1. Traditional Approach to Requirements Data Flow Diagram (DFD)
Slides from Prof. Peter Khaiter

2. Lecture Outline Traditional Approach vs. OOA Data Flow Diagrams (DFDs)
DFD and Levels of Abstraction
Context Diagrams ‏
DFD Fragments
Evaluating DFD Quality
Documentation of DFD Components

3. Traditional Approach vs. OOA
Two approaches differ in the way the system is modeled and implemented:
• Traditional approach:
– views a system as a collection of processes (like computer programs, a set of instructions that execute in sequence)
– when the process executes it interacts with data (reads data values and then writes data values back to the data file
– emphasizes processes, data, inputs/outputs
• Object Oriented approach (OOA):
– views a system as a collection of interacting objects which are capable of their own behavior (called methods) which allow the objects to interact with each other and with people using the system
– there are NO conventional processes and data files, just interacting objects

4. Traditional Approach vs. OO Approach

5. Requirements for the Traditional and OO Approaches

6. 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 (only 5 symbols used)
DFD integrates processing triggered by events (event table) with the data entities modeled by ERD

7. DFD Symbols
The square is an external agent (a person or organization, outside the boundary of a system that provides data inputs or accepts data outputs)
The rectangle with rounded corners is a process (named “Look up item available” and can be referred to by its number, 1)
A process defines rules (algorithms or procedures) for transforming inputs into outputs
The lines with arrows are data flows (represents movement of data).
The flat three-sided rectangle is a data store (a file or part of a database that stores information about data entity)

8. Data Flow Diagram Symbols

9. DFD Fragment Showing Use Case Look Up Item Availability from the RMO

10. DFD Integrates Event Table and ERD

11. DFD and Levels of Abstraction
DFD is a modeling technique that breaks the system into a hierarchical set of increasingly more detailed models
DFD may reflect the processing at either a higher level (more general view of the system) or at lower level (a more detailed view of one process)
These different views of the system (higher level versus low level) creates the levels of abstraction
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

12. Layers of DFD Abstraction for Course Registration System

13. 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

14. Context Diagrams for a Course Registration System

15. Notes on Context Diagrams
• Useful for showing system boundaries (represents the system scope within the single process plus external agents)
• External agents that supply or receive data from the system are outside the system scope
• Data stores are not usually shown in the context diagram since they are considered to be within the system scope
• It is the highest level of DFD
• Context diagram does not show any details of what takes place within the system

16. Context Diagrams for RMO CSS

17. 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

18. Three Separate DFD Fragments for Course Registration System

19. 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
Shows the entire system on a single DFD (in greater detail than on the context diagram)
Sometimes called “diagram 0”
Used primarily as a presentation tool
Decomposed into more detailed DFD fragments

20. Combining DFD Fragments to Create Event- Partitioned System Model

21. Layers of DFD Abstraction

22. RMO Subsystems and Use Cases/Activities from Event Table

23. Context Diagram for RMO Order-Entry Subsystem

24. Five Separate DFD Fragments for RMO Order-Entry Subsystem

25. The event-partitioned model of the Order-Entry subsystem (diagram 0)

26. 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 numbering scheme
Hierarchical decomposition
DFD Fragment 2 is decomposed into Diagram 2
Diagram 2 has processes 2.1, 2.2, 2.3, 2.4 –four steps to complete the activity

27. Detailed DFD for Create new order DFD fragment

28. FIGURE 6-20 Incorrect and correct way to draw DFD.

29. Physical and Logical DFDs
Logical model
Assumes implementation in perfect technology
Does not tell how system is implemented
Physical model
Describes assumptions about implementation technology
Developed in last stages of analysis or in early design

30. Physical DFD for Scheduling Courses

31. Evaluating DFD Quality
Readable
Internally consistent and balanced
Accurately represents system requirements
Reduces information overload – rule of 7 +/- 2
Single DFD should not have more than 7 +/-2 processes
No more than 7 +/- 2 data flows should enter or leave a process or data store in a single DFD
Minimizes required number of interfaces

32. Data Flow Consistency Problems
Differences in data flow content between a process and its decomposition
Data outflows without corresponding inflows
Data inflows without corresponding outflows
Results in unbalanced DFDs

33. Black Hole and Miracle
black hole, i.e. a process with data input that is never used to produce data output.
Miracle - a process with a data output that is created out of nothing ( “miraculously appears”)

34. Unnecessary Data Input: Black Hole

35. Process with Impossible Data Output: a Miracle

36. Process with Unnecessary Data Input

37. Process with Impossible Data Output

38. Documentation of DFD Components
DFDs show three types of internal system component: processes, data flows and data stores
The details of each component need to be described
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

39. Process Descriptions • Each process on a DFD must be formally defined
• These description methods include:
- Structured English
- Decision tables
- Decision trees
• These models describe the process as an algorithm.

40. 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

41. Structured English Example

42. Process 2.1 and Structured English Process Description

43. A structured English process description for calculating shipping charges

44. 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

45. Decision Table for Calculating Shipping Charges

46. Decision Tree for Calculating Shipping Charges

47. A Decision Table with Multiple Action Rows

48. Components of a Traditional Analysis Model
Four components of a traditional analysis model are
– Data flow diagrams
– Entity-relationship diagram
– Process definitions
– Data definitions
They form an interlocking set of specifications for system requirements
DFD shows highest-level view of the system
Other components describe some aspect of DFD
These models were created in the 1970’s and 1980’s as a part of the structured analysis methodology

49. Components of a Traditional Analysis Model