Chapter 6 Structuring System Requirements Process Modeling Learning
- Slides: 73
Chapter 6 Structuring System Requirements: Process Modeling
Learning Objectives Understand the logical modeling of processes through studying data flow diagrams ü How to draw data flow diagrams using rules and guidelines ü How to decompose data flow diagrams into lower-level diagrams ü Balancing of data flow diagrams ü 2
Learning Objectives (Cont. ) Explain the differences among four types of DFDs: current physical, current logical, new physical, and new logical. ü Discuss process modeling for electronic commerce applications. ü 3
Process Modeling 4
Structuring System Requirements n Modeling a system’s process ¨ The two sub-phases of the analysis phase of the SDLC n n Requirement determination Requirements structuring In the requirement structuring phase, the SA must n organize and utilize the information gathered during requirements determination into a meaningful representation of the IS that exists (Process Modeling) n Model the processing logic and the timing of the events (logic Modeling). n Structure of the data within the system (Data Modeling) 5
Traditional vs. Object-Oriented Approaches 6
Requirements for the Traditional and OO Approaches 7
Process Modeling Graphically represent the functions or processes that capture, manipulate, store and distribute data between a system and its environment and among system components n A common form of process model is Data flow diagram (DFD) n 8
Process Modeling n Data flow diagrams enable us to model how data flows through an information system (i. e. between external entities and the processes and data stores within a system), ¨ the relationship among data flows, and ¨ how data is stored at specific locations. ¨ the processes that change or transform data. ¨ 9
Process Modeling n As data flow diagrams concentrate on the movement of data between processes, these diagrams are called process models. 10
Process Modeling n Deliverables and Outcomes ¨ Context data flow diagram (DFD) n Scope of the system, indicating which elements are inside and outside the system. ¨ DFDs of current physical system (Adequate detail only) ¨ DFDs of current logical system n n n 8. 11 Technology-independent DFD Shows what data processing functions are performed by the current information system. Enables analysts to understand current system 11
Process Modeling n Deliverables and outcomes (continued) ¨ DFDs of new logical system n Technology independent n Show data flows, structure and functional requirements of new system 8. 12 12
Data Flow Diagramming Mechanics DFDs are versatile diagramming tools. n Four symbols are used to represent both physical and logical IS. n ¨ Two different standard n De. Marco and Yourdan n Gane and Sarson 8. 13 sets can be used 13
Definitions and Symbols 14
Data Flow Diagramming Mechanics n Data Flow ¨ ¨ ¨ Represent data in motion and moving from one place to another in the system. A data flow is data that move together. It can be composed of many individual pieces of data that are generated at the same time and flow together to common destinations. Drawn as an arrow Select a meaningful name to represent the data Example: n n Data on a customer order form or a payroll check. The results of a query to a database. The contents of a printed report Data on a data entry computer display form. 15
Data Flow Diagramming Mechanics n Data Store Represent data at rest ¨ May represent one or many different physical locations for data e. g. ¨ n n n File folder Computer-based file(s) Notebook Represented by a rectangle with a vertical line on the left side. At the left end, the number for the data store is written. At the right side, a meaningful label for the data store is written. ¨ Examples : ¨ n Data about the customers, students, customer orders, or suppliers invoices, roster of classes 16
Data Flow Diagramming Mechanics n Process Represent work or action performed on data so that they are transformed, stored or distributed ¨ Represented by a rectangle with rounded corners, and a line drawn through the top. The upper portion is used to indicate the number of process, and the lower portion indicates the name for the process. ¨ When modeling data processing of a system, it doesn’t matter whether a process is performed manually or by a computer. ¨ Examples: ¨ n n n Generate paycheck Calculate overtime pay Compute grade point average 17
Data Flow Diagramming Mechanics n Source/Sink ¨ Represent the origin and/or destination of the data i. e. Source which produces data or Sink which consumes or accepts data. Sometimes referred to as an external entity, as they are outside the system. ¨ Drawn as a square symbol ¨ Name states what the external agent is 18
Data Flow Diagramming Mechanics (Source/Sink) ¨ Because they are external to the Information system, many characteristics are not of interest to us ¨ Examples of sources or sinks are as: n n n A person inside or outside the business units supported by the system the SA is analyzing and who interacts with the system e. g. customer Another information system with which the system (the SA is analyzing) exchanges information e. g. instructor information system and student information system Another Organization that sends data to or receives information from the system ( the SA is analyzing) e. g. book supplier and the university. 19
Do not consider the following characteristics of sources and sinks When drawing Data Flow Diagrams, do not show n Direct Interactions that occur between source and sinks. n How information is used in the Source or Sink ( i. e. show a source or sink a black box) n How to control or redesigning a source or sink from the perspective of the information system. n Sources or Sink directly accessing stored data since external agents cannot directly manipulate data stored within the system i. e. processes within the system must receive or distribute data between the system and its environment. 20
Example : Hoosier Burger’s Customer Order System n n Hoosier Burger uses an information system known as customer order information system which is used to ¨ take Customer order from the Customer. and gives ¨ the Food order to the kitchen, ¨ the Receipt is to the Customer ¨ Management Reports to the Restaurant Manager Draw the highest level Data flow Diagram i. e. context diagram 21
Data Flow Diagramming Definitions n Context Diagram ¨A data flow diagram (DFD) of the scope of an Information system that shows the n n n Whole system boundaries, external entities that interact with the system and major data flows between the external entities and the system ¨ Highest level view of ¨ The context diagram the system contains only one process labeled as “ 0” representing the entire system. ¨ The data stores of the system are conceptually inside the one process, no data stores appear on a context diagram. ¨ The sources/sinks represent its environment 22
Context Diagram 23
Context Diagram for RMO Customer Support System 24
Data Flow Diagramming Definitions Level-0 Diagram (Diagram after the Context diagram) n Next step is to expand the context diagram to show the breakdown of processes ¨A data flow diagram (DFD) that represents a n n n system’s major individual processes (functions), data flows and data stores at a high level of detail ¨ Each process has a number that ends in. 0 ¨ Capturing data from different sources ¨ Maintaining data stores ¨ Producing and distributing data to different sinks ¨ High level descriptions of data transformation operations. 25
DFD Fragment Showing Use Case Look Up Item Availability from the RMO 26
DFD Integrates Event Table and ERD 27
Developing DFDs: An Example ¨ The system begins by receiving and transform the customer order ( name the process as 1. 0) and outputs four data flows. 1. 2. 3. 4. Food order send to the kitchen. goods sold data. inventory data a receipt given to the customer. 28
Developing DFDs: An Example n n n Goods sold data obtained from process 1. 0 is used to update the Goods Sold File by adding the new good sold data obtained in process 1. 0 (Name the process as P 2. 0 and the data file as D 2). Inventory data obtained from process 1. 0 is used to update the Inventory File by deducting the inventory data (no. of items from the item stock) from the inventory file( Name the process as P 3. 0 and the data store as D 1) The Management reports are produced by reading the information from the updated D 1 and D 2 i. e. daily goods amount sold file and inventory data file ( Name the process as P 4. 0). These Management reports are given to the Restaurant Manager 29
Figure 7 -5 Level-0 DFD of Hoosier Burger’s food ordering system 30
Data Flow Diagramming Rules n There are two DFD guidelines that apply: ¨ The inputs to a process are different from the outputs of that process. n Processes purpose is to transform inputs into outputs. ¨ Objects n on a DFD have unique names. Every process has a unique name.
Data Flow Diagramming Rules 32
Decomposition of DFDs n 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 subprocess can logically be broken down any further. 33
Decomposition of DFDs (Cont. ) Primitive DFD is the lowest level of a DFD. n Level-1 diagram results from decomposition of Level-0 diagram. n Level-n diagram is a DFD diagram that is the result of a n nested decompositions from a process on a level-0 diagram. n 34
Decomposition of level-0 DFD of Hoosier Burger System Level -1 Data flow Diagram n The first process 1. 0 called Receive and Transform Customer Food order is decomposed into five 5 logically separate processes or functions. They are Receive customer order ( P 1. 1) Transform the order into a kitchen format used for the kitchen (P 1. 2) ¨ Transform the order into a printed receipt for the customer ( P 1. 3) ¨ Transform the order into goods sold data ( P 1. 4) ¨ Transform the order into inventory data ( P 1. 5) ¨ ¨ n n No source and sinks are represented are not shown in level -1 DFD. Decomposing Process 1. 0, 2. 0 or 3. 0 or 4. 0 would create level-1 diagram. 35
Level - 1 36
Decomposition of level-0 DFD of Hoosier Burger System n n n Process 2. 0 and Process 3. 0 use data input to update data stores. Since updating a data store is a singular logical function , neither of the processes need to be decomposed further. Each level-1, -2 or –n DFD represents one process on a level-(n-1) DFD. Each DFD should be on a separate page. Process 4. 0 (Produce Management reports) can be decomposed into three sub-processes. Access Goods Sold and Inventory data (P 4. 1) ¨ Aggregate Goods Sold Inventory Data ( P 4. 2) ¨ Prepare Management Reports ( P 4. 3) ¨ 37
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) DFDs if necessary. 38
Decomposition of level-1 DFD of Hoosier Burger System n Process 4. 3 should be further decomposed into sub processes ¨ Format Management Reports (P 4. 3. 1) ¨ Print Management Reports ( P 4. 3. 2) 39
Level-2 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. 40
Layers of DFD Abstraction for Course Registration System 41
DFD Fragments n Created for each use case in the event table n Represent system response to one event within a single process symbol n Self-contained models n Focus attention on single part of system n Show only data stores required in the use case 42
Three Separate DFD Fragments for Course Registration System 43
Balancing DFDs n n n When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition This is called balancing Example: Hoosier Burgers In Figure 7 -4, notice that there is one input to the system, the customer order ¨ Three outputs: ¨ n n n 8. 44 Customer receipt Food order Management reports 44
Balancing DFDs n Example (Continued) ¨ Notice Figure 7 -5. We have the same inputs and outputs ¨ No new inputs or outputs have been introduced ¨ We can say that the context diagram and level -0 DFD are balanced 8. 45 45
Balancing DFDs n n Notice Fig 7 -7, where Process 1. 0 from the level 0 DFD has been decomposed. Process 1. 0 has one input and four outputs. The inputs and outputs in level-1 diagram are the same. No new inputs and outputs have been added. Compare Process 4. 0 in fig 7 -5 to its decomposition in fig 7 -8, the inputs and outputs are the same. 46
Balancing DFDs (Cont. ) n 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 47
Balancing DFDs (Cont. ) 1 input 1 output 2 inputs 1 output 48
Balancing DFDs The DFDs are balanced when we split a data flow going to a process into separate data flows going to two processes (see Figure 7 -11). These diagrams are still balanced since exactly the same data are included in each diagram. n Balancing leads to four additional advanced rules (See Table 8 -3) n 8. 49 49
Advanced Rules governing data flow diagrams n n n A composite data flow on one level can be split into component data flows at the next level, but no new data can be added. The inputs to a process must be sufficient to produce the output from the process. To avoid having data flow lines cross each other, repeat data stores or source/sinks on a DFD. Use an additional symbol, like a double line on the middle vertical line of a data store symbol, or a diagonal line in a corner of a sink/source square 50
Balancing DFDs (Cont. ) © 2008 by Prentice Hall 51
Four Different Types of DFDs n 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. n Current Logical ¨ Physical aspects of system are removed as much as possible. ¨ Current system is reduced to data and processes that transform them. 52
Four Different Types of DFDs (Cont. ) n New Logical ¨ Includes additional functions. ¨ Obsolete functions are removed. ¨ Inefficient data flows are reorganized. n New Physical ¨ Represents the physical implementation of the new system. 53
Physical DFD for Scheduling Courses 54
Evaluating DFD Quality Readable n Internally consistent and balanced n Accurately represents system requirements n Reduces information overload – rule of 7 +/ -2 n ¨ 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 n Minimizes required number of interfaces 55
Data Flow Consistency Problems n Differences in data flow content between a process and its process decomposition n Data outflows without corresponding inflows n Data inflows without corresponding outflows n Results in unbalanced DFDs 56
Consistency Rules n All data that flows into a process must ¨ Flow out of the process, or ¨ Be used to generate data that flows out of the process n All data that flows out of a process must ¨ Have flowed into the process, or ¨ Have been generated from data that flowed into the process 57
Unnecessary Data Input: Black Hole 58
Process with Impossible Data Output: a Miracle 59
Process with Unnecessary Data Input 60
Process with Impossible Data Output 61
n Rules for stopping decomposition ¨When each process has been reduced to a single decision, single calculation or single database operation ¨When each data store represents data about a single entity, such as a customer, employee, product or order. ¨When the system user does not care to see any more detail 62
n Rules for stopping decomposition (continued) ¨ When the analyst have documented sufficient detail to do subsequent systems development task. ¨ 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 63
Documentation of DFD Components n Lowest-level processes need to be described in detail n Data flow contents need to be described n Data stores need to be described in terms of data elements n Each data element needs to be described n Various options for process definition exist 64
Structured English n Method of writing process specifications n Combines structured programming techniques with narrative English n Well-suited for lengthy sequential processes or simple control logic (single loop or if-then-else) n Ill-suited for complex decision logic or few (or no) sequential processing steps 65
Structured English Example 66
Process 2. 1 and Structured English Process Description 67
Decision Tables and Decision Trees n Can summarize complex decision logic better than structured English n Incorporate logic into the table or tree structure to make descriptions more readable 68
Decision Table for Calculating Shipping Charges 69
Decision Tree for Calculating Shipping Charges 70
Data Flow Definitions n Textual description of data flow’s content and internal structure n Often coincide with attributes of data entities included in ERD plus computed values n Algebraic notion describes data elements on data flow plus data structure 71
Summary n In this chapter you learned how to: ü Understand logical process modeling via data flow diagrams (DFDs). ü Draw data flow diagrams of well structured process models. ü Decompose data flow diagrams into lowerlevel diagrams. 72
Summary (Cont. ) ü Balance high-level and low-level data flow diagrams. ü Explain differences between current physical, current logical, new physical, and new logical data flow diagrams. ü Use data flow diagrams for analyzing information systems. 73
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