national diploma in systems analysis and design data flow modelling
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National Diploma
in
Systems Analysis and Design
Data Flow Modelling
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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
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Components of DFD’s
• External Entities
• Data Flows
• Processes
• Data Stores
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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
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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
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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
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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
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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
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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.
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External Entity to Process, i.e. receivingdata from outside the system
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Process to ExternalEntity, i.e. sending data out of the system
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Process to Data Store,i.e. writing data to adata store
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Data Store to Process. i.e.reading from a data store
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Process to Process, i.e simply passing data onwithin the system
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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
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Data Store to External Entity
This is the logical equivalent of giving customers and suppliersdirect access to read your files, eg using an editor
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Data Store to Data Store
In this case even a direct to file copy is considered to be carriedout by a process
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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;
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•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.
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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
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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
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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
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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
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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!
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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.
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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)