Transcript Data Flow Modelling

National Diploma
in
Systems Analysis and
Design
Data Flow Modelling
Data Flow Modelling
• A data flow model (DFM) defines the
passage of data through a system, The
DFM comprises:
• A consistent set of Hierarchic Data Flow
Diagrams (DFD)
• Associated documentation
Components of DFD’s
• External Entities
• Data Flows
• Processes
• Data Stores
External Entities
Identifier
A
Customer
Sends data into
or receives data
from a system
Name
• An external entity is a
person, organisation,
department, computer
system or anything else
which either sends data
into a system or which
receives data from a
system, but which for the
purposes of the project in
question are outside the
scope of the system itself.
Data Flows
Customer
Details
A route by which data
can flow from one element
to a DFD to another
• A data flow is a route
by which data may
travel from one
element of a DFD to
another. Data flows
are represented by
arrows which are
labelled with a simple
meaningful name
Processes
Identifier
1.
Location
Accts
Check Customer
Details
Transformations
Process
which change
incoming data flows Description
to outgoing data flows
• Processes are
transformations which
change incoming data
flows into outgoing data
flows. Processes are
represented as rectangles
which contain a simple
description of the process,
e.g. verify customer
details. Each process has
a unique reference
number.
Processes - continued
• In the early stages it is possible to show
where in the organisation the process takes
place, e.g. Accounts Section, however this
is a physical constraint imposed by the
existing system and should not appear in a
completed ‘logical’ data flow diagram
Data Stores
Identifier
D1
Name
Orders
A repository for data
• A data store is a repository
for data. A data store is
represented by an open
ended rectangle containing
the name of the data store
(usually a plural noun
such as customers), each
data store has a unique
reference number prefixed
by the letter D
Data Flow Diagrams
• Having described the basic components of
DFD’s , the next stage is to look at how the
components can be fitted together to form a
complete DFD. Essentially the External
Entity, Process and Data Store components
can be interlinked using certain valid data
flows.
External Entity to
Process, i.e. receiving
data from outside the
system
Process to External
Entity, i.e. sending
data out of the system
Process to Data Store,
i.e. writing data to a
data store
Data Store to Process. i.e.
reading from a data store
Process to Process, i.e
simply passing data on
within the system
External Entity to Data Store
This is the logical equivalent of giving customers and suppliers
direct access to update/delete your files, eg using an editor.
There has to be a process to transfer data from external entity to
data store
Data Store to External Entity
This is the logical equivalent of giving customers and suppliers
direct access to read your files, eg using an editor
Data Store to Data Store
In this case even a direct to file copy is considered to be carried
out by a process
How are DFD’s Constructed?
There are no hard and fast rules and many re-drafts will be necessary
as your understanding improves and new/changed requirements are
identified, however, the following steps may be useful:
•Establish the major inputs and outputs of the system, their sources,
recipients and represent them in a context diagram;
•Establish a process which handles each data flow on its arrival into
the system;
•Establish a process that generates each output data flow;
•Identify the data stores which are required to link the input and output
processes, ie the data stores which need to be read from and written to;
•Rationalise the Level 1 DFD so that it includes 6-10 processes
(This may be done by combining and /or splitting processes;
•For each level 1 process draw a level 2 DFD and if necessary draw
any level 3 DFD’s required;
•Review the entire DFD set against the identified requirements and
re-draft if necessary.
Data Flow Diagram Levels
Level 0
External
Entity
Level 1
Level2
2
1
Process
2.1
Process
Overall
2
Process
Process
2.2
Process
3
External
Entity
Process
2.3
Process
Data Flow Diagram Levels
Level 3
2.2
Level4
2.2.2
2.2.1
Elementary
Process
Process
Description
2.2.2
Process
2.2.3
Process
An Example of a simple DFD
Consider for a moment that you are reading a book, and then
passing the information that you have gained onto a friend.
You have all the required components to construct a simple
DFD:
Process:
You
Data Store:
Book
External Entity:
A Friend
Data Flow:
Book Information
An Example of a simple DFD
Now if we put this diagram together we now have:
M1
Book
(Data Store)
A
Friend
(External Entity)
1
Book
Information
You
Gather and explain
the contents of the
book
(Process)
This is an example of a physical Diagram
Book
Information
An Example of a simple DFD
DFD’s can be used to show the logical view of a situation:
M1
Recipient
1
Communication Data
Communicator
Communicate
It does not matter anymore who or where the data is coming
from or to!
Conclusion
DFD’s can be used to represent any situation a system can present.
They can represent both physical and logical views of a situation.
There are no hard or fast rules for the construction of DFD’s and
Your design will change a number of times.
Normally evolve from the context diagram, (identifying the major
Inputs/outputs).
Each level of your DFD should hold 5 or so process box’s.
Conclusion
There are commonly only 3 levels used.
The lowest level (Level 4) that can be reached is the
Elementary Process Description (EPD’s)
EPD’s commonly use Decision trees, decision tables or
Structured English.
Further Reading:
Systems Analysis Techniques by Mark Lejk and David Deeks,
pages 23 - 57 (Introducing Data Flow Diagrams and
Creating DFD’s)