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DATA FLOW DIAGRAMS DFD. Three words. DATA FLOW DIAGRAMS. Therefore: def. Does this mean ... . DFD’s are depictions of data flows within a system?. Yes and Yes ... . “DFD’s are graphical representations of data flows within an information system”
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DATA FLOW DIAGRAMSDFD Three words
DATA FLOW DIAGRAMS Therefore: def
Does this mean ... DFD’s are depictions of data flows within a system? Yes and Yes... “DFD’s are graphical representations of data flows within an information system” - collaboration of the user and the System Analyst Levels of Complexity
Levels of complexity High Level Data Flow Diagramsoverview Low Level Data Flow Diagramsmore detailed Parts
Parts within the Data Flow Diagram External Entities Data Flows Data Stores Processes External identities
External Entities sources of data fall outside of the DFD -Customers -Department Store -Inventory System Interact with business processes destination of data Data flows
Data Flows Represented by arrows Given appropriate titles The directions of the arrow also illustrates the direction in movement Examples: Purchase Order Customer Profile Account Number Purchase Order Data stores
Data Stores • Collection of data needed by that business process • Represented by two unequal sized conjoined boxes - first box: unique identifier - second box: identify the store type * Data Stores are not Databases * processes
Processes Represented by the figure below: -The process takes in the data as an input and does its specific chore and produces an output -Processes could be repeated -Example: Cash withdrawal practises
Common Practise # 1 “A process must have at least one data flow in and one data flow out” there needs to be a way out
Common Practise # 2 “A process should transform the data coming in into a new form of data going out” • the process should make a modification or then it would not be needed
Common Practise # 3 “A data store must have at least one data flow” An collection of data has to have a direction to flow towards. If data had no where to flow, it would almost be like it was locked in a vault
Common Practise # 4 “An external entity must be involved with one data flow ” The fact that external entities external, there needs to be a way for this data to flow into the interiors of the DFD’s
Common Practise # 5 “A data flow must be attached to at lease one process” a flow in data needs a destination to go to, if its not attached to a process, the data will not have an output, it will remain as an input organizing
Organizing what you have • DFD’s can be very complex in nature and in turn can look like this: • Keep it small • Avoid unnecessary processes • Prune your DFD’s (they may change) *make sure they add value advantages
Disadvantages • No set standard for the symbols used - Gane & Sarson use the rectangle to represent processes - DeMarco & Yourdan use eclipses • Takes a while to get to the users, because of the alternation making the process slow advantages
Advantages • Visual instead of all content • Boundaries are established • Communication tool for new users • When done correctly, DFD’s will depict a continuous flow of data always leading up to something Example
Refer to Example • A data flow diagram of a student who applies for a university, either getting approved or denied and flowing through with the registration process Parts or example
Parts • 1 external entity • Student • 3 processes • Applies to university, approval, registration • 6 data flows • Progress report, application, student information (2x), denial letter, acceptance letter • 1 data store • Student database Questions?
