17/03/2018. Meltem Özturan

Transcription

1 Meltem Özturan 2 1

2 Traditional Approach to Requirements Traditional Analysis Model Data flow diagrams Process description Data flow definiton Data store definition (Entity-Relationship diagram) External entity definition Workflow Modeling 3 Data Flow Diagram (DFD) A graphical system model that shows all of the main requirements for an information system: inputs, outputs, processes and data storage Everyone working on the project (and end users) can see all the aspects of the project in the diagram with minimal training (simple only 5 symbols) 4 2

3 5 Process : A symbol that represents an algorithm or procedure by which data inputs are tranformed into data outputs Data flow : Represents movement of data among processes, data stores, and external agents External agent : A person or organization, outside the boundary of a system that provides data inputs or accepts data outputs Data store : A place where data is held pending future access by one or more processes Real-time link : Communication between an external agent and a process as the process is executing 6 3

4 Rules for Drawing a Data Flow Diagram Data flows are used to make the necessary connections within a DFD A process must have at least one incoming and one outgoing data (spontaneous, black hole, gray hole) A data store must have at least one incoming and one outgoing data A data store must be connected to an outside source through a process Two data stores can not be connected by a data flow without an intervening process 7 Context Diagram: A DFD that summarizes all processing activity within the system in single process symbol Describes highest level view of a system All external agents and all data flows into and out of a system are shown in the diagram The whole system is represented as one process 8 4

5 Notes on Context Diagram Useful for showing system boundaries External agents are outside the system scope (which is represented by the single process) Data stores are not usually shown in the context diagram since they are considered to be in the system scope (i.e. the single process) It is the highest level DFD Context diagram does not show any details of what takes place within the system (i.e. that single process) 9 0 Level Diagram: Zooms in on the context diagram and shows major processes, data flows, and data stores. Data stores are included Context diagram is the parent Decomposition of 0 level diagram can create child diagrams 10 5

6 Lower Level Diagram: Zooms in on the processes of 0 level diagram Functional primitive process : The process that consists of a single function that is not exploded further. Leveling : The process of draiwng a series of increasingly detiled diagrams, unti the desired degree of detail is reached. Balancing : The consistency among the entire serie of diagrams, including input and output data flows, data definition, and process descriptions 11 Evaluating DFD Quality A quality set of DFDs is Readable Internally consistent Accurately represents system requirements Minimizing complexity Want to avoid information overload Occurs when too much information is presented to a user at one time Two ways to avoid information overload use 7 (+ or ) 2 rule (which limits the number of components) and interface minimization (which minimizes the number of interfaces and connections between components) 12 6

7 Avoid common errors: Differences in data flow for a process and its decomposition (want to have balancing: equivalence of data content between data flows entering and leaving a process or its decomposition) Black hole : A process with data input that is never used to produce a data output Miracle : A process with a data output that is created out of nothing (I.e. miraculously appears ) Black hole and miracle problems apply to both processes and stores Most CASE tools perform data flow consistency checking 13 (Process Description) Process Description Each process on a DFD needs to be defined Can keep breaking down DFD to more detailed DFD but at some point have to describe the process in structured English, decision table or decision tree 14 7

8 (Process Description) (Process Description) Structured English Uses instructions, repetition and if-then-else logic (an algorithm that describes the process) Not so good for showing complex decision logic Not so good if there are few or no sequential steps Decision Table A tabular representation of processing logic containing decision variables, decision variable values and actions (or formulas) Decision Tree A graphical description of process logic that uses lines organized like branches of a tree 16 8

9 (Data Flow Definition) Data flow : a collection of data elements Data flow definition : a textual description of a data flow s content and internal structure Lists all the elements Most of these are stored and correspond to the attributes of data entities In addition to just listing elements can use algebraic notation Data Element : String, integer, floating point, Boolean, etc. Lengths are usually defined for strings Numeric values usually have a minimum and maximum value (a valid range) Might define special codes (e.g. code A means ship immediately etc.) 17 (Data Flow Definition) Elements make up the data structure and algebraic notations are used for describing the data structure: Equal sign (=), meaning consists of Plus sign (+), meaning "and" Braces {} meaning repetitive elements, a repeating element or group of elements Brackets [] for an either/or situation Parentheses () for an optional element For each element, Element Description Form should be prepared. 9

10 (Data Flow Definition Element) (Data Store Definition) A data store on the DFD represents a data entity on the ERD Can say to look at the ERD for details on this If no ERD, can define the data store as a collection of elements (like did for data flows) 20 10

11 (Data Store Definition) (External Entity Definition) 11

12 (Activity-Data Matrix) A table that describes stored data entities, the activities from which they are accessed and the nature of the accesses. CRUD An acronym for Create, Read, Update and Delete For each activity and data communication, does activity; Create new data Read existing data Update existing data Delete existing data 23 (Workflow Modeling) Workflow : The flow of control through a processing activity as it moves among people, organizations, computer programs, and specific processing steps Workflow models directly model the sequence of processing activities Uses some symbols from flow charts DFD are good at capturing flow of data within a workflow Flow charts and activity charts can represent control flow but don t represent data flow 24 12