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Systems Analysis Requirements structuring Process Modeling

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Presentation on theme: "Systems Analysis Requirements structuring Process Modeling"— Presentation transcript:

1 Systems Analysis Requirements structuring Process Modeling
Representing the system with data flow diagrams Logic Modeling Represents the logic of the DFD’s processes Data Modeling Represents the contents and structure of the DFD’s data flows and stores

2 Process Modeling Graphically represent the processes that capture, manipulate, store and distribute data between a system and its environment and among system components Data flow diagrams (DFD) Graphically illustrate movement of data between external entities and the processes and data stores within a system

3 Process Modeling Modeling a system’s process Deliverables and Outcomes
Utilize information gathered during requirements determination Structure of the data is also modeled in addition to the processes Deliverables and Outcomes Set of coherent, interrelated data flow diagrams

4 Deliverables and outcomes
Context data flow diagram (DFD) Scope of system DFDs of current system Enables analysts to understand current system DFDs of new logical system Technology independent Show data flows, structure and functional requirements of new system Project dictionary and CASE repository

5 Data Flow Diagram Symbols
Data Store Process Source/Sink

6 DFD Mechanics Data Flow
Depicts data that are in motion and moving as a unit from one place to another in the system. Drawn as an arrow Select a meaningful name to represent the data

7 DFD Mechanics Data Store Depicts data at rest May represent data in
File folder Computer-based file Notebook Drawn as two horizontal parallel lines The name of the store as well as the number are recorded in between lines

8 DFD Mechanics Process Depicts work or action performed on data so that they are transformed, stored or distributed Drawn as a circle Number of process as well as name are recorded

9 DFD Mechanics Source/Sink
Depicts the origin and/or destination of the data Sometimes referred to as an external entity Drawn as a square symbol Name states what the external agent is Because they are external, many characteristics are not of interest to us

10 DFD Syntax Context Diagram Level-O Diagram
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 Level-O Diagram A data flow diagram (DFD) that represents a system’s major processes, data flows and data stores at a higher level

11 Context Diagram: Automated food ordering system

12 Level-0 diagram: Automated food ordering system

13 DFD Guidelines Inputs to a process are always different than outputs
The same input may go in and out of a process but the process also produces other new data flows as a result of manipulating the inputs Objects always have a unique name In order to keep the diagram uncluttered, you can repeat data stores and data flows on a diagram

14 Incorrect DFD (process is shown as a sink)

15 Decomposition of DFDs Functional decomposition Level-N Diagrams
Act of going from one single system to many component processes Repetitive procedure Lowest level is called a primitive DFD Level-N Diagrams A DFD that is the result of n nested decompositions of a series of sub processes from a process on a level-0 diagram

16 Level 1 Level 2

17 Balancing DFDs When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition Process 4.0 in the Level-0 diagram had one input and one output. The level-1 and level-2 diagrams also have just one input and output. The inputs and outputs are conserved in the lower level diagram, so the DFDs are balanced. In the Automated food ordering system, there was one input to the system (the customer order), and three outputs (Customer receipt, Food order, Management reports). In the level-0 DFD, no new inputs or outputs have been introduced. So the context diagram and level-0 DFDs are balanced. Balancing ensures that all information in one level of a DFD is accurately and appropriately (not identically) represented at the next level of DFD. When a parent process is broken into children processes the children processes should wholly and completely make up the parent process e.g. after slicing a pie, can put the pieces back together and nothing is omitted.

18 Unbalanced DFDs In the context diagram, we have one input to the system, A and one output, B. The level 0 diagram has one additional data flow, C, coming from a different source than A These DFDs are not balanced. Note that data stores and flows to and from them are internal to the system.

19 Decomposing a Process

20 Use Case A set of activities the system performs to produce some output result. Describe the tasks in the process Identify the triggers for each task List all inputs and outputs

21 Building use cases Identify the major use cases
Ask “Who?”, “What?”, “When?” and “Where?” about the tasks and their inputs and outputs Identify the major steps within each use case Ask “How?” about each use case Identify the major inputs and outputs for each step Ask “How?” about each step

22 Data Flow Diagramming Rules
Process No process can have only outputs (a miracle) No process can have only inputs (black hole) A process has a verb phrase label Data Store Data cannot be moved from one store to another. Data cannot move from an outside source to a data store Data cannot move directly from a data store to a data sink Data store has a noun phrase label

23 Data Flow Diagramming Rules
Source/Sink Data cannot move directly from a source to a sink A source/sink has a noun phrase label Data Flow A data flow has only one direction of flow between symbols. A fork means that exactly the same data go from a common location to two or more processes, data stores or sources/sinks

24 Data Flow Diagramming Rules
Data Flow (Continued) A join means that exactly the same data come from any two or more different processes, data stores or sources/sinks to a common location A data flow cannot go directly back to the same process it leaves A data flow to a data store means update A data flow from a data store means retrieve or use A data flow has a noun phrase label

25 Advanced DFD Rules: Balancing
Composite data flow can be spilt into components data flows at the next level Inputs to a process must be sufficient to produce the output At the lowest level, new data flows may be added To avoid having data flow lines cross each other, can repeat data stores or sources/sinks on a DFD

26 Types of DFDs Current physical Current logical New logical
Process labels include the users and technology involved Data flows and stores are labeled with the name of the actual physical media Current logical Physical aspects of the system are removed New logical Additional functions Obsolete functions removed Inefficient flows re-organized New physical Defer until several alternative solutions have been considered.

27 Conservation of Data Data stays at rest until moved by a process
Processes cannot consume or create data Black hole error (no output) Miracle error (no input)

28 Guidelines for Drawing DFDs
Completeness DFD must include all components necessary for system Each component must be fully described in the project dictionary or CASE repository

29 Guidelines for Drawing DFDs
Consistency The extent to which information contained on one level of a set of nested DFDs is also included on other levels Timing Time is not represented well on DFDs Best to draw DFDs as if the system has never started and will never stop.

30 Guidelines for Drawing DFDs
Iterative Development Analyst should expect to redraw diagram several times before reaching the closest approximation to the system being modeled Primitive DFDs Lowest logical level of decomposition Decision has to be made when to stop decomposition

31 When to Stop Rules for stopping decomposition
When each process has been reduced to a single decision, calculation or database operation When each data store represents data about a single entity When the system user does not care to see any more detail

32 When to Stop (continued)
When every data flow does not need to be split further to show that data are handled in various ways When you believe that you have shown each business form or transaction, on-line display and report as a single data flow When you believe that there is a separate process for each choice on all lowest-level menu options

33 Final DFD Notes ·  when to show a direct data flow between processes and when to decouple these with a data store ·  what activities to encompass with each process ·  how to distinguish processes from sinks and sources

34 Using DFDs as Analysis Tools
Gap Analysis The process of discovering discrepancies between two or more sets of data flow diagrams or discrepancies within a single DFD Inefficiencies in a system can often be identified through DFDs

35 Using DFDs in Business Process Reengineering
Characteristics of a DFD that imply areas for improvement: Processes that simply collect and pass on information (rather than transforming data) Collecting the same information into several processes Placing untransformed data into data stores (causing unknown delays in processing this data) Cycles or loops that have no apparent termination

36 Logic Modeling Data flow diagrams do not show the logic inside the processes Logic modeling involves representing internal structure and functionality of processes depicted on a DFD Methods Structured English Decision Tables & Trees

37 Modeling Logic with Structured English
Modified form of English used to specify the logic of information processes Uses a subset of English Action verbs Noun phrases No adjectives or adverbs No specific standards

38 Modeling Logic with Structured English
Similar to programming language Sequence statements Conditional statements (choice) Iterative statements (repetition)

39 Modeling Logic with Decision Tables
A matrix representation of the logic of a decision Specifies the possible conditions and the resulting actions Best used for complicated decision logic

40 Modeling Logic with Decision Tables
Consists of three parts Condition stubs Lists condition relevant to decision Action stubs Actions that result for a given set of conditions Rules Specify which actions are to be followed for a given set of conditions

41 Modeling Logic with Decision Tables
Indifferent Condition Condition whose value does not affect which action is taken for two or more rules Standard procedure for creating decision tables Name the condition and values each condition can assume Name all possible actions that can occur List all rules Define the actions for each rule Simplify the table


43 Decision trees Depicts a decision as a connected series of nodes and actions Each node is a decision point with at least two branches Branches are the available options for each node Endpoints are the actions

44 Which logical model? Criteria SE D Table D Tree
Portraying simple logic Better Worst Best Portraying complex logic Transforming conditions & actions into sequence Checking consistency & completeness Worse

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