Chapter 9 - Using Data Flow Diagrams

1. The Data Flow Approach to Requirements Determination

   Advantages of the Data Flow Approach

   1. Freedom from committing to the technical implementation of the system too early.
   2. Further undrstanding of the interrelatedness of systems and subsystems.
   3. Communicating current system knowledge to users through data flow diagrams.
   4. Analysis of aproposed system to determie if the necessary data and processes have been defined.

   Conventions Used in Data Flow Diagrams

   1. Assign the name of the whole system, when naming a high-level process, i.e. INVENTORY CONTROL SYSTEM.
   2. To name a major subsystem, use a name like INVENTORY REPORTING SUBSYSTEM.
   3. Use a verb-adjective-noun format for detailed processes.
   4. Use a noun or adjective-noun for entities and data stores.

2. Developing Data Flow Diagrams

   1. Make a list of business activities and use it to determine various
      • external entities
      • data flows
      • processes
      • data stores
   2. Create a context diagram which shows external entities and data flows to/from the system. Do not show any detailed processes or data stores.
   3. Draw diagram 0, the next level. Show processes, but keep them general. Show data stores at this level.
   4. Create a child diagram for each of the processes in Diagram 0.
   5. Check for errors and make sure the labels you assign to each process and data flow are meaningful.
   6. Develop a physical data flow diagram from the logical data flow diagram. Distinguish between manual and automated processes, describe actual files and reports by name, and add controls to indicate which processs are complete or errors occur.
   7. Partition the physical data flow diagram by separating or grouping parts of the diagram in order to facilitate programming and implementation.

3. Logical and Physical Data Flow Diagrams

   Design Features	Logical	Physical
   What the model depicts	How the business operates	How the system will be implemented (or how the current system operates)
   What the processes represent	Business activities	Programs, program modules, and manual processes
   What the data stores represent	Collections of data, regardless of how the data is stored	Physical files and databases, manual files
   Type of data stores	Show data stores representing permanent data collections	Master files, transaction files. Any processes that operate at two different times must be connected by a data store.
   Systems controls	Show business controls	Show controls for validating input data, for obtaining a record (record found status), for ensuring successful completion of a process, and for system security (i.e. journal records)

   Developing Logical Data Flow Diagrams

   1. better communication with users
   2. more stable systems
   3. better understanding of the business by analysts
   4. flexibility and maintenance
   5. elimination of redundancies and easier creation of the physical model

   Developing Physical Data Flow Diagrams

   1. clarifying which processes are manual and which are automated
   2. describing processes in more detail than do logical DFDs
   3. sequencing processes that have to be done in a particular order
   4. identifying temporary data stores
   5. specifying actual names of files and printouts
   6. adding controls to ensure the processes are done properly

4. Partitioning Data Flow Diagrams

   reasons for partitioning DFDs
   1. partition if different user groups
   2. partition if processes execute at different times
   3. combine if two processes perform similar tasks
   4. combine for efficient processing
   5. combine for consistency of data
   6. partition for security reasons

5. A Data Flow Diagram Example - FilmMagic [Fig. 9-13, 9.14, 9.15]

6. Creating a Physical Data Flow Diagram [Fig. 9.16, 9.17, 9.18]

7. A Second Data Flow Diagram Example - World's Trend Catalog Division [Fig. 9.20, 9.21, 9.22, 9.23, 9.24]

8. Using Data Flow Diagrams

   - when to stop?

Transparencies

Exercises (pg. 273): Problems #1, 4(due - noon Oct. 20, 1998)
e-mail your answers to summers_wayne@ColumbusState.edu
Explore the MRE HyperCase company and e-mail the answers to the questions on page 272[This can be done in a group of 2-3]