section 04dfd - top level1 04 data flow diagrams - top level dfd and franchise colleges by mansha...
TRANSCRIPT
Section 04 DFD - Top Level 1
04 Data Flow Diagrams
- Top Level DFD
And Franchise Colleges
By MANSHA NAWAZ
Section 04 DFD - Top Level 2
Learning Aims
• Aim– to be able to construct a top level data flow
diagram for a simple information system
• Objectives– identify processes from an event list– create a set a dfd fragments– combine the fragments consistent with the
context diagram
Section 04 DFD - Top Level 3
Lecture Outline
• the nature & purpose of DFDs
• syntax for DFDs
• identifying system components
• constructing a DFD fragment
• combining dfd fragments
• checking for consistency with the CD
Section 04 DFD - Top Level 4
Data Flow Diagrams
• a DFD is a graphical model of the proposed system– some methods model the actual system
• it represents what happens to data within the system not the actual flow of physical objects
Section 04 DFD - Top Level 5
DFD Syntax Reminder
• there a numerous notations
• the rules are essentially the same
• we shall use that of YOURDON
• once mastered you should have no difficulty drawing or understanding another notation
• do not mix notations on one diagram
Section 04 DFD - Top Level 6
External Entities
• alias Terminators• square• numbered a..z• named single noun• duplicates indicated by a slash• can connect only to processes
a
Customer
Section 04 DFD - Top Level 7
Processes
• alias data transform• circle• numbered 1..n• no duplicates• named ‘verb-noun’• connects to all• must have in and out data flow
1
checkstock
1.
Section 04 DFD - Top Level 8
Data Store
• parallel lines• numbered 1..n• named pural noun• duplicates indicated by
vertical line• connect to process• data flow in = write• data flow out = read
1 orders
{reading}
{writing}
Section 04 DFD - Top Level 9
Data Flows
• arrows• headed in direction of flow• named to reflect data carried• connect other components• duplicates possible
neworder
Section 04 DFD - Top Level 10
Identifying Components I
• External Entities... on the CD– things outside the system with which it interacts
• Processes... on the Event List– things inside the system that must be done
• Data Stores... from fact finding– information held inside the system over time
Section 04 DFD - Top Level 11
Identifying Components II
• Data Flows
• Inputs... on the CD– data into the system from external entities
• Outputs... on the CD– data out of the system to external entities
• Internal... from fact finding– data moved around inside the system
Section 04 DFD - Top Level 12
Building the DFD fragments
• there are many ways to build the dfd
• no-one gets it right first time– be prepared to review and rewrite
• this method is called event partitioning
• the starting point is the context diagram & event list
Section 04 DFD - Top Level 13
Fragments Contd.
• for each event on the Event List
• draw a process, number & name it
• if that process interacts with a terminator add it
• connect the terminator and process with the appropriate data input or output
Section 04 DFD - Top Level 14
Fragments Contd.
• if that process reads or writes to needed stored data add a data store, number & name it
• connect the process to the store with a data flow, name it
• You should now have a set of DFD fragments, one for each event
Section 04 DFD - Top Level 15
Fragments Contd.
• DFD Fragments are working out
• Drafts to help us build the system
• Generally paper based
• You may decide to model and document each fragment
Section 04 DFD - Top Level 16
Combining the Fragments
• The top level DFD is created by joining the fragments together
• usually this is done through the data stores
• try to place the terminators on the outside of the diagram
• place the stores centrally
• fit the processes accordingly
Section 04 DFD - Top Level 17
The Top-Level DFD
• The Top-Level DFD is the diagram of interest.
• Best modelled in Ascent
• DFD Fragment views are obtained by exploding each process to a lower level
• Ascent menu <level><explode-process> and click on the
process for a exploded – fragment view of the process
Section 04 DFD - Top Level 18
Consistency Checking with the Context Diagram
• external entities on the context diagram must match those on the dfd– numbers and names must be the same– an external entity on the context diagram may
be duplicated on the DFD
• in/outputs on the context diagram must match those on the dfd– names must be the same
Section 04 DFD - Top Level 19
Validating the DFD
• check the diagram against the Terms of Reference and in particular update the Requirements List
• at this stage of your knowledge – check that your model carries out any stated
calculations – check that your model produces the required
outputs
Section 04 DFD - Top Level 20
Development : Top-Level DFD
– Top-Level Data Flow Diagram (DFD)• Provide a supporting text description of the
system components view as a introduction to the Top-Level DFD
• Provide the Top-Level DFD• Provide the DFD Fragments view
– Explode each process to the lower level for a fragment view.
– Modelling the exploded process view gives us the low-level DFD’s
– We cover low-level DFD in section 06 and 07
Section 04 DFD - Top Level 21
Document : Analysis Specification – Terms Of Reference lecture 02
– Context Diagram lecture 03
• Context Diagram Support Text• Context Diagram • Event List
– Top-Level Data Flow Diagram lecture 04 lecture 05
• Top-Level DFD Support Text• Top-Level DFD• Simple Data Flow Fragments
– Low-Level Data Flow Diagrams lecture 06 lecture 07