13/11/2017. Meltem Özturan misprivate.boun.edu.tr/ozturan/mis515

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1 Meltem Özturan misprivate.boun.edu.tr/ozturan/mis

2 Traditional Approach to Requirements 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) 3 4 2

3 External agent A person or organization, outside the boundary of a system that provides data inputs or accepts data outputs 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 Data stores A place where data is held pending future access by one or more processes 5 Data Flow Diagrams and Levels of Abstraction Levels of abstraction Particular to any modeling technique that breaks the system into a hierarchical set of increasingly more detailed models A DFD fragment showing one process in response to one event Other diagrams show the processing at a higher level (more general) or lower level (a more detailed view of one process) Higher level processes in a DFD can be decomposed into separate lower level DFD (or some other diagram) 6 3

4 Context Diagrams 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 7 Notes on Context Diagram Useful for showing system boundaries External agents are outside the system scope (which is represented by the single process) (But not from System Analysis 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) 8 4

5 DFD Fragments DFD fragment: A DFD that represents the system response to one event within a single process symbol A fragment is created for each event in the event list it is a self-contained model showing how the system responds to a single event Created one at a time Each fragment represents all processing for an event within a single process symbols The data stores in the DFD fragment represent entities in the ERD Not Necessarily! 9 The Event-Partitioned System Model Event-partitioned system model: a DFD that models requirements using a single process for each event The entire set of DFD fragments can be combined into this single DFD called the event-partitioned system model or diagram 0 Diagram 0 shows the entire system on a single DFD 10 5

6 Dividing the system into subsystems When a system involves n events, the eventpartitioned system model (diagram 0) would contain n processes This can get to be a crowded diagram! Solution is to divide the system into subsystems Events are grouped into related subsystems based on similar: Interactions with external agents Interactions with database stores Required processing 11 Relationship of all DFD s The figure starts off with the context diagram, which breaks down to the subsystem diagram (one for each subsystem) The subsystem diagram is turn broken down into the event-partitioned subsystem diagrams Using this same principle of breaking down the model to more detailed level, we can take a DFD fragment and decompose it into subprocesses and number the leveling accordingly etc. 12 6

7 13 Physical and Logical DFDs A DFD can be a physical system model, a logical system model or a blend of the two If the DFD is a logical model then it leaves out low level physical details and focuses on a perfect technology model If the DFD is a physical model then it includes details of the information technology (this is embedded in the model) 14 7

8 Physical and Logical DFDs With the logical model you should not be able to tell how the system is implemented In contrast, a physical model contains Technology-specific process Actor-specific process names Technology or actor-specific process orders Redundant processes, data flows and files Note during Analysis physical DFDs should be avoided 15 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) 16 8

9 Evaluating DFD Quality Data Flow Consistency 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 17 Documenting DFD Components Process Descriptions 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 18 9

10 Process Descriptions 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 19 Data Flow Definitions 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 20 10

11 Data Element Definitions Describe a data type Eg. String, integer, floating point, or Boolean 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.) 21 Data Store Definitions 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) 22 11

12 DFD Summary Four components of a traditional analysis model Data flow diagrams Entity-relationship diagram Process definitions Data definitions Form an interlocking set of specifications (for system requirements) DFD shows highest level of system Other components describe some aspect of DFD Created in the 1970 s and 1980 s (Yourdon) 23 Representing Locations and Communications Need to consider Number of locations of users Processing and data access requirements Volume and timing of processing and access Location diagram A diagram or map that identifies all of the processing locations of a system Activity-location matrix A table that describes the relationship between processes and the locations where they are performed 24 12

13 Activity-data matrix A table that describes stored data entities, the locations from which they are accessed, and the nature of the accesses CRUD An acronym for create, read, update and delete Shows what each activity does to the data For each activity and data combination Does activity create new data Read existing dataq Update existing data Delete existing data 25 Workflow Modeling Workflow The flow of control through a processing activity as it moves among people, organizations, computer programs, and specific processing steps Encompasses Trigger The processing steps that respond to a trigger Participants (or actors ) can be people and machines Flow of data 26 13

14 Workflow models directly model the sequence of processing activities Can develop and check with users to gain better understanding of a system or organization Can also be developed during the transition between analysis and structured design Can be used to describe complex interactions Can be used to describe alternative approaches Uses some symbols from flow charts DFD are good at capturing flow of data within a workflow (but not control) Flow charts and activity charts can represent control flow but don t represent data flow 27 14