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					The Data Flow Diagram




                        1
     The Data Flow Diagram
• A graphic tool used to portray the flow of
  data through a system.
• For documenting the old system as well as
  beginning to create the new one.
• Shows a highly useful partitioning of the
  system into tasks (activities, functions) and
  subtasks.
  – Smaller problems are easier to handle.
  – “Leveling” allows top down reasoning.
                                              2
     The Data Flow Diagram
• Looks at the system from point of view of a
  single piece of data.
  – Not reiterative -- no loops shown.
  – As a result, we cannot program directly from a
    DFD.




                                                 3
    The Data Flow Diagram
• Four symbols:
  – terminator
  – data store
  – process bubble
  – data flow




                            4
                 Symbols
• Terminator
  – Person or organization that lies
    outside the system and that is a net   EMPLOYEE
    originator or receiver of data.

   – Key - outside the area of our concern
     and control.



                                                 5
               Symbols
– Source (originator of data) or sink (receiver
  of data).
– Prime sources on the left side of the DFD,
  prime sinks to right.
– Name inside box.
– Also called an external entity.




                                                  6
                Symbols
                               EMPLOYEE
• Data store (file)
  – Same as the data store in the data
    dictionary.
  – Could be a computer file, card file, file
    cabinet, etc.
  – Note that EMPLOYEES here is the data
    store that contains the employee
    information, while EMPLOYEE (the
    terminator) is the actual person.
                                                7
                  Symbols
                                                 1
                                              PRODUCE-
                                             EMPLOYEE-
• Process (bubble, transform)                PAYCHECK
  – An activity, task, function, etc.
  – Shows work being done against the data.
    • Transforms incoming data into outgoing data.
    • Changes status (logical) or content, format, or
      medium (physical)




                                                         8
                 Symbols
– Each bubble has a unique number and name.
  • The name must be an active verb followed by
    object clause:
         » EDIT-CUSTOMER-PAYMENT
         » WRITE-PAYMENT-REPORT
  • If no active verb, it’s not a process!




                                                  9
                   Symbols
– Partitioning:
   • Breaking into smaller bubbles.
   • Hard to choose bubble boundaries.
   • Black box--a process for which we know the net
     inputs, net outputs, and general function, but we
     don't know exactly how that function is
     accomplished.
      – Highly useful--can be fully exploited without knowledge of
        what is inside the black box, since it behaves predictably.




                                                                 10
                  Symbols
                                  DATA-FLOW-NAME
• Data flow
  – The data interface between bubbles, terminators,
    and data stores.
  – Should be a packet of logically related data items.
     • good--CUSTOMER-PAYMENT-TRANSACTION
     • bad--MISCELLANEOUS-STUFF
  – No excess data passed around.
     • Tramp data is not acceptable.
     • Data flows should be lean and mean.




                                                          11
                  Symbols
– Arrows show direction of data movement.
– Into and out of a data store...


  Write to data store               Read from data store


                        EMPLOYEES


    • The access to a data store (request or key) is
      not shown, only the net result.                      12
                  Symbols
– Naming
  • Unique, descriptive.
  • Data dictionary naming conventions (because all of
    these names need to be in the DD, too).
  • No loops, so never GET-NEXT-CUST.
  • No flags.
  • Avoid vague names like -INFO, -DATA.
     – Can usually (but not always) be more specific.
     – Real test--can you write a DD entry?



                                                        13
        Context Level DFD
• Upper-most level, most abstract view of
  system.
• The “outside” view of the system.
• Shows a single process bubble, the net
  inputs and outputs of entire system, and
  the terminators with which they
  communicate.
• Purpose is to delineate the domain
  (scope) of the system.
                                             14
                                                               Context Diagram



                                         PAYROLL-AUDIT-TRAIL
                                         EMPLOYEE-MAINTENANCE-
                                         AUDIT-TRAIL

                                                                   MANAGEMENT
           EMPLOYEE-MAINTENANCE-
           TRANSACTION
           EMPLOYEE-HOURS-WORKED-
           TRANSACTION                             EMPLOYEE-PAY-RATE-
                                                   TRANSACTION
EMPLOYEE                           PAYROLL

                                                   GENERAL-LEDGER-
           EMPLOYEE-PAYCHECK
                                                   ACCOUNT-NUMBER



                                                                    GENERAL-
                                                                     LEDGER

                                             PAYROLL-VOUCHER
                                             PAYROLL-AUDIT-TRAIL
                                                                                15
                                                                                     15
                            Clearly labeled                 Context Diagram
                          “Context Diagram”
                                                               Terminator

                         Process bubble
                                   PAYROLL-AUDIT-TRAIL

 Terminator                              EMPLOYEE-MAINTENANCE-
                                         AUDIT-TRAIL

                                                                   MANAGEMENT
           EMPLOYEE-MAINTENANCE-
           TRANSACTION
           EMPLOYEE-HOURS-WORKED-
           TRANSACTION                             EMPLOYEE-PAY-RATE
                                                   EMPLOYEE-PAY-RATE-
                                                   TRANSACTION
EMPLOYEE                           PAYROLL
                                                                    Terminator
                                                   GENERAL-LEDGER-
           EMPLOYEE-PAYCHECK
                                                   ACCOUNT-NUMBER



                                                                    GENERAL-
                                                                     LEDGER
       Data flows...                         PAYROLL-VOUCHER
                                             PAYROLL-AUDIT-TRAIL
                                                                                16
                                                                                     16
        Context Level DFD
• We will go through each of the four
  components (bubble, data flow, data store,
  terminator) with each diagram.




                                           17
                                                            Context Diagram
 • Process bubbles
   • Here, just one, which represents the
     entire system.
   • Numbered 0, or number is omitted.
                               PAYROLL-AUDIT-TRAIL
   • Anything wrong here? EMPLOYEE-MAINTENANCE-
                                         AUDIT-TRAIL

                                                                   MANAGEMENT
           EMPLOYEE-MAINTENANCE-
           TRANSACTION
           EMPLOYEE-HOURS-WORKED-
           TRANSACTION                             EMPLOYEE-PAY-RATE-
                                                   TRANSACTION
EMPLOYEE                           PAYROLL

                                                   GENERAL-LEDGER-
           EMPLOYEE-PAYCHECK
                                                   ACCOUNT-NUMBER



                                                                    GENERAL-
                                                                     LEDGER

                                             PAYROLL-VOUCHER
                                             PAYROLL-AUDIT-TRAIL
                                                                                18
                                                                                     18
• Terminators                                             Context Diagram
  • Remember, they are outside of our control.
  • In this case, each terminator is both a source
    and a sink.
  • Prime sources on the left and prime sinks on
                                      PAYROLL-AUDIT-TRAIL
                                      EMPLOYEE-MAINTENANCE-
    the right.                        AUDIT-TRAIL
        •Can also show above and below.                     MANAGEMENT
  • Shown here, then never again on lower levels.
               EMPLOYEE-MAINTENANCE-
               TRANSACTION
               EMPLOYEE-HOURS-WORKED-
               TRANSACTION                          EMPLOYEE-PAY-RATE-
                                                    TRANSACTION
    EMPLOYEE                        PAYROLL

                                                    GENERAL-LEDGER-
               EMPLOYEE-PAYCHECK
                                                    ACCOUNT-NUMBER



                                                                    GENERAL-
                                                                     LEDGER

                                              PAYROLL-VOUCHER
                                              PAYROLL-AUDIT-TRAIL
                                                                               19
                                                                                    19
• Data stores                                                      Context Diagram
  • Internal to our system, so none on
    this level ever.

                                             PAYROLL-AUDIT-TRAIL
                                             EMPLOYEE-MAINTENANCE-
                                             AUDIT-TRAIL

                                                                       MANAGEMENT
               EMPLOYEE-MAINTENANCE-
               TRANSACTION
               EMPLOYEE-HOURS-WORKED-
               TRANSACTION                             EMPLOYEE-PAY-RATE-
                                                       TRANSACTION
    EMPLOYEE                           PAYROLL

                                                       GENERAL-LEDGER-
               EMPLOYEE-PAYCHECK
                                                       ACCOUNT-NUMBER



                                                                        GENERAL-
                                                                         LEDGER

                                                 PAYROLL-VOUCHER
                                                 PAYROLL-AUDIT-TRAIL
                                                                                    20
                                                                                         20
                                                                   Context Diagram
• Data flows
  • Long, descriptive, singular names.
  • A "packet" of logically related data.
                                             PAYROLL-AUDIT-TRAIL
                                             EMPLOYEE-MAINTENANCE-
                                             AUDIT-TRAIL

                                                                       MANAGEMENT
               EMPLOYEE-MAINTENANCE-
               TRANSACTION
               EMPLOYEE-HOURS-WORKED-
               TRANSACTION                             EMPLOYEE-PAY-RATE-
                                                       TRANSACTION
    EMPLOYEE                           PAYROLL

                                                       GENERAL-LEDGER-
               EMPLOYEE-PAYCHECK
                                                       ACCOUNT-NUMBER



                                                                        GENERAL-
                                                                         LEDGER

                                                 PAYROLL-VOUCHER
                                                 PAYROLL-AUDIT-TRAIL
                                                                                    21
                                                                                         21
                                                                   Context Diagram
• Anything wrong?



                                             PAYROLL-AUDIT-TRAIL
                                             EMPLOYEE-MAINTENANCE-
                                             AUDIT-TRAIL

                                                                       MANAGEMENT
               EMPLOYEE-MAINTENANCE-
               TRANSACTION
               EMPLOYEE-HOURS-WORKED-
               TRANSACTION                             EMPLOYEE-PAY-RATE-
                                                       TRANSACTION
    EMPLOYEE                           PAYROLL

                                                       GENERAL-LEDGER-
               EMPLOYEE-PAYCHECK
                                                       ACCOUNT-NUMBER



                                                                        GENERAL-
                                                                         LEDGER

                                                 PAYROLL-VOUCHER
                                                 PAYROLL-AUDIT-TRAIL
                                                                                    22
                                                                                         22
         Context Level DFD
• Duplicate data flow names acceptable if
  two or more identical copies of the same
  item going to two or more destinations.
  – To show how the system relates to the world,
    we must show each copy.
  – On level below, treat as a single data flow.
     • Whether one or multiple copies is irrelevant except
       to outside world; we process the same regardless.


                                                         23
                 Leveling
• If a system is too large to be shown on a
  single diagram, break into subsystems and
  sub-subsystems.
• Called leveling or top-down partitioning.
• Each partitioning (breaking up) of a bubble
  to a lower level is done to show more
  detail.
  – Called an explosion in engineering
    terminology.
                                           24
                 Leveling
• Parent/child relationship
  – A parent bubble can have a child diagram.
• How do we decide upon partitioning
  boundaries?
  – Use the same techniques as when partitioning
    programs into subroutines.




                                                25
        Overview Diagram
• Child of the single bubble on the Context
  Diagram.
• Shows major functions, major data stores
  and major data flows.




                                          26
                                                     Overview Diagram


                                                              PAYROLL-AUDIT-TRAIL
EMPLOYEE-HOURS-WORKED-TRANSACTION
                                        1
                                     PRODUCE-                   PAYROLL-VOUCHER
GENERAL-LEDGER-ACCOUNT-NUMBER       EMPLOYEE-
                                    PAYCHECK                  EMPLOYEE-PAYCHECK




                                    EMPLOYEES




EMPLOYEE-MAINTENANCE-TRANSACTION
                                        2
                                                EMPLOYEE-MAINTENANCE-AUDIT-TRAIL
                                    MAINTAIN-
EMPLOYEE-PAY-RATE-TRANSACTION       EMPLOYEE-
                                     RECORD



                                                                             27
                                                                                  27
                                                          Overview Diagram


                                                                  PAYROLL-AUDIT-TRAIL
EMPLOYEE-HOURS-WORKED-TRANSACTION
                                         1
                                      PRODUCE-                      PAYROLL-VOUCHER
GENERAL-LEDGER-ACCOUNT-NUMBER        EMPLOYEE-   • Process bubbles
                                     PAYCHECK                      EMPLOYEE-PAYCHECK
                                                   • Here, two major
                                                      functions (bubbles).
                                                   • May have up to seven
                                    EMPLOYEES
                                                      bubbles on a
                                                      diagram.
                                                   • What about 1
EMPLOYEE-MAINTENANCE-TRANSACTION                      bubble?
                                         2
                                     MAINTAIN-
                                                   • EMPLOYEE-MAINTENANCE-AUDIT-TRAIL
                                                      Numbered 1, 2, 3, etc.
EMPLOYEE-PAY-RATE-TRANSACTION        EMPLOYEE-
                                      RECORD
                                                   • Names have an active
                                                      verb & object clause.
                                                   • Avoid vague verbs 28
                                                      like PROCESS.                 28
         Overview Diagram
• Partition the Overview Diagram based on:
  – Different major functions.
     • Don’t put trivial functions (like EDIT, FORMAT,
       WRITE, etc.) on Overview.
  – Different major inputs.
  – Different time frames.
  – Different equipment.

  – Note: know all four of these criteria for tests.
                                                         29
                                                         Overview Diagram


                                                                  PAYROLL-AUDIT-TRAIL
EMPLOYEE-HOURS-WORKED-TRANSACTION
                                         1
                                      PRODUCE-                      PAYROLL-VOUCHER
GENERAL-LEDGER-ACCOUNT-NUMBER        EMPLOYEE-
                                     PAYCHECK                     EMPLOYEE-PAYCHECK




                                                 • Terminators
                                    EMPLOYEES


                                                   • Shown only on Context,
                                                     so not here.
EMPLOYEE-MAINTENANCE-TRANSACTION
                                         2
                                                    EMPLOYEE-MAINTENANCE-AUDIT-TRAIL
                                     MAINTAIN-
EMPLOYEE-PAY-RATE-TRANSACTION        EMPLOYEE-
                                      RECORD



                                                                                 30
                                                                                      30
                                                          Overview Diagram


                                                                  PAYROLL-AUDIT-TRAIL
EMPLOYEE-HOURS-WORKED-TRANSACTION
                                         1
                                      PRODUCE-                      PAYROLL-VOUCHER
GENERAL-LEDGER-ACCOUNT-NUMBER        EMPLOYEE-
                                     PAYCHECK                     EMPLOYEE-PAYCHECK



                  Read
                                    EMPLOYEES
                                                 • Data flows
                                                   • Unique, descriptive
                  Write                              names, generally long.
EMPLOYEE-MAINTENANCE-TRANSACTION
                                                   • No reiteration, flags,
                                         2
                                     MAINTAIN-
                                                     or decisions.
                                                     EMPLOYEE-MAINTENANCE-AUDIT-TRAIL
EMPLOYEE-PAY-RATE-TRANSACTION        EMPLOYEE-
                                      RECORD
                                                   • Show direction of
                                                     data flow into/out of
                                                     data stores.                31
                                                                                    31
         Overview Diagram
• No labels on data flows into and out of
  data stores when using the entire record.
  – Always need to use the entire record on a
    write, so writes are never labeled.
  – On reads, if using just one or two fields, then
    label as such.




                                                      32
         Overview Diagram
• Placement of data flows
  – Try to move left to right, top to bottom if
    possible.
  – Inputs and outputs to edge of page.
  – Avoid line crossings by rearranging.


• Stop Here


                                                  33

				
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