section 04dfd - top level1 04 data flow diagrams - top level dfd and franchise colleges by mansha...

Section 04 DFD - Top Level 1

04 Data Flow Diagrams

- Top Level DFD

And Franchise Colleges

By MANSHA NAWAZ


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


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


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


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


External Entities

• alias Terminators• square• numbered a..z• named single noun• duplicates indicated by a slash• can connect only to processes

a

Customer


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.


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}


Data Flows

• arrows• headed in direction of flow• named to reflect data carried• connect other components• duplicates possible

neworder


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


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


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


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


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


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


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


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


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


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


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


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


Summary & Self Study

Summary• model of what

happens to data• Yourdon syntax• event = dfd fragment• combine fragments• check with cd & rd

Self Study• review the lecture• complete 4a & b from

the course workbook• can you achieve the

objectives set out in slide 2?

section 04dfd - top level1 04 data flow diagrams - top level dfd and franchise colleges by mansha...

Documents

system slide

level dfd

cd data

process data flow

event slide

set of dfd fragments

data stores

stored data