logical process models describe processes
without suggesting how they are conducted
 Physical models include information about how
the processes are implemented
e
Manager
External
Entities P.O.
Data Flow Diagrams aid communication
Purchase Order
1
Stock List
Stock Clerk
Order
Stock
M2
Stock List
Stock
File
d
Processes
Supplier
Data Stores
Purchase Order
Delivery
2
Stock Clerk
Receive
Stock
e
Orders
Delivered Goods
3
Cashier
Sell
Stock
Sold Goods
*
Customer
Purchase Order
Cabinet
*
Matched Orders
Manager
a
M1
Bought Goods
M2
Stock
File
Data Flow Diagrams
DFD Notation
 The DFD is a diagram that consists
principally of four symbols, namely
the external entity, the data flow, the
process and the data store
 Additionally, a physical flow can be
shown on the DFD of the current
system
Data Flow Diagrams
External Entities
d
Supplier
Data Flow Diagrams
Data Flows
Data Flow (usual)
Customer Details
Bi-directional Flow (rare)
Flow Between External Entities
(for convenience)
Resource Flow (for convenience)
Goods
Cosmetics
Data Flow Diagrams
Process
3
Cashier
Sell
Stock
Data Flow Diagrams
Data Stores
Digitised
D3
Manual
M1
T1
Transient
Duplicate
D1
Orders
Suppliers
Stock
File
Unpaid Invoices
D1
Orders
e
Manager
Data Flow Diagrams
Decomposition
Purchase Order
1
Stock List
Stock Clerk
Order
Stock
P.O.
M2
Stock List
Stock
File
d
Supplier
Purchase Order
Delivery
2
Stock Clerk
Receive
Stock
e
Orders
Delivered Goods
3
Cashier
Sell
Stock
Sold Goods
*
Customer
Purchase Order
Cabinet
*
Matched Orders
Manager
a
M1
Bought Goods
M2
Stock
File
Data Flow Diagrams
Decomposing Data Flow Diagrams
 Any process on a DFD may be broken up
into several sub-processes which, when
viewed collectively, make up that process
 Thus for example we may break-up process
1 of the Small Stock System into that shown
on the next slide:
Data Flow Diagrams
Decomposing Data Flow Diagrams
e
Manager
1
Order Stock
Stock List
1.1
1.2
Produce
Stock
List
Stock List
M2
Purchase Order
Stock
File
Record
Purchase
Order
*
*
Purchase Order
M1
Purchase Order
Cabinet
Data Flow Diagrams
Decomposing Data Flow Diagrams
 The decomposition of a DFD into lower level
DFDs is known as levelling
 The DFD that shows the entire system is
known as the ‘top level’ or ‘level 1’ DFD
 The DFDs that contain more detailed views of
the level 1 processes make up ‘level 2’ DFDs
 Any level 2 process that is further
decomposed gives rise to a level 3 DFD and
so on
Data Flow Diagrams
Decomposing Data Flow Diagrams
Elementary Process Description
System: Small Stock
DFD Type: Current
Process Name: Record Purchase Order
Process Id: 1.2
Managers give the stock clerk a ready-made purchase order. The stock
clerk places this order in the Purchase Order Cabinet.
It is the managers’ responsibility to send the order directly to the
supplier they have chosen. Each purchase order contains product
information taken from the supplier’s price list. The date after which a
delivery of goods will be unacceptable is also included.
Data Flow Diagrams
Decomposing Data Flow Diagrams
1.2
Record
Purchase
Order
*
Data Flow Diagrams
Context Diagrams
Data Flow
Stock List
Purchase Order
Data Item
product name
quantity in stock
supplier name
supplier address
supplier’s product code
product name
quantity ordered
purchase order date
latest acceptable delivery date
Remarks
Purchase order
contains one ‘supplier
name’ but many
‘product name’
Data Flow Diagrams
Starting from the Context Diagram

To develop a Context Diagram we carry out the
following tasks:
(i) Identify all sources and recipients of data from
the system, i.e. external entities
(ii) Identify the major data flows to and from the
external entities
(iii)Convert each source or recipient into an
external entity symbol
(iv)Add the data flows between each external
entity and a single box representing the entire
system
Data Flow Diagrams
Starting from the Context Diagram
External Entity
Supplier
Purchaser
Customer
Sales & Marketing
Accounts
S or R
s
r
s
s
s
r
r
s
r
r
Data Flow
Delivery Note
Purchase Order
Delivery Details
Invoice
P.O. Quantities
Stock Report
Dispatch Note
Customer Order
Matched C.O. #1
Matched Invoices
Data Flow Diagrams
Starting from the Context Diagram
a
Supplier
Payment
Delivery Note
Purchase Order
Delivery
Details
e
Matched Invoice
Accounts
Invoice
ZigZag
Warehouse
System
Stock Report
d
Despatch Note
Matched C.O.
Copy #1
Customer
Customer Order
P.O.Quantities
b
Purchaser
Customer Order
c
Sales and
Marketing
Document Flow Diagrams
i. Identify
all recipients and sources of
documents, whether inside or outside the
system boundary
ii. Identify the documents that connect them
iii. Convert each source and recipient into an
external entity symbol
iv. Add data flow arrows to represent each
connecting document
v. Add the system boundary to exclude the
external entities identified in the context
diagram
Document Flow Diagrams
Source
Supplier
Supplier
Stock Clerk
Stock Clerk
Despatch Clerk
Customer
Sales & Marketing
Despatch Clerk
Despatch Super.
Despatch Clerk
….
Document
Invoice
Delivery Times
Stock Report
Stock Report
Despatch Note
Customer Order
Customer Order
Despatch Report
Matched Dsp Rep
Matched CO #1
Recipient
P.O.Clerk
Stock Clerk
Purchaser
Despatch Supervisor
Customer
Sales & Marketing
Despatch Clerk
Despatch Supervisor
Despatch Clerk
Sales & Marketing
Document Flow Diagrams
Despatch
Supervisor
Sales and
Marketing
Matched
Despatch Rpt
Customer Order
Despatch Report
Matched C.O.
Copy #1
Despatch
Clerk
Data Flow Diagrams
Converting Document Flow Diagrams
To transform the Document Flow Diagram into a DFD
we follow each document flow in turn, asking the
following questions:
What process generates this document flow?
What process receives this document flow?
Is the document stored by a process?
Where is the document stored?
Is the document created from stored data?
What business activity triggers the process?
Is the document a source of new data?
Data Flow Diagrams
Context Diagrams
 A level
higher than
level 1,
showing the
whole
system as a
single
process with
external
entities
around it, is
also
possible:
e
P.O.
Manager
Stock List
Purchase Order
Matched Orders
d
Small
Stock
System
Bought Goods
a
Customer
Delivery
Supplier
Data Flow Diagrams
Decomposing Data Flow Diagrams
Decomposition Diagram
Data Flow Diagrams
Starting from the Context Diagram


We can now follow each flow into and
identify
the
elementary
process
responsible for it
A grouping of these elementary processes
can then give us a first glimpse of the
system’s Data Flow Model
Data Flow Diagrams
Converting Business Activity Models
Check
Delivery
2
Goods Receiving
b
Supplier
Check
Delivery
Delivery Note
Place Goods
in Delivery
Dock
P.O. Copy
M1
Matched P.O.
Purchase Orders
Allocate
Stock
Location
T2
Matched P.O.’s
Matched P.O.
Remove Goods
from Delivery
Dock
M2
Stock
New Stock
3
Stock Keeping
Store
Stock
Store
Goods in
Depot
Data Flow Diagrams
Tips
 For a process to be complete, it needs to have both an
input and an output (shown by data flows going into and
coming out of it)
2
Goods Receiving
Check
Delivery
2
T2
Matched P.O.’s
T2
Matched P.O.’s
Matched P.O.
Goods Receiving
b
Supplier
Check
Delivery
Delivery Note
2
Goods Receiving
b
Supplier
Delivery Note
Check
Delivery
Matched P.O.
Data Flow Diagrams
Tips
f
M2
Someone
A data store
Something
WHY?
2
f
Someone
Something
Do
something
with it
M2
Same something
A data store
Creating Use Cases
Elements of a Use Case
 Trigger -- event that causes the
scenario to begin
 External trigger
 Temporal trigger
 All possible inputs and outputs
 Individual steps
 Show sequential order
 Show conditional steps
Scenario Template (Use Case)
Scenario Description
Scenario Name:
Short Description:
ID Number: _ __
Trigger: _______________________________________________________________
Type: External / Temporal
Major Inputs:
Description
________________________
________________________
________________________
________________________
________________________
________________________
Major Steps Performed
Major Outputs:
Source
___________
___________
___________
___________
___________
___________
Description
Destination
_______________________
_______________________
_______________________
_______________________
_______________________
____________
____________
____________
____________
____________
Information for Steps
Creating Data Flow
Diagrams
Steps in Building DFDs
 Build the context diagram
 Create DFD fragments for each
scenario
 Organize DFD fragments into level 0
 Decompose level 0 DFDs as needed
 Validate DFDs with user
A DFD Fragment Example
DFD – Common Errors
Error
Error
Error
DFD – Packet Concept
Telephone Service
Provider
Item ized calls
& invoice
Incorrect use
of the packet
concept
Item ized calls
Invoice
1
Pay phone
bill
Correct use
of the packet
concept
A Second DFD Fragment Example
Level 0 Tips
 Generally move from top to bottom, left to right
 Minimize crossed lines
 Iterate as needed
 The DFD is often drawn many times before it is finished, even
with very experienced systems analysts
Composite & Elementary Flows
Tips for Level 1 and Below
 Sources for inputs and outputs listed at
higher level
 List source and destination of data flows
to processes and stores within each DFD
 Depth of DFD depends on overall system
complexity
 Two processes generally don’t need
lower level
 More than seven processes become
overly complex and difficult to read
Flows to & from Data Stores
Illegal Data Flows
Validating the DFD
 Syntax errors
 Assure correct DFD structure
 Semantics errors
 Assure accuracy of DFD relative to
actual/desired business processes
 User walkthroughs
 Role-play processes
 Examine lowest level DFDs
 Examine names carefully
How should we gather requirements? Use cases?
1. Use cases hold functional
requirements in an easy-to-read text
format
2. They make a good framework for nonfunctional requirements & project
details.
3. Use cases show only the Functional
req’ts.
4. Design is not done only in use case
units.
Use cases do not collect formulae, state, cardinality,
performance, uptime, ...
 Examples:
1. Order cost = order item costs * 1.06 tax
2. Promotions may not run longer than 6 months.
3. Customers only become Preferred after ...
4. A customer has one and only one sales contact.
5. Response time is ...
6. Uptime requirement is ...
7. Number of simultaneous users will be ...
Use the scenarios directly
to Design and Test the system.
What if...?
Scenarios
“Knows how to...”
“Knows how to...” “Knows how to...”
Summary
 The Data Flow Diagram (DFD) is an essential
tool for creating formal descriptions of
business processes and data flows.
 Use cases record the input, transformation,
and output of business processes.
 Eliciting scenario descriptions and
modeling business processes are critically
important skills for the systems analyst to
master.