Transcript Chapter 8
CHAPTER 8 STRUCTURED APPROACH WITH THE DATA & PROCESS MODELING DATA FLOW DIAGRAM Def : shows how data moves through an information system but does not show program logic or processing steps. DFD represents logical model that shows what the system does, not how it does it ! DFD SYMBOLS 4 basic symbols in DFD: Process Data flow Storage External entity 2 basic version of DFD symbol ; Gane & Sarson symbol set Yourdon symbol set Gane & Sarson APPLY PAYMENT BANK DEPOSIT STUDENTS CUSTOMER Symbol Name Process Data Flow Yourdan APPLY PAYMENT BANK DEPOSIT Data Store STUDENTS External Entity CUSTOMER PROCESS SYMBOL Def : receives input data & produces output that has a different content, form or both. Process contains the business logic , also called business rules that transform the data & produce the required results. Symbol for process is a rectangle with rounded corners. The processes name identifies a specific function. FILL ORDER DATA FLOW SYMBOL Def : is a path for data to move from one part of the information system to another. Represents one or more data items. Symbol for data flow is a line with single or double arrowhead, & the name appears above the line. 3 data flow & process combination to be avoided; Spontaneous generation – a process with no input data flow Black hole – process that has no output Gray hole – process has an input that is obviously unable to produce output. DATA STORE SYMBOL Def ; represent a situation in which the system must retain data because one or more processes need to use the stored data at a later time. Symbol for data store is a flat rectangle that is open on the right side & closed on the left side. Name that identifies the content appears inside the rectangle. A data store must be connected to a process with a data flow. Rule : data store must have at least one incoming and one outgoing data flow. EXTERNAL ENTITY SYMBOL Defines a person, department, outside organization or other information system that provides data to the system or receives output from the system. Symbol for an external entity is a rectangle usually shaded to give the 3dimensional look with the name of the entity appears in the center. External entities shows the boundaries of the information system & how the information system interacts with the outside world. An external entity must be connected by a data flow to a process & not directly to data store / another entity. RULES FOR DFD Data flow that connects Okay to use? A process to another process YES A process to an external entity YES A process to a data store YES An entity to a data store NO An entity to another entity NO A data store to another data store NO CONTEXT DIAGRAM Start by placing a single process symbol in the center of the page & that symbol represents the entire information system. (identify as process 0) Place the external entities around the page & use data flows to connect the entities to the central process. You wont show any data stores in context diagram because data stores are internal to the system! Student Record System Submitted Work Final Grade Class Router Student 0 Grading System Graded Work Grade Report Grading Parameters Instructor CONVENTIONS / RULES FOR DFD Each context diagram must fit in one page The process name in the context digram must be the name of the information system Use unique names within each set of the symbols. Do not cross lines Use unique reference number for each process symbol. DIAGRAM 0 Is like a black box of a system & to show the details inside the black box, you create DFD Diagram 0 Diagram 0 is a more detailed / expanded version of process 0 on the context diagram (exploded view of diagram 0). All the processes are numbered for documentation purposes, but the numbers do not suggest that the processes are accomplished in sequential order. Diverging data flow – same data travels to two or more different locations. LOWER LEVEL DIAGRAM Also called child diagram are needed to show detail, it is essential that they be leveled & balanced. Leveling – process of drawing a series of increasingly detailed diagrams, until the desired degree of detail is reached. Balancing – maintains consistency among the entire series of diagrams, including input & output data flows, definitions & process descriptions. LEVELING Displays the information system as a single process & then shows more detail until all processes are functional properties. Also called exploding, partitioning or decomposing because analyst create DFDs as a series of top down pictures of an information system, with each lower level provides additional detail. BALANCING Effective DFDs are accurate, clear & consistent. One way to gain consistency is to define each functional primitive with a process description & see that each data store, data flow, external entity & process is defined in the data dictionary To achieve this, you must balance your DFDs properly. Balanced set of DFD = preserves the input & output data flows of the parent on the child DFD. STRATEGIES FOR DEVELOPING DFD’S A set of DFD is a graphical, top down model, so most analyst create the context diagram then diagram 0, then all the child diagrams for diagram 0 & so on. Some analyst may also prefer to use bottom-up strategy. Bottom-up strategy – first identify all functional primitives, data stores, external entities & data flows. Then you group processes with other related symbols to develop the lowest level diagrams. Next you group them in a logical way to form the next higher level & continue until you reach diagram 0. STRATEGIES FOR DEVELOPING DFD’S Regardless of the strategy you choose, the main objective is to ensure that your model is accurate & easy to understand. You may also use the combination of top down & bottom up strategy if the system is too complex.