Jump to first page Chapter 2 System Analysis - Process Modeling.
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Jump to first page Chapter 2 System Analysis - Process Modeling
Jump to first page Learning Objectives Understand the logical modeling of processes through studying data flow diagrams How to draw data flow diagrams using rules and guidelines How to decompose data flow diagrams into lower-level diagrams 8.2
Jump to first page Learning Objectives Balancing of data flow diagrams Explain the differences among four types of DFDs: current physical, current logical, new physical and new logical Discuss the use of data flow diagrams as analysis tools 8.3
Jump to first page Process Modeling n Graphically represent the processes that capture, manipulate, store and distribute data between a system and its environment and among system components n Data flow diagrams (DFD) u Graphically illustrate movement of data between external entities and the processes and data stores within a system 8.4
Jump to first page Process Modeling n Modeling a system’s process u Utilize information gathered during requirements determination u Structure of the data is also modeled in addition to the processes n Deliverables and Outcomes u Set of coherent, interrelated data flow diagrams 8.5
Jump to first page Process Modeling n Deliverables and outcomes (continued) u Context data flow diagram (DFD) F Scope of system u DFDs of current system F Enables analysts to understand current system u DFDs of new logical system F Technology independent F Show data flows, structure and functional requirements of new system 8.6
Jump to first page Process Modeling n Deliverables and outcomes (continued) u Project dictionary and CASE repository 8.7
Jump to first page Data Flow Diagramming Mechanics n Four symbols are used u See Figure 8-2 u Two different standard sets can be used F DeMarco and Yourdan F Gane and Sarson 8.8
Jump to first page Figure 5-1 Comparison of DeMarco & Yourdan and Gane & Sarson DFD symbol sets 8.9
Jump to first page Data Flow Diagramming Mechanics n Data Flow u Depicts data that are in motion and moving as a unit from one place to another in the system. u Drawn as an arrow u Select a meaningful name to represent the data 8.10
Jump to first page Data Flow Diagramming Mechanics n Data Store u Depicts data at rest u May represent data in F File folder F Computer-based file F Notebook u The name of the store as well as the number are recorded in between lines 8.11
Jump to first page Data Flow Diagramming Mechanics n Process u Depicts work or action performed on data so that they are transformed, stored or distributed u Number of process as well as name are recorded 8.12
Jump to first page Data Flow Diagramming Mechanics n Source/Sink u Depicts the origin and/or destination of the data u Sometimes referred to as an external entity u Drawn as a square symbol u Name states what the external agent is u Because they are external, many characteristics are not of interest to us 8.13
Jump to first page Data Flow Diagramming Definitions n Context Diagram u A data flow diagram (DFD) of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system n Level-0 Diagram u A data flow diagram (DFD) that represents a system’s major processes, data flows and data stores at a high level of detail 8.14
Jump to first page Developing DFDs: An Example n Hoosier Burger’s automated food ordering system n Context Diagram (Figure 8-4) contains no data stores n Next step is to expand the context diagram to show the breakdown of processes (Figure 8-5) 8.15
Jump to first page Figure 8-4 Context diagram of Hoosier Burger’s food ordering system 8.16
Jump to first page Figure 8-5 Level-0 DFD of Hoosier Burger’s food ordering system 8.17
Jump to first page Data Flow Diagramming Rules n Basic rules that apply to all DFDs u Inputs to a process are always different than outputs u Objects always have a unique name F In order to keep the diagram uncluttered, you can repeat data stores and sources/sinks on a diagram 8.18
Jump to first page DFD Diagramming Rules: Process No process can have only outputs or only inputs…processes must have both outputs and inputs. Process labels should be verb phrases.
Jump to first page DFD Diagramming Rules: Data Store Data store labels should be noun phrases. All flows to or from a data store must move through a process.
Jump to first page DFD Diagramming Rules: Source/Sink Source and sink labels should be noun phrases. No data moves directly between external entities without going through a process. Interactions between external entities without intervening processes are outside the system and therefore not represented in the DFD.
Jump to first page DFD Diagramming Rules: Data Flow Bidirectional flow between process and data store is represented by two separate arrows. Forked data flow must refer to exact same data item (not different data items) from a common location to multiple destinations.
Jump to first page DFD Diagramming Rules: Data Flow (cont.) Joined data flow must refer to exact same data item (not different data items) from multiple sources to a common location. Data flow cannot go directly from a process to itself, must go through intervening processes.
Jump to first page DFD Diagramming Rules Data Flow (cont.) n Data flow from a process to a data store means update (insert, delete or change). n Data flow from a data store to a process means retrieve or use. n Data flow labels should be noun phrases.
Jump to first page Decomposition of DFDs n Functional decomposition u Act of going from one single system to many component processes u Repetitive procedure u Lowest level is called a primitive DFD n Level-N Diagrams u A DFD that is the result of n nested decompositions of a series of subprocesses from a process on a level-0 diagram 8.25
Jump to first page Balancing DFDs n When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition n This is called balancing n Example: Hoosier Burgers u In Figure 8-4, notice that there is one input to the system, the customer order u Three outputs: F Customer receipt F Food order F Management reports 8.26
Jump to first page Balancing DFDs n Example (Continued) u Notice Figure 8-5. We have the same inputs and outputs u No new inputs or outputs have been introduced u We can say that the context diagram and level-0 DFD are balanced 8.27
Jump to first page Balancing DFDs n An unbalanced example u Figure 8-10 u In context diagram, we have one input to the system, A and one output, B u Level-0 diagram has one additional data flow, C u These DFDs are not balanced 8.28
Jump to first page Figure 8-10 An unbalanced set of data flow diagrams (a) Context diagram (b) Level-0 diagram 8.29
Jump to first page Balancing DFDs n We can split a data flow into separate data flows on a lower level diagram (see Figure 8-11) 8.30
Jump to first page Balanced DFD These are balanced because the numbers of inputs and outputs of context diagram process equal the number of inputs and outputs of Level-0 diagram. 1 input 2 outputs
Jump to first page Balanced DFD (cont.) These are balanced because the numbers of inputs and outputs to Process 1.0 of the Level-0 diagram equals the number of inputs and outputs to the Level-1 diagram. 1 input 4 outputs
Jump to first page Data Flow Splitting A composite data flow at a higher level may be split if different parts go to different processes in the lower level DFD. This remains balanced because the same data is involved, but split into two parts.
Jump to first page Four Different Types of DFDS n Current Physical u Process label includes an identification of the technology (people or systems) used to process the data u Data flows and data stores are labeled with the actual name of the physical media on which data flow or in which data are stored 8.34
Jump to first page Four Different Types of DFDS n Current Logical u Physical aspects of system are removed as much as possible u Current system is reduced to data and processes that transform them n New Logical u Includes additional functions u Obsolete functions are removed u Inefficient data flows are reorganized 8.35
Jump to first page Four Different Types of DFDS n New Physical u Represents the physical implementation of the new system 8.36
Jump to first page Guidelines for Drawing DFDs n Completeness u DFD must include all components necessary for system u Each component must be fully described in the project dictionary or CASE repository n Consistency u The extent to which information contained on one level of a set of nested DFDs is also included on other levels 8.37
Jump to first page Guidelines for Drawing DFDs n Timing u Time is not represented well on DFDs u Best to draw DFDs as if the system has never started and will never stop. n Iterative Development u Analyst should expect to redraw diagram several times before reaching the closest approximation to the system being modeled 8.38
Jump to first page Guidelines for Drawing DFDs n Primitive DFDs u Lowest logical level of decomposition u Decision has to be made when to stop decomposition 8.39
Jump to first page Guidelines for Drawing DFDs n Rules for stopping decomposition u When each process has been reduced to a single decision, calculation or database operation u When each data store represents data about a single entity u When the system user does not care to see any more detail 8.40
Jump to first page Guidelines for Drawing DFDs n Rules for stopping decomposition (continued) u When every data flow does not need to be split further to show that data are handled in various ways u When you believe that you have shown each business form or transaction, on-line display and report as a single data flow u When you believe that there is a separate process for each choice on all lowest-level menu options 8.41
Jump to first page Using DFDs as Analysis Tools n Gap Analysis u The process of discovering discrepancies between two or more sets of data flow diagrams or discrepancies within a single DFD n Inefficiencies in a system can often be identified through DFDs 8.42
Jump to first page Summary n Data flow diagrams (DFD) u Symbols u Rules for creating u Decomposition u Balancing n Four different kinds of DFDs u Current Physical u Current Logical u New Logical u New Physical 8.43