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System Analysis Overview. Document functional requirements by creating models Two concepts help identify functional requirements in the traditional approach and object-oriented approach Events that trigger use cases Things in the users’ work domain. Models and Modeling.
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System Analysis Overview • Document functional requirements by creating models • Two concepts help identify functional requirements in the traditional approach and object-oriented approach • Events that trigger use cases • Things in the users’ work domain
Models and Modeling • Analyst describes information system requirements using a collection of models • Complex systems require more than one type of model • Models represent some aspect of the system being built • Process of creating models helps analyst clarify and refine design • Models assist communication with system users
Why modeling? • Learning from modeling process • Reduce complexity by abstraction • Remembering the details • Communicating with other team members, users, and stakeholders • Documenting what was done for future maintenance/enhancement
Types of Models • Different types of models are used in information systems development • Mathematical – formulas that describe technical aspects of the system • Descriptive– narrative memos, reports, or lists that describe aspects of the system • Graphical– diagrams and schematic representations of some aspect of the system
Events • Business events trigger elementary business processes (EBPs) • EBPs are at correct level of analysis for use cases • Business events are memorable, can be described, and occur at a specific time and place
Sequence of “Transactions” for One Specific Customer Resulting in Many Events
Types of Events • External • Outside system • Initiated by external agent or actor • Temporal • Occur as result of reaching a point in time • Based on system deadlines • State • Something inside system triggers processing need
Events modeling • Identify business events to decompose system into activities/use cases • Use cases (activities) are identified from user goals and business events that trigger elementary business processes • Event decomposition is, therefore, used by • Traditional approach to identify activities • OO approach to identify use cases • Event table records event, trigger, source, use case, response, and destination
Things • “Things” are what user deals with and system remembers, such as an order placed by a customer • Analysts identify these types of things by considering each use case in the event table • What things does the system need to know about and store information about?
Modeling things • Traditional approach uses entity-relationship diagrams (ERD) for data entities, attributes of data entities, and relationships between entities • Object-oriented approach uses UML class diagrams for classes, attributes, methods of class, and associations among classes
Data Flow Diagrams (DFDs) • Graphical system model that shows all main requirements for an IS in one diagram • Inputs/outputs • Processes • Data storage • Easy to read and understand with minimal training
DFD and Levels of Abstraction • Data flow diagrams (DFDs) are decomposed into additional diagrams to provide multiple levels of detail • Higher-level diagrams provide general views of system • Lower-level diagrams provide detailed views of system • Differing views are called levels of abstraction
Context Diagrams • DFD that summarizes all processing activity for the system or subsystem • Highest level (most abstract) view of system • Shows system boundaries • System scope is represented by a single process, external agents, and all data flows into and out of the system
Context Diagram Data Flow External entity Data Flow External entity The System Data Flow Data Flow External entity
DFD Fragments • Created for each use case in the event table • Represent system response to one event within a single process symbol • Self-contained models • Focus attention on single part of system • Show only data stores required in the use case
Event-Partitioned System Model • DFD to model system requirements using single process for each use case/activity in system or subsystem • Combines all DFD fragments together to show decomposition of the context-level diagram • Sometimes called “diagram 0” • Used primarily as a presentation tool • Decomposed into more detailed DFD fragments
Combining DFD Fragments to Create Event- Partitioned System Model
Decomposing DFD Fragments • Most DFD fragments can be further described using structured English • Sometimes DFD fragments need to be diagrammed in more detail • Decomposed into subprocesses in a detailed DFD
DFD Practice • Precision tools sells a line of high-quality woodworking tools. When customers place orders on the company’s web site, the system checks to see if the items are in stock, issues a status message to the customer, and generates a shipping order to the warehouse, which fills the order. When the order is shipped, the customer is billed. The system also produces various reports.
DFD Practice • Draw a context diagram • System scope is represented by a single process, external agents, and all data flows into and out of the system • Draw Level-0 diagram • Identify the major use cases • Draw DFD fragment for each use case • Combine DFD fragments together at the same level of detail
Evaluating DFD Quality • Readable • Internally consistent and balanced • Accurately represents system requirements • Reduces information overload – rule of 7 +/- 2 • Minimizes required number of interfaces
Data Flow Consistency Problems • Differences in data flow content between a process and its process decomposition • Data outflows without corresponding inflows • Data inflows without corresponding outflows • Results in unbalanced DFDs
Consistency Rules • All data that flows into a process must • Flow out of the process, or • Be used to generate data that flows out of the process • All data that flows out of a process must • Have flowed into the process, or • Have been generated from data that flowed into the process
Documentation of DFD Components • Lowest-level processes need to be described in detail • Data flow contents need to be described • Data stores need to be described in terms of data elements • Each data element needs to be described • Various options for process definition exist
Structured English • Method of writing process specifications • Combines structured programming techniques with narrative English • Well-suited for lengthy sequential processes or simple control logic (single loop or if-then-else) • Ill-suited for complex decision logic or few (or no) sequential processing steps
Decision Tables and Decision Trees • Can summarize complex decision logic better than structured English • Incorporate logic into the table or tree structure to make descriptions more readable