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Structuring System Process Requirements -- Process Modeling --

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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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

6 A simple process for making lunch

7 DFD Elements Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

15 Context Diagram Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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 Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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

24

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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 Chapter 7 © 2008 by Prentice Hall

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 Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

31 Balancing DFD (Cont.) Chapter 7 © 2008 by Prentice Hall

32 Balancing DFD: More DFD Rules
Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall

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 Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall 58 58

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 Chapter 7 © 2008 by Prentice Hall 59 59

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. Chapter 7 © 2008 by Prentice Hall

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. Chapter 7 © 2008 by Prentice Hall


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