SYSTEMSDESIGNANALYSIS 1 Chapter 9 Data Flow Diagrams Jerry Post Copyright © 1997

SYSTEMSDESIGN 2 A Systems Approach  Systems  Input, Process, Output  Divide and Conquer  Goals and Objectives  Control and Feedback  Diagramming Systems  Common Systems Problems  System models  Physical Process re-engineering  Abstract/logical Data flow diagram  Existing system  New/proposed system Basic Systems Input Process Output

SYSTEMSDESIGN 3 Data Flow Diagram Objects External Entity Data Flow Feedback and Control Data External Entity Process Data Store (file) Data Flow Feedback and Control Data  External Entity  Objects we cannot change.  We exchange data  Nouns Customer, Vendor, Investor  Process  Activities that alter data  Must have input and output  Verbs Collect, Modify, Delete  Data Store  Storage for data  File, database, paper … Product catalog, credit file  Data Flow  Inputs and Outputs  Must always be labeled

SYSTEMSDESIGN 4 Goals and Feedback  Every system must have a goal.  Some processes and data flows are feedback and control loops. Feedback Control Analyze Process Input Output Goal

SYSTEMSDESIGN 5 Context Diagram: The Zoo Members Donors Visitors Other Zoos Education Visitor Counts Educational Materials Baby Animals Registration Papers Health Data The Zoo Money Suppliers Requests & Comments Animal Feed Invoices Management  Context Diagram  Title page  Describe the main system.  One process.  All external entities.  Primary data flows.

SYSTEMSDESIGN 6 Subsystems  Each diagram can explode to a new subsystem to show detail.  Top down  Work from general to specific.  Start with the main problem.  Identify all external agents.  Bottom up  Identify small problems.  Extend to other areas.  As diagram gets big, change detail. More than 7-9 processes. Split and add a higher level. Context Diagram Level 0 Diagram Level 1 Diagram Level 1 Diagram Level 1 Diagram

SYSTEMSDESIGN 7 Primary Subsystems: The Zoo  Display the primary subsystems.  Look for major processes.  Corporation by function/ division.  Focus on data.  Level 0  Number each process (any order).  Include all data flows from higher level. Animal Care Donor & Public Relations Manage Facilities Produce Management Reports Manage Human Resources

SYSTEMSDESIGN 8 Level 0: The Zoo public/ zoo visitors donors zoo booster members 1 donor and public relations 2 produce mgt. reports 3 manage human resources 4 animal care 5 manage facilities other zoos & breeders certification agencies management money & requests PR data receipts etc. public requests needs & budgets animal requests health research maintenance schedule maint. & building request specialist request employee schedule specialist request emp. schedule emp. reports employees animal status hours, benefits, etc. pay data, requests receipts Usage reports Management reports

SYSTEMSDESIGN 9 Level 1: The Zoo public/ zoo visitors donors zoo booster members 1.2 handle donor requests 1.1 produce PR & outreach programs 1.3 org. booster services & meetings 1.4 track needs and donor programs 1.5 produce accounting & reports adopt an animal files receipts money & comments PR data receipts etc. money & requests newsletter notices, etc. money & suggestions booster requests needs & plans public requests needs & budgets needs & plans donor requests donor lists expenses & budget money, data, visitor statistics accounting reports public requests animal needs

SYSTEMSDESIGN 10 System Problems Do not include data flows between external entities. Drop the line or make one of the entities internal. A process cannot invent data. It must have a data flow coming in to it. A process cannot be a black hole--data must flow out of the process. Maybe it should be a file instead. ManufacturerCustomer Sell Products Invoicessales receipt orders purchase orders warranty registration Modify Reports weekly report daily sales Save Reports Store reports Inventory list

SYSTEMSDESIGN 11 Problem Boundary Boundaries Distribute Educational Materials Create Educational Materials Visitors Problem Boundary Problem: Shortages of Educational Materials Zoo system boundary

SYSTEMSDESIGN 12 Common System Problems  Defective subsystems  Wrong Data  Errors in Data  Missing feedback and control  Not responding to environment Receive Orders Check Customer Credit Check Orders for Errors

SYSTEMSDESIGN 13 Example Use: Finding Causes donors zoo booster members money & requests public requests needs & budgets adopt an animal files money & suggestions newsletter, notices, etc. donor lists donor requests needs & plans needs & plans booster requests expenses & budget donor list & animal needs & plans Problems Some animal budgets have excess $ Some animal budgets have no money 1.2 Handle donor requests 1.3 Booster services & meetings PR data receipts etc. 1.4 Track needs & donor programs

SYSTEMSDESIGN 14 Common Errors  Genesis  Process generates data without input.  Black hole  Process receives data and has no output.  No data flow between entities.  Need process.  Cannot connect entity to data store.  Must process it first.  Wrong labels  You do not understand system.  Readers will be misled.  Data flow is omitted.  Informal flows.  Diagram is too cluttered.  Split and add new level.  Unbalanced flows.  Data from prior levels must be listed on lower levels.  Can split to more detail.

SYSTEMSDESIGN 15 Examples  Book, p. 244  Proposed physical system.  Book, p  Video store  Class/groups  University bookstore-- textbooks.  Fast food restaurant.  Car dealership  Assignments  Physical process for you job (any job).