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