System Design Example
=====================

1. System to be described:
   ----------------------

   A printer system used by students to print projects, homeworks, etc.
   Students approach a room where the printer resides and where an employee
   hands out a print job submitted by students.  Students submit requests from
   printer requests consoles.





   Diagram of the System:

                                Technician
                                  
                                    | 
                                    |
                                    v

           Employee ----->  Printer Controller ----> Printer

                                    ^
                                    |
                                    |

                             Printer Console 

                                    ^
                                    |
                                    |

                             Person (student)


2. Uses Cases:
   -----------

   Use Case #1 (Person printing a document)
   ----------------------------------------

      Prerequisites:  A document exist and is ready to be printed.

      Set of actions: 

           If there is space in the printer spool
              specify name of document to print via the Console
              printer controller receives document and process it
           else
              printing of the document is denied and appropriate message 
              generated in console.

      Effects:
 
           A document has been printed or a message to the effect
           the printing was not possible has been generated. 


   Use Case #2 (Technician fixing printer problem)
  ------------------------------------------------

      Prerequisites: A problem has been identified in the printer controller.

      Set of actions:

           A technician enters his password in the printer controller (to gain 
                        acces to the system).
           The technician asks for a status report.
           The printer problem area is identified.
           Technician proceeds to repair the problem area in the printer.
            
      Effects:

           The printer is now operational. Controller reflects this status.


   User #3 (Employee picks up printed document for student)
   --------------------------------------------------------

      Prerequisites: A job has been sent to the printer by a student.
                     Student approached printer consoler for submission of
                     job.

      Set of actions: 
           A student inquires about a particular job.
           The employee checks the stack of printed jobs in the printer tray.
           If student document is found
              document is handed to student
           else
              student is informed about missing document

      Effects:

           A student has a printed document or has been informed
           the document is not present.

3. For each part of the system:
   ----------------------------

  
   Name: Printer 
   Description:  Provide paper printouts of documents.  
   Must keep track of: Paper quantity and other printer status information.
   
   Name: Printer Controller
   Description: Used by the Employee and Technician to interact with printer.
   Must Keep track of: Must keep track of documents to be processed,
                       documents already finished, and status of the
                       printer. 
 
   Name: Printer Request Console
   Description: Used by students to submit a document to print.  
   Must Keep track of: Users allowed in the system, and currently in the system.


4. For each interaction between parts:
   -----------------------------------

   Interaction #1  (Controller and Printer)
       The Controller sends documents to process to the printer.  It also 
       gathers status information from the printer which makes available to 
       the Employee or technician.

   Interaction #2  (Console and Controller)

       Console sends job to the controller which determines whether the 
       document can be printed or not.  It schedules the job to printed.  
       Status information about the job is send back to the Console.