Section 04DFD - Top Level1 04 Data Flow Diagrams - Top Level DFD And Franchise Colleges By MANSHA NAWAZ

Section 04DFD - Top Level2 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 04DFD - Top Level3 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 04DFD - Top Level4 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 04DFD - Top Level5 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 04DFD - Top Level6 External Entities alias Terminators square numbered a..z named single noun duplicates indicated by a slash can connect only to processes a Customer

Section 04DFD - Top Level7 Processes alias data transform circle numbered 1..n no duplicates named ‘verb-noun’ connects to all must have in and out data flow 1 check stock 1.

Section 04DFD - Top Level8 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 04DFD - Top Level9 Data Flows arrows headed in direction of flow named to reflect data carried connect other components duplicates possible neworder

Section 04DFD - Top Level10 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 04DFD - Top Level11 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 04DFD - Top Level12 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 04DFD - Top Level13 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 04DFD - Top Level14 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 04DFD - Top Level15 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 04DFD - Top Level16 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 04DFD - Top Level17 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 and click on the process for a exploded – fragment view of the process

Section 04DFD - Top Level18 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 04DFD - Top Level19 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 04DFD - Top Level20 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 04DFD - Top Level21 Document : Analysis Specification –Terms Of Reference lecture 02 lecture 02 –Context Diagram lecture 03 lecture 03 Context Diagram Support Text Context Diagram Event List –Top-Level Data Flow Diagram lecture 04lecture 05 lecture 04lecture 05 Top-Level DFD Support Text Top-Level DFD Simple Data Flow Fragments –Low-Level Data Flow Diagrams lecture 06lecture 07 lecture 06lecture 07

Section 04DFD - Top Level22 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?