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

• 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.

A

Customer

Identifier

NameSends data intoor receives datafrom a system

Data Flows

• 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

Customer Details

A route by which datacan flow from one elementto a DFD to another

Processes

• 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.

1. Accts

Check Customer Details

ProcessDescription

LocationIdentifier

Transformationswhich changeincoming data flowsto outgoing data flows

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

• 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

D1 Orders

Identifier Name

A repository for data

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. receivingdata from outside the system

Process to ExternalEntity, i.e. sending data out of the system

Process to Data Store,i.e. writing data to adata store

Data Store to Process. i.e.reading from a data store

Process to Process, i.e simply passing data onwithin the system

External Entity to Data Store

This is the logical equivalent of giving customers and suppliersdirect 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 suppliersdirect 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 carriedout by a process

How are DFD’s Constructed?

There are no hard and fast rules and many re-drafts will be necessaryas your understanding improves and new/changed requirements areidentified, 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 outputprocesses, 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 andre-draft if necessary.

Data Flow Diagram Levels

External Entity

External Entity

Level 0 Level 1 Level2

Overall

Process

1

Process

2

Process

3

Process

2.1

Process

2.2

Process

2.3

Process

2

Data Flow Diagram Levels

Level 3 Level4

Elementary

Process

Description

2.2.2

2.2.1

Process

2.2

2.2.2

Process

2.2.3

Process

An Example of a simple DFDConsider 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 DFDNow if we put this diagram together we now have:

A

Friend

1 You

Gather and explain the contents of the

book

M1 Book

(Data Store)

(Process)

(External Entity)

(Dat

a Fl

ow)

Data Flow)Book

Information

Book Information

This is an example of a physical Diagram

An Example of a simple DFDDFD’s can be used to show the logical view of a situation:

Recipient 1 Communicator

Communicate

M1 Communication Data

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 andYour design will change a number of times.

Normally evolve from the context diagram, (identifying the majorInputs/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)