Data Flow Diagrams DFDs Data Flow Diagrams

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					Data Flow Diagrams (DFDs)


   Analysis tool
   System modeling
   Excellent documentation tool
   Data flow, aka, business process
   Provide a way to see the pattern
       Systems Thinking
       Systems View
       Systems Approach
Data Flow Diagrams (DFDs) -
continued

 Advantages for systems view,
  working with users:
     Four simple symbols:

  (1) External Entity
     - source and
    destination of
                             or
    data


  (2) Process                or


  (3) Data Store        #




  (4) Data Flow
Data Flow Diagrams (DFDs) -
continued

 Simple rule:
     left to right, top to bottom,
      basically (like English)
     Show all processes
     data flows must start or end in a
      process
     beware of black holes - every
      process and data store must
      produce data
     no “magic processes” - must enter
      all needed data to get data out
Data Flow Diagrams (DFDs) -
continued



     * Key: Simple
        Symbols

     Most common
     error: Trying to
       do too much
        DFD Example
In the following example, a DFD will be
created based on a paragraph about Bebop
Records Co.’s method of receiving and
filling orders.

In this example, the information necessary
to create the DFD is provided for us. In
Real Life, this information comes from
extensive interviewing, document
gathering, and other research.

Depending upon the system being studied,
the number of people working on it, etc.,
the development of a detailed DFD can
take anywhere from a few hours to several
months (paid for by your client with NO
CODE TO SHOW FOR IT). Yet it is a
very standard and meaningful part of
system documentation.
DFD Example
Bebop Records is a mail-order company that
distributes CDs and tapes at discount price to
record-club members. When an order
processing clerk receives an order form, he or
she verifies that the sender is a club member
by checking the Member file. If the sender is
not a member, the clerk returns the order along
with a membership application form. If the
customer is a member, the clerk verifies the
order item data by checking the Item file. Then
the clerk enters the order data and saves it to
the Daily Orders file. The clerk also prints an
invoice and shipping list for each order, which
are forwarded to Order Fulfillment.
DFD Example - Step 1


FIRST SENTENCE:
Bebop Records is a mail-order company that
  distributes CDs and tapes at discount price
  to record-club members.

RESULT: initial title


            Bebop Records
DFD Example - Step 2

SECOND SENTENCE:
When an order processing clerk receives an
  order form, he or she verifies that the
  sender is a club member by checking the
  Member file.

RESULT:
    Bebop Records Order Processing
                              1
  EE-1
                 order   Verify member   member   D-1 Member Master
  Record Club                 status      data
    Member                 OR Clerk




  EE-2

  Customer
  (non-member)
DFD Example - Step 2 continued


SECOND SENTENCE:
When an order processing clerk receives an
  order form, he or she verifies that the
  sender is a club member by checking the
  Member file.

RULES FOLLOWED:
 left to right, top to bottom
 data flows start or end in a process
DFD Example - Step 3

THIRD SENTENCE:
If the sender is not a member, the clerk
    returns the order along with a membership
    application form.

RESULT:
    Bebop Records Order Processing

                                        1
  EE-1
                   order        Verify member   member   D-1 Member Master
  Record Club                        status      data
    Member                         OR Clerk




  EE-2

  Customer
  (non-member)    non-member order
                 and application form
DFD Example - Step 3 continued

THIRD SENTENCE:
If the sender is not a member, the clerk
    returns the order along with a membership
    application form.


RULES FOLLOWED:
 shows all processes without conditions
DFD Example - Step 4
FOURTH SENTENCE:
If the customer is a member, the clerk verifies
    the order item data by checking the Item
    file.

RESULT:
    Bebop Records Order Processing
                                       1
 EE-1
                  order        Verify member          member   D-1 Member Master
  Record Club                       status             data
    Member                        OR Clerk




                                             member
 EE-2                                         order

 Customer
 (non-member)    non-member order
                and application form

                                       2

                                 Verify order          item    D-2 Item Master
                                  item data            data

                                  OR Clerk
DFD Example - Step 4 continued

FOURTH SENTENCE:
If the customer is a member, the clerk verifies
    the order item data by checking the Item
    file.


RULES FOLLOWED:
 left to right, top to bottom
 data flows start or end in a process
 shows all processes without conditions
DFD Example - Step 5

FIFTH SENTENCE:
Then the clerk enters the order data and saves
  it to the Daily Orders file.

RESULT:
DFD Example - Step 5 continued
RESULT:
    Bebop Records Order Processing

                                       1
  EE-1
                  order        Verify member          member   D-1 Member Master
  Record Club                       status             data
    Member                        OR Clerk




                                             member
 EE-2                                         order

 Customer
 (non-member)    non-member order
                and application form

                                       2

                                 Verify order          item    D-2 Item Master
                                  item data            data

                                  OR Clerk

                                       verified
                                       member
                                        order


                                       3

                                 Enter order
                                                      order    D-3   Daily Orders Detail
                                  into Daily
                                    Orders
                                  OR Clerk
DFD Example - Step 5 continued

FIFTH SENTENCE:
Then the clerk enters the order data and saves
  it to the Daily Orders file.

RULES FOLLOWED:
 left to right, top to bottom
 data flows start or end in a process
DFD Example - Step 6

SIXTH SENTENCE:
The clerk also prints an invoice and shipping
  list for each order, which are forwarded to
  Order Fulfillment.

RESULT:
DFD Example - Step 6 continued
RESULT:
    Bebop Records Order Processing
                                       1
 EE-1
                  order        Verify member          member          D-1 Member Master
 Record Club                        status             data
   Member                         OR Clerk




                                             member
 EE-2                                         order

 Customer
 (non-member)    non-member order
                and application form

                                       2

                                 Verify order          item           D-2 Item Master
                                  item data            data

                                  OR Clerk

                                       verified
                                       member
                                        order


                                       3

                                 Enter order
                                                      order           D-3   Daily Orders Detail
                                  into Daily
                                    Orders
                                  OR Clerk

                                                              order
                                                              data



                                       4

                                Print invoice                                  EE-3
                                                       invoice and
                                     and               shipping list           Order
                                shipping list
                                                                               Fulfillment
                                 OR Clerk
DFD Example - Step 6 continued

SIXTH SENTENCE:
The clerk also prints an invoice and shipping
  list for each order, which are forwarded to
  Order Fulfillment.

RULES FOLLOWED:
 left to right, top to bottom
 data flows start or end in a process
 shows all processes without conditions
 beware of black holes - every process and
  data store must produce data