A system or subsystem as it exists within the corporate organization may be graphically depicted in several ways. The various graphical models show the boundaries of the system and the information used in the system.<\/p>\n\n\n\n
The first model is the context-level data flow diagram (also called an environmental model). Data flow diagrams focus on the data flowing into and out of the system and the processing of the data. These basic components of every computer program can be described in detail and used to analyze the system for accuracy and completeness.<\/p>\n\n\n\n
As shown in table below, the context-level data flow diagram employs only three symbols: (1) a rectangle with rounded corners, (2) a square with two shaded edges, and (3) an arrow. Processes transform incoming data into outgoing information, and the content level has only one process, representing the entire system. The external entity represents any entity that supplies or receives information from the system but is not a part of the system. This entity may be a person, a group of people, a corporate position or department, or other systems. The lines that connect the external entities to the process are called data flows, and they represent data.<\/p>\n\n\n\n
An example of a context-level data flow diagram is shown in the figure below the table. In this example, the most basic elements of an airline reservation system are represented. The passenger (an entity) initiates a travel request (data flow). The context-level diagram doesn\u2019t show enough detail to indicate exactly what happens (it isn\u2019t supposed to), but we can see that the passenger\u2019s preferences and the available flights are sent to the travel agent, who sends ticketing information back to the process.We can also see that the passenger reservation is sent to the airline. The context-level data flow diagram serves as a good starting point for drawing the use case diagram (discussed later in this chapter).<\/p>\n\n\n\n