1. Structuring System Process Requirements -- Process Modeling --

2. Learning Objectives
Understand the logical modeling of processes by studying examples of data flow diagrams (DFDs).
Draw data flow diagrams following specific rules and guidelines that lead to accurate and well-structured process models.
Decompose data flow diagrams into lower-level diagrams.
Balance higher-level and lower-level data flow diagrams.
Explain the differences among four types of DFDs: current physical, current logical, new physical, and new logical.
Use data flow diagrams as a tool to support the analysis of information systems.
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3. Process Modeling
Graphically represent the processes that capture, manipulate, store, and distribute data between a system and its environment and among system components.
Utilize information gathered during requirements determination.
Illustrates the activities that are performed and how data moves among them
Processes and data structures are modeled.
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4. DATA FLOW DIAGRAMS (DFD)
Technique for creating process models
DATA FLOW DIAGRAMS (DFD)

5. Data Flow Diagram
Represent both physical and logical information systems.
Only four symbols are used.
Useful for depicting purely logical information flows.
DFD that detail physical systems differ from system flowcharts which depict details of physical computing equipment.
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6. A simple process for making lunch

7. DFD Elements
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8. DFD Elements
Process: work or actions performed on data (inside the system).
Data store: data at rest (inside the system).
Source/sink: external entity that is origin or destination of data (outside the system).
Data flow: arrows depicting movement of data.
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9. DFD Elements Process Data flow
An activity or function performed for a specific business reason
Manual or computerized
Data flow
A single piece of data or a logical collection of data
Always starts or ends at a process

10. DFD Elements Data Store Source (External entity)
A collection of data that is stored in some way
Data flowing out is retrieved from the data store
Data flowing in updates or is added to the data store
Source (External entity)
A person, organization, or system that is external to the system but interacts with it.

11. Naming and Drawing DFD Elements
Process
Data flow
Data store
Source (external
Entity)

12. Depicting Business Processes with DFDs
Business processes are too complex to be shown on a single DFD
Decomposition is the process of representing the system in a hierarchy of DFD diagrams
Child diagrams show a portion of the parent diagram in greater detail
Balancing involves insuring that information presented at one level of a DFD is accurately represented in the next level DFD.

13. Relationship Among DFD levels
Context diagram
Level 0 diagram
Decomposition
Balancing
Level 1 diagram
Level 2 diagram

14. Context Diagram First DFD in every business process
Shows the overall business process as just one process (process 0)
Context diagram is an overview of an organizational system that shows:
the system boundaries;
external entities that interact with the system;
Major information flows between the entities and the system.
Note: only one process symbol, and no data stores shown.
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15. Context Diagram
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16. Level-0 Diagram
Level-0 diagram is a data flow diagram that represents a system’s major processes, data flows, and data stores at a high level of detail.
Processes are labeled 1.0, 2.0, etc. These will be decomposed into more primitive (lower-level) DFDs.
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17. Level 0 Diagram
Shows all the major processes that comprise the overall system – the internal components of process 0
Shows how the major processes are interrelated by data flows
Shows external entities and the major processes with which they interact
Adds data stores
Example: DFD Level 0

18. Level-0 Diagram
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19. Level 1 Diagrams
Generally, one level 1 diagram is created for every major process on the level 0 diagram
Shows all the internal processes that comprise a single process on the level 0 diagram
Shows how information moves from and to each of these processes
If a parent process is decomposed into, for example, three child processes, these three child processes wholly and completely make up the parent process

20. Level-1 DFD
Level-1 DFD shows the sub-processes of one of the processes in the Level-0 DFD.
This is a Level-1 DFD for Process 4.0.
Processes are labeled 4.1, 4.2, etc. These can be further decomposed in more primitive (lower-level) DFD if necessary.
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21. Level-n DFD
Level-n DFD shows the sub-processes of one of the processes in the Level n-1 DFD.
This is a Level-2 DFD for Process 4.3.
Processes are labeled 4.3.1, 4.3.2, etc. If this is the lowest level of the hierarchy, it is called a primitive DFD.
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22. Level 2 Diagrams
Shows all processes that comprise a single process on the level 1 diagram
Shows how information moves from and to each of these processes
Level 2 diagrams may not be needed for all level 1 processes
Correctly numbering each process helps the user understand where the process fits into the overall system

23. Relationship Among DFD levels
Context diagram
Level 0 diagram
Level 1 diagram
Level 2 diagram .
Level-n diagram

25. DFD Rules There are two DFD guidelines that apply:
The inputs to a process are different from the outputs of that process.
Processes purpose is to transform inputs into outputs.
Objects on a DFD have unique names.
Every process has a unique name.
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26. Decomposition of DFD
Functional decomposition is an iterative process of breaking a system description down into finer and finer detail.
Creates a set of charts in which one process on a given chart is explained in greater detail on another chart.
Continues until no sub-process can logically be broken down any further.
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27. Decomposition of DFD (Cont.)
Primitive DFD is the lowest level of a DFD.
Level-1 diagram results from decomposition of Level-0 diagram.
Level-n diagram is a DFD diagram that is the result of a n nested decompositions from a process on a level-0 diagram.
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28. Balancing DFD
Conservation Principle: conserve inputs and outputs to a process at the next level of decomposition.
Balancing: conservation of inputs and outputs to a data flow diagram process when that process is decomposed to a lower level.
Balanced means:
Number of inputs to lower level DFD equals number of inputs to associated process of higher-level DFD
Number of outputs to lower level DFD equals number of outputs to associated process of higher-level DFD
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29. Balancing DFDs (Cont.) 1 input
1 output
This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs.
2 inputs
1 output
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30. Balancing DFD (Cont.)
Data flow splitting is when a composite data flow at a higher level is split and different parts go to different processes in the lower level DFD.
The DFD remains balanced because the same data is involved, but split into two parts.
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31. Balancing DFD (Cont.)
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32. Balancing DFD: More DFD Rules
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33. CREATING DATA FLOW DIAGRAMS

34. Steps in Building DFDs Build the context diagram
Create DFD fragments for each activity
Organize DFD fragments into level 0 diagram
Decompose level 0 processes into level 1 diagrams as needed; decompose level 1 processes into level 2 diagrams as needed; etc.
Validate DFDs with user to ensure completeness and correctness

35. Build the Context Diagram
Draw one process representing the entire system (process 0)
Find all inputs and outputs that come from or go to external entities; draw as data flows
Draw in external entities as the source or destination of the data flows

36. A Context Diagram Example

37. Creating DFD Fragments
Each activity is converted into one DFD fragment
Change process name into verb phrase
Design the processes from the viewpoint of the organization running the system
Add data flows to show use to data stores as sources and destinations of data
Layouts typically place
processes in the center
inputs from the left
outputs to the right
stores beneath the processes

38. A DFD Fragment Example

39. Creating the Level 0 Diagram
Combine the set of DFD fragments into one diagram
Generally move from top to bottom, left to right
Minimize crossed lines
Iterate as needed
DFDs are often drawn many times before being finished, even with very experienced systems analysts

40. A Level 0 DFD Example

41. Creating Level 1 Diagrams (and Below)
Each use case is turned into its own DFD
Inputs and outputs become data flows on DFD
Include sources and destinations of data flows to processes and stores within the DFD
May also include external entities for clarity

42. Creating Level 1 Diagrams (and Below)
Input data flows shown on a parent DFD are often unbundled on the child diagram using splits
Output data flows shown on a child DFD are often bundled using joins and shown as a larger data flow on the parent diagram
When to stop decomposing DFDs?
Ideally, a DFD has at least 3 processes and no more than 7-9.

43. Validating the DFD Syntax errors – diagram follows the rules
Assure correct DFD structure
For each DFD:
Check each process for:
A unique name: action verb phrase; number; description
At least one input data flow
At least one output data flow
Output data flow names usually different than input data flow names
Between 3 and 7 processes per DFD

44. Validating the DFD For each DFD: Check each data flow for:
A unique name: noun; description
Connects to at least one process
Shown in only one direction (no two-headed arrows)
A minimum number of crossed lines
Check each data store for:
A unique name: noun; description
At least one input data flow
At least one output data flow
Check each external entity for:
A unique name: noun; description
At least one input or output data flow

45. Validating the DFD Across DFDs: Context Diagram: Decomposition:
Every set of DFDs must have one Context Diagram
Decomposition:
Every process is wholly and complete described by the processes on
its children DFDs
Balance:
Every data flow, data store, and external entity on a higher level DFD
is shown on the lower level DFD that decomposes it
No data stores or data flows appear on lower-lever DFDs that do not
appear on their parent DFD

46. Validating the DFD Semantics errors – diagram conveys correct meaning
Assure accuracy of DFD relative to actual/desired business processes
To verify correct representation, use
User walkthroughs
Role-play processes
Examine lowest level DFDs to ensure consistent decomposition
Examine names carefully to ensure consistent use of terms

47. Four Different Types of DFD
Current Physical
Process labels identify technology (people or systems) used to process the data.
Data flows and data stores identify actual name of the physical media.
Current Logical
Physical aspects of system are removed as much as possible.
Current system is reduced to data and processes that transform them.
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48. Four Different Types of DFD (Cont.)
New Logical
Includes additional functions.
Obsolete functions are removed.
Inefficient data flows are reorganized.
New Physical
Represents the physical implementation of the new system.
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49. Guidelines for Drawing DFD (Cont.)
Completeness
DFD must include all components necessary for system.
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.
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50. Guidelines for Drawing DFD (Cont.)
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.
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51. Guidelines for Drawing DFD (Cont.)
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.
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, online 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.
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52. Using DFD as Analysis Tools
Gap Analysis is 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.
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53. Example: A Video Sales System - step by step
The system accepts video orders from customers. These customer orders are checked against a video file (i.e. title and distributor match etc) to ensure they are correct. Also, another file is used to check the customers credit worthiness.
Once a valid order is received it is stored in a pending order file, until a batch of orders is assembled to be sent to a specific distributor.
Each distributor send a delivery note with the video and this is checked with the customer order. Videos are then delivered with a delivery note to the customer. An invoice is also sent for all the orders that a customer has been sent. a copy of this is sent is stored for use by the accounts department. Payment is outside the scope of this investigation.

54. Context Diagram Purchase order customer order Distributor
customer order
Distributor
A Video Sales System:
Process
Orders
delivery note
Customer
delivery note, invoice

55. DFD Level 0 Customer Verify Order Valid 1.0 D2 Customer Data D1
Video File
Video details
customer
order D3
Pending Orders
Create
Purchase
Orders
2.0
valid
Distributor
purchase
batched order
Assemble
3.0
credit
status
invoice
Video
Order details D4
Distributor file
Distributor details
delivery note
address

56. DFD Level 1 (Process 3.0) D3 Pending Orders Video order details
Distributor
Assign
delivery to
pending
orders
3.1
delivery note
Customer
Create
Invoice
3.3
Verify
Correct
Delivery
3.2 D5
A/c Receivable
invoice copy
invoice
of indiv.
assembled
Note
3.4
Del. note D6
Customer Data
Customer details

57. An example -- Electronic Commerce Application: Process Modeling using DFD
Figure 7-19 Level-0 data flow diagram for the WebStore
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58. Deliverables and Outcomes
Context data flow diagram (DFD)
Scope of system.
DFD of current physical system
Adequate detail only.
DFD of current logical system
Enables analysts to understand current system.
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59. Deliverables and Outcomes (Cont.)
DFD of new logical system
Technology independent.
Show data flows, structure, and functional requirements of new system.
Thorough description of each DFD component
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60. Summary In this chapter you learned how to:
Understand logical process modeling via data flow diagrams (DFDs).
Draw data flow diagrams of well structured process models.
Decompose data flow diagrams into lower-level diagrams.
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61. Summary (Cont.) Balance high-level and low-level data flow diagrams.
Explain differences between current physical, current logical, new physical, and new logical data flow diagrams.
Use data flow diagrams for analyzing information systems.
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