Data Flow Diagramming
Data Flow Diagramming Used to represent processes that: … capture, manipulate, store, & distribute data … between a system & its environment … & among system components.
Data Flow Diagramming Source input retrieval storage output Sink Data Store # Process storage output Sink 2.0 Connector Show that data flow continues to another process
Context Diagram An overview - shows the relationship between the external agents and the system Customers Sales orders Reject notice Order Entry System Accepted sales order file Rejected sales order report Inventory System Sales Manager
Level-0 Diagram Shows the decomposition of the main system in to its major processes & the data that flows between them. Do not include all data stores here. Only the ones shared between the major processes. Will get too busy! Customers Sales orders 1.0 Screen sales orders Reject notice Accepted sales order file Rejected sales order file 2.0 Sort rejected sales order file Inventory System Sorted rejected sales order file 3.0 Prepare rejected sales order report Rejected sales order report Sales Manager
Level 1 Diagram (for Process 1.0) aka: Diagram 1.0 Customers Sales orders 1.1 Verify item number Master item number list Master item number list Item numbers Verified item numbers 1.2 Verify price Reject notice Master price list Item prices Master price list Accepted sales order file Inventory System Rejected sales order file 2.0
DFD Practice Draw a Level-0 Diagram Notes: Time card ≠ Time sheet Treat “Employee” as external to this system. Employees provide the departmental secretary with their weekly timecards. The secretary prepares a weekly time sheet that lists each employee, the total hours worked, and the hourly rate. The timecards are filed in a Time card history file. The departmental supervisor signs the time sheet if everything seems to be in order. If any figures are unacceptable, the supervisor returns the sheet to the secretary. The acceptable sheets from the departments are sent in the company mail to the accounting department. The accounting clerk uses the acceptable time sheets to calculate payroll earnings and writes the earnings on the time sheets. Another clerk uses the time sheets to prepare the payroll checks, which are forwarded to the employees. After this operation, the time sheets are filed in an off-line Time sheet history file.
Balancing! Level 1 Diagram (Process 1.0) Context Diagram Level 0 Diagram Level 1 Diagram (Process 2.0)
Consistency! Context Diagram Level 0 Diagram Watch out for inconsistent names!
DFD Rules Naming Conventions Rules By Type DFDs should: Processes Accurately represent the system Be Balanced Be Consistent Be Readable Naming Conventions Processes Verb phrase = verb + object e.g. ValidateAccount, CalculateTax Data Flows Noun/noun phrase = adj + object e.g. payment, verified payment Data Stores e.g. Employee, HourlyEmployee External Agent Customer, Regional Warehouse, e.g. Billing System Rules By Type Process Must have both inputs & outputs At least one output must be different Data Flow Has only one direction Cannot return directly to the process Data must be moved via a process
Other Concepts Retrieval Updates Packet Insert Update Delete Price Inventory Item Item # Price Inventory Item New item Inventory Item Quantity on hand Inventory Item Purged item Inventory Item Student Application & resume Student application When items travel together resume # P1.1 # P1.1