Data Flow Diagram Developed By Larry Constantine as

Data Flow Diagram: Developed By Larry Constantine as a way of expressing system requirements in graphical Form: ü Data Flow Models (DFMs) are easy to understand and, with a little practice, reasonably quick and straightforward to develop ü They consist of two parts: a set of Data Flow Diagrams (DFDs) and a set of associated textual descriptions ü… that provide us with the truly effective tool for understanding the information processes of a system

Objectives of Data Flow Diagram: ü To define what is meant by structured analysis. ü To define the symbols used in a Data Flow Diagram (DFD). ü To draw a context diagram (Level 0 DFD) for a given scenario. ü To refine a DFD to a more detailed (lower- level) view.

Using Data Flow Diagrams: ü structured approach - take a top-down approach to system development. ü system is defined first at a general level – overview. ü successive refinement occurs until the bottom (primitive) levels are defined. ü primitive level - point where specifications can be translated into lines of code. ü So. . . system is decomposed into small modules that perform simple tasks.

Structured Development: ü definition is from top to bottom in increasing levels of detail. ü major flows and processes identified. ü These are exploded into subprocesses. ü Subprocesses are exploded into more detail. ü This process can continue to the primitive level, where programming begins directly from the exploded diagram.

Structured Terms: ü data elements - lowest level of information on which a process can act i. e. DB attributes/record fields - e. g. unit price. ü data stores - places where data are stored; e. g. files; microfiche, filing cabinets. ü data flows - represent movement of data in a system; consist of data input and data output e. g. forms, reports, invoices, letters. ü show movement of data about a physical “thing”.

Logical DFDs: ü shows sources and sinks (destinations) of data ü identifies and names the logical functions (processes) of the system. ü identifies and names the groups of data elements that connect one process to another. ü identifies the data stores. ü each function broken down into more detailed DFD (levels). ü descriptions of processes, flows, stores, elements recorded in data dictionary.

Logical DFDs: ü All of the above documentation comprises a logical functional specification for an existing or new system. ü A detailed statement of what the system does/is to do. ü Free from physical considerations of how it will be implemented.

Data Flow Diagrams Symbol: The following four symbols are used in data flow diagrams: External entity Process Data store Data flow

Data flow diagram of part of an order processing system: Order Available stock Customer Invoice stock Process order Unfilled order Out-of-stock notice Despatch note Warehouse backorders

Data flow diagram of a travel agent booking system: Customer Travel-query Available flights Book flight Booking confirmation Booking