Content Chapter 7 Structuring System Process Requirements Understand the logical (&physical) process modeling by using data flow diagrams (DFDs) Draw DFDs & Leveling Balance higher-level and lower-level DFDs Explain the differences among four types of DFDs: current physical, current logical, new physical, and new logical Use DFDs as a tool to support the analysis of ISs Discuss process modeling for e-commerce applications Process Description ** 2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6 th Edition 1/68 2/68 1. Process Modeling Process Modeling (Cont.) 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 Processes and data structures are modeled 3/68 4/68
1.1 Deliverable and Outcome Context data flow diagram (DFD) Scope of system DFDs of current physical system Adequate detail only DFDs of current logical system Enables analysts to understand current system DFDs of new logical system Technology independent Show data flows, structure, and functional requirements of new system DFDs of new physical system Description of each DFD component 2. Data Flow Diagramming Mechanics Represent both physical and logical ISs Only four symbols are used 5/68 6/68 Definitions and Symbols (Cont.) Process work or actions performed on data Data store data at rest (inside the system) Source/sink (destination); terminator; actor; boundary external entity that is origin or destination of data (outside the system) Data flow arrows depicting movement of data Data Flow Diagramming Mechanics (Cont.) Useful for depicting purely logical information flows DFDs that detail physical systems differ from system flowcharts which depict details of physical computing equipment 7/68 8/68
Different between Sources/ Sinks and Processes 2.1 Developing DFDs: an Example 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 9/68 10/68 Context Diagram Developing DFDs (Cont.) 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. Leveling & Balancing 11/68 12/68
Level-0 Diagram 2.2 Data Flow Diagramming Rules Two DFD guidelines 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 13/68 14/68 Data Flow Diagramming Rules (Cont.) Data Flow Diagramming Rules (Cont.) 15/68 16/68
Illegal Data Flows 2.3 Decomposition of DFDs B2 Functional decomposition is an iterative process of breaking a system description down into finer and finer detail 0 PROJECT GOAL 1 PHASE 2 PHASE 3 PHASE 2.1 ACTIVITY 2.2 ACTIVITY 2.3 ACTIVITY 2.2.1 2.2.2 2.2.3 TASK TASK TASK Whitten 17/68 18/68 Decomposition of DFDs (Cont.) Level-1 DFD 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. 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 Balancing!!! Processes are labeled 4.1, 4.2, etc. These can be further decomposed in more primitive (lower-level) DFDs if necessary. 19/68 20/68
Level-n DFD 2.4 Balancing DFDs Not Necessary for this level!! 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. 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. 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 21/68 22/68 Balancing DFDs (Cont.) Balanced means (in general): 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 Balancing DFDs (Cont.) 1 input 1 output 2 inputs 1 output This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs. 23/68 24/68
Balancing DFDs (Cont.) Balancing DFDs (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 25/68 26/68 Balancing DFDs: More DFD Rules Exercise Document your project requirements by using Context diagram (scope) L-0 DFD (main activities) ++ Leveling & Balancing ++ Process description 27/68 28/68
3. Four Different Types of DFDs 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 Four Different Types of DFDs (Cont.) New Logical Include additional functions Remove obsolete functions Reorganize inefficient data flows New Physical Represents the physical implementation of the new system 29/68 30/68 System Development Model (Classical Model) 4. Using Data Flow Diagram in the Analysis Process AS IS (Current) 1 Physical Model 2 Logical Model TO BE (New) 4 Physical Model 3 Logical Model 4.1 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 Consistency The extent to which information contained on one level of a set of nested DFDs is also included on other levels 31/68 32/68
Guidelines for Drawing DFDs (Cont.) Timing Time is not represented well on DFDs Best to draw DFDs as if the system has never started and will never stop 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 33/68 34/68 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 each 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 4.2 Using DFDs 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 35/68 36/68
4.3 Using DFDs in BPR Using DFDs in BPR (Cont.) FIGURE 7-16 FIGURE 7-17 IBM Credit Corporation s primary work process before BPR IBM Credit Corporation s primary work process after BPR (Source: Based on Hammer and Champy, 1993.) (Source: Based on Hammer and Champy, 1993.) 37/68 38/68 5. e-commerce Application: Process Modeling using Data Flow Diagram e-commerce Application: Process Modeling using Data Flow Diagrams (Cont.) Process modeling for Pine Valley Furniture s Webstore Completed JAD session Began translating the Webstore system structure into data flow diagram Identified six high-level processes 39/68 40/68
e-commerce Application: Process Modeling using Data Flow Diagrams (cont.) 6. Process Description Use Structured English as a tool for representing steps in logical processes in DFDs Use decision tables to represent the logic of choice in conditional statements Select among Structured English and decision tables for representing processing logic Understand how logic modeling techniques apply to the development of your project study Figure 7-22 Level-0 data flow diagram for the WebStore 41/68 2011 by Prentice Hall: J.A.Hoffer et.al., Modern Systems Analysis & Design, 6 th Edition 42/68 Logic Modeling: process description Deliverable and Outcome Data flow diagrams (DFDs) do not show the logic inside the processes Logic modeling involves representing internal structure and functionality of processes depicted on a DFD Logic modeling can also be used to show when processes on a DFD occur Structured English representation of process logic Decision Tables representation OOT Interaction diagram Sequence diagram & Communication diagram Activity diagram Plus: decision tree & flowchart 43/68 44/68
Structured English Structured English Structured English: modified form of English language used to specify the logic of IS processes Typically relies on action verbs and noun phrases and contains no adjectives or no specific standards 45/68 46/68 Structured English Structured English Constructs (Part 1) 47/68 Whitten 48/68
Structured English Constructs (Part 2) Process Description: Modeling Logic * * * 49/68 Process #: Process Name Description:... Created Date:... Input Actor form P#:... attribute,... D#:... (attribute,...) Output Actor report P#:... attribute,... D#:... (attribute,...) Tasks and Activities (show logic not brief description) Use: Structured English or Activity diagram Flowchart Decision table/ tree etc. p output D# data store P# process input boundary p 50/68 Decision Table Decision Table Decision table: a matrix representation of the logic of a decision which specifies the possible conditions for the decision and the resulting actions Best used for complicated decision logic 1 2 3 51/68 52/68
Decision Table Modeling Logic with Decision Table Condition stubs: list conditions that relevant to the decision Action stubs: list actions that result for a given set of conditions Rules: define which actions are to be followed for a given set of conditions 53/68 54/68 Logic Modeling (Cont.) Logic Modeling (Cont.) 55/68 56/68
Summary Understand the logical process modeling by using data flow diagrams (DFDs) Draw DFDs & Leveling Balance higher-level and lower-level DFDs Explain the difference among four types of DFDs: current physical, current logical, new physical, and new logical Process Description Questions & Answers 57/68 58/68 Exercise What is a data flow diagram? Why do system analysts use data flow diagrams (DFDs)? Explain the rules of drawing good DFDs. What unique rules apply to drawing context diagram? Case Study A simple banking system 1 Open new a/c 2 Deposit and withdraw 3Close a/c Draw the context diagram and DFD of this simple banking system. 59/68 60/68
Main Activity New customers are registered; and accounts are closed by bank manager When accounts are closed, customers are notified by a system Cash deposits into an account are made by the customer, and salary are paid by the employer The bank clerk performs a balance check before allowing a withdrawal by a customer at the bank counter List of system logical elements List of System Logical Elements Wichian 61/68 Wichian 62/68 Data Flow Diagram : a simple banking system Context Diagram : A simple banking system Wichian 63/68 64/68 Wichian
Data Flow Diagram : a simple banking system Wichian 65/68 66/68 Wichian Problems of System Development Poor system thinking Poor system integration Poor verifying the model Physical Data Flow Diagrams (PDFDs) Technical and human decisions to be implemented as part of an information system Finding missing algorithm Syntax checking Tracing data element Cross-referencing Tracing objective 67/68 68/68
Example: Classify the following aspects into functional (F), non-functional (NF), or not be a requirement. 1. How information about flights, passengers and booking are entered. 2. What information appears on tickets and reports. 3. How fares are calculated. 4. What information must be stored in the database that travel agents and others access. 5. The system should be designed so it can be extended to handle a frequent flier plan. 6. The system must be available at all times. Only 2 minutes downtime a week is to be permitted. 7. A merge sort algorithm must be used to sort the flights by departure time. 69/68