06 Data Flow Diagrams - Exploding

Section 06 DFD - Exploding 1

06 Data Flow Diagrams - Exploding

And Franchise Colleges

By MANSHA NAWAZ


Learning Aims• Aim

– to be able to explode a process on a top-level DFD

• Objectives– to distinguish primitive/non primitive processes– to identify appropriate sub-processes– to identify local data stores– to construct the exploded DFD consistent with

the parent DFD


Lecture Outline

• selecting processes for exploding

• initial inputs and outputs

• identifying sub-processes

• local data stores

• diagram construction

• consistency checking

• diagram hierarchy


Checking Processes on the DFD

• check each process, can it be broken down into meaningful sub-processes? – no

• a functional primitive• do not explode

– yes• explode the process

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1

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Inputs & Outputs to Explosions

• each exploded process has its own inputs and outputs

• the data flows into the process must all appear as inputs to the exploded process

• the data flows out of the process must all appear as outputs from the exploded process


Modelling Sub-processes

• the task is to identify all the sub-processes that must take place to achieve the purpose of the parent process

• start by identifying the normal sequence of events– list the processes

• identify the error or abnormal events– add to the process list


Local Data Stores

• it may become apparent that a new data store is required– if only the sub-processes interface with it

then add it to the exploded diagram– if other processes use it, add to the top

level

• if a data store on the top level DFD only interfaces with one process– remove it and place inside the exploded

process


Diagram Construction

• the exploded process is represented by a large frame, named and numbered

• on the left outside are placed the inputs

• on the right outside are placed the outputs

• sub-processes & local data stores are placed inside the frame


Contd.

• add the processes connecting them to the in/outputs

• numbering the processes– n.1, n.2 etc where n is the number of the

parent

• numbering the data stores– continue the sequence from the top level

• make sure all inputs & outputs are connected to the sub-processes


Consistency Checking

• check numbering correct

• check no name clashes

• check number of inputs and outputs are at least as many as those on the top level

• if there are more check they are logically the same– flow duplication and splitting is possible


Diagram Hierarchy

• it is possible to explode the processes in an exploded diagram

• this leads to a hierarchy of levelled DFDs


Development : Low-Level DFD – Low-Level Data Flow Diagram (LL-DFD)

• For each process – Low-Level DFD description– Low-Level DFD Diagram

– Top-Level Data Flow Diagram (TL-DFD)• Amend your diagrams and supporting text after consistency

checking with the low-level DFD’s.– Top-Level DFD description– Top-Level DFD Diagram

– Context Diagram (CD-DFD)• Amend your diagrams and supporting text after consistency

checking with the low-level DFD’s.– Context Diagram description– Context Diagram


Document : Analysis Specification – Terms Of Reference lecture 02

– Context Diagram lecture 03

– Top-Level Data Flow Diagram lecture 04

• Top-Level DFD Support Text lecture 05

• Top-Level DFD lecture 06

• Data Flow Fragments

– Low-Level Data Flow Diagrams lecture 06 lecture 07

For each Top-Level process

• Low-Level DFD Support Text• Low-Level DFD


Summary & Self Study

Summary• check processes• capture in/outputs• check local data

stores• build DFD• check consistent• levelled hierarchy

Self Study• review the lecture• complete 6a from the

workbook • can you achieve the

objectives set out in slide 2?


Further Reading

• http://outranet.scm.tees.ac.uk/users/u0018257/SDV/SDevWk11.doc

There are many ways to develop a proposed computer system model… see what you can utilise from the following guides

Guide to documenting DFDs using Toy Sales Order Example Toy Sales Ordering System

Guide to modelling multilayered systems DFD Review 1

Guide to modelling basic systems DFD Review 2

Common Functions, Features, Events and Sub-Events SOP

Common Functions, Features, Events and Sub-Events from PowerQuotes Case Study

powerquote

Sample DFD Models Ascent Solutions


Further Reading

• Notes from Annette Marshall



http://outranet.scm.tees.ac.uk/users/u0018257/SDV/SDevWk3.doc




06 Data Flow Diagrams - Exploding

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