05 data flow diagrams - completing the top-level

Section 05 DFD - Complete Top-Level 1

05 Data Flow Diagrams

- Completing the top-level

And Franchise Colleges

By MANSHA NAWAZ


Learning Aims

• Aim– to be able to complete a top-level DFD

• Objectives– model all read/write access to data stores– model abnormal situations– to check consistency between diagrams– to validate against requirements


Lecture Outline

• Read/Write access to data stores

• Error and abnormal situations

• Consistency checking

• Validation

• Structured Walkthroughs


Read Access to Data Stores

• all data stores must be read at least once– else why have them?

• check each data store, if not read– has it been missed?– is it needed?

• note that duplicates are the same store


Write Access to Data Stores

• all data stores must be written to at least once– else how does the data get in there?

• check each data store, if not written to– has it been missed?– add process to input initial/change of data


Error & Abnormal Situations

• check the DFD to see if all reject/fail situations are modelled– eg existing library member tries to join

library

• check to see if appropriate success/fail messages are sent to user

• check to see if any data sent out of system for manual checking can be re-activated


Consistency Checking

• Changes made to the top-level DFD might make it inconsistent with the Context Diagram

• Check the Context Diagram and...– add any new external entities– add any new inputs– add any new outputs– add any new events to the event list


Validation with the Requirements Document

• Check each functional requirement in the Specification Document– is it modelled on the DFD?

• Check the DFD for extra functionality over the requirements– if needed add to Spec’ Doc.

• Add traceability comments to both documents


Structured Walkthroughs

• Systems are rarely built by one person

• Large systems are built by teams

• A mechanism is needed for peer review– check on quality

• Helps to spot errors early

– check on progress• Insurance against personnel loss


Procedure for SW

• Author of dfd distributes material in advance of meeting

• In meeting author talks through the diagram

• peers comment– identify correct parts– identify errors or unsure (DON’T SOLVE)

• Action list drawn up for author to do


Personnel for a SW

• Author

• Chair– runs the meeting

• Scribe– admin support

• Peers

• About 6 people is best

• Qualified & knowledgeable


Merits for SW

• helps for better and cheaper product

• train junior staff• team building


Problems with SW

• increases

development time• take criticism ?• meetings need good

chair so not waste time


Development : Top-Level DFD – Top-Level Data Flow Diagram (TL-DFD)

• Amend your diagrams and supporting text after consistency checks.

• Top-Level DFD description• Top-Level DFD Diagram• DFD Fragments view

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

consistency checking with the Top-Level DFD’s.– Context Diagram description– Context Diagram


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

• Top-Level DFD Support Text lecture 05

• Top-Level DFD lecture 06

• Simple Data Flow Fragments

– Low-Level Data Flow Diagrams lecture 06 lecture 07


Summary & Self Study

Summary• add read/write access• add error events• check consistent• validate• SW to review• find errors early

Self Study• review the lecture• complete 5a from the

course workbook • can you achieve the

objectives set out in slide 2?

05 data flow diagrams - completing the top-level

Documents