Logical Dataflow Diagrams (DFDs)
Vocabulary of DFDs Context DFD Physical DFD Logical DFD Bubble Square The System Internal Entity External Entity External Entity External Entity Process N/AFile Data Flow Data Flow Data Flow or “connector”
Guidelines for DFDs The idea is that we would like to group activities that “belong together” into processes. Note that a process is a just set of related activities. The SYSTEM itself is just a set of activities, too. There are no ENTITIES now.
Guidelines for DFDs The approach is easy - –Sort the list of activities chronologically - I usually just write down the numbers in chronological order (we actually already did this) –Break it into processes by deciding upon “division points” between processes. This is TOTALLY up to you. There is not necessarily a right answer. The only rule is that every process must have at least ONE activity. –Write F over each number that is a file activity and E over each number that is a data flow to or from an external entity. Do not worry about data flows between internal entities.
Guidelines for DFDs An Example - Causeway E F F F E F E 1, 2, 3, 4, 5, 6, 7, 8, 17, 13, 14, 15, 16, 11, 12, 18, 19, 9, 10
Guidelines for DFDs You would decide how many processes there were by intuition (but you do not need to worry about that for this class). For real life situations, the general rule of thumb is 5 to 7 bubbles. You will typically have between 3 and 4 for this class. There would be one bubble for each process and they would be numbered 1.0, 2.0, 3.0, etc. (again, these will be given for this class). Draw arrows from bubble 1.0 to bubble 2.0 and from bubble 2.0 to 3.0, etc. THESE WILL NOT BE LABELED. Draw necessary arrows to and from the “appropriate bubble” to and from (1) external entities and (3) files.
Causeway 1.0 Open Mail 2.0 Record A/R 3.0 Prepare Deposit 4.0 Complete Cash Receipts Customers Bank G/L Office Txn Data Acc Rec. E F F F E F E 1, 2, 3, 4, 5, 6, 7, 8, 17, 13, 14, 15, 16, 11, 12, 18, 19, 9, 10 Checks & R/As Deposit Total of C/R 10