Chapter 5

Chapter 5

ESSENTIALS OF SYSTEMS ANALYSIS AND DESIGN FOURTH EDITION Chapter 6 Structuring System Requirements: Process Modeling 6.1 6.1 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Learning Objectives Explain process modeling Discuss data-flow diagramming mechanics, definitions, and rules Discuss balancing data-flow

diagrams Discuss the use of data-flow diagrams as analysis tools Examine decision tables used to represent process logic 6.2 6.2 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Process Modeling Graphically represents the processes that capture, manipulate, store, and distribute data between a system and its environment and among system components Data-flow Diagrams (DFD): The most popular

method of Process Modeling Graphically illustrate movement of data between external entities and the processes and data stores within a system. Movement of data BUSINESS PROCESS. Your job is to make systems efficient by understand data flows. 6.3 6.3 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Process Modeling (continued) Modeling a Systems Process Utilize information gathered during requirements determination. Find inefficiencies in current system Re-engineer current system Structure of the data is also modeled in addition to the processes Deliverables and Outcomes Set of coherent, interrelated data-flow 6.4 6.4 diagrams Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Process Modeling (continued) Deliverables and Outcomes (continued) Context data-flow diagram (DFD) Scope of system DFDs of current system Enable analysts to understand current system 6.5 6.5 DFDs of new logical system Technology independent

Show data flows, structure and functional requirements of new system Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall data store Process Interface Source/ Sink 6.6 6.6 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Mechanics Data Flow

data in motion marks movement of data through the system - a pipeline to carry data connects the processes, external entities and data stores Unidirectional originate OR end at a process (or both) name as specifically as possible - reflect the composition of the data - a noun do not show control flow! Control flow is easy to identify- a signal 6.7 6.7 with only one byte - (on/off). HINT: if you can't name it: either it's control flow, doesn't exist or

you need to get more information! Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Mechanics (continued) Data Store Depicts data at rest data store represents holding areas for collection of data, processes add or retrieve data from these stores May represent data in: File folder, Computer-based file, Notebook Label includes name of the store as well as the number only processes are connected to data stores

name using a noun (do not use file) show net flow of data between data store and process. For 6.8 6.8 instance, when access a DBMS, show only the result flow, not the request Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Mechanics (continued) Process Process Depicts work or actions performed on data so

that they are transformed, stored, or distributed Drawn as a rectangle with rounded corners Number of process as well as name are recorded name with a strong VERB/OBJECT combination; examples: create_exception_report validate_input_characters calculate_discount 6.9 6.9 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Mechanics (continued)

Interface Source/ Source/Sink Sink Depicts the origin and/or destination of the data Sometimes referred to as an external entity Any class of people, an organization, or another system which exists outside the system you are studying. Form the boundaries of the system Drawn as a square symbol Name states what the external agent is NOUN Because they are external, many characteristics 6.10 6.10 are not of interest to us

Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Two Source/ Sink Cannot Communicate without a process 6.11 6.11 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Definitions Context Diagram A data-flow diagram of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major

information flows between the entities and the system Level-O Diagram A data-flow diagram that represents a 6.12 6.12 systems major processes, data flows, and data stores at a higher level FOR THE PURPOSE OF THIS CLASS WE WILLMAINLY CONSIDER TILL THIS LEVEL Copyright 2009 Pearson Education,

Inc. Publishing as Prentice Hall Developing DFDs: An Example Hoosier Burgers Automated Food Ordering System Context Diagram (Figure 6-4) contains no data stores 6.13 6.13 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall 6.14

6.14 The System uses one process (Food Ordering System), four data flows and three sources and sinks Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Developing DFDs: An Example (continued) Next step is to expand the context diagram to show the breakdown of processes (Figure 6-5) Food ordering System has 4 subprocesses Receive and Transform Order Update goods sold file (for materials

ordering) Update inventory file Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall 6.16 6.16 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Data-Flow Diagramming Rules Basic rules that apply to all DFDs: Inputs to a process are always different than outputs . THUS INPUTS OUTPUTS

Objects always have a unique name In order to keep the diagram uncluttered, you can repeat data stores and data flows on a diagram You dont want the DFD looking like a Spaghetti. 6.17 6.17 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall DFD Rules -- Process A. No process can have only outputs (Miracle) B. No process can have only inputs (Black hole) C. (1) Use Verb phrase

labelEvery Process has Input & Output DFD Rules -- Process Incorrect Miracle A. Black Hole B. Correct DFD Rules -- Data Store

D. Data cannot move directly from one data store to another data store . You NEED A PROCESS. E.Data cannot move directly from an outside source to a data store. AGAIN YOU NEED A PROCESS F. Data cannot move directly to an outside sink from a data store. AGAIN YOU NEED A PROCESS One end of a Data Flow must be a Process G. Use a Noun phrase label (Entity Name) DFD Rules -- Source /

Sink H. Data cannot move directly from a source to a sink. It must be moved by a process. I. Noun phrase label. (External) DFD Rules -- Data Flow J. A data flow has only one direction of flow between symbols; a data flow may flow in both directions to and from a data store

(usually two symbols) K. A fork in a data flow means that exactly the same data goes to two different processes or data stores. L. A join in a data flow means that exactly the same data comes from two different processes and data stores. DFD Rules -- Data Flow Incorrect J. K. L.

Correct DFD Rules -- Data Flow M. A data flow cannot go directly back to the same process it leaves N. A data flow to a data store means create, update or delete O. A data flow from a data store means retrieve or use P. Use a Noun phrase label. Contents are attributes of entities and data items Decomposition of DFDs

Functional Decomposition Act of going from one single system to many component processes Repetitive procedure Lowest level is called a primitive DFD Level-n Diagrams A DFD that is the result of n nested decompositions of a series of sub-processes from a process on a level-0 diagram 6.25 6.25 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Balancing DFDs When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition This is called balancing The number of inputs and Outputs must be balanced across levels. 6.26 6.26 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Balancing DFDs An Unbalanced Example

In context diagram, we have one input to the system, A and one output, B Level-0 diagram has one additional data flow, C These DFDs are not balanced Draw a Border around the System. Check for the number of entries and exists to and from the systems. THEYSHOULD MATCH ACROSS LEVELS 6.27 6.27

Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Balancing DFDs We can split a data flow into separate data flows on a lower level diagram 6.28 6.28 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Guidelines for Drawing DFDs 1. Completeness

DFD must include all components necessary for system Each component must be fully described in the project dictionary or CASE repository 2. Consistency 6.29 6.29 The extent to which information contained on one level of a set of

nested DFDs is also included on other levels Balancing is a part of this. Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Guidelines for Drawing DFDs (continued) 3. Timing Time is not represented well on DFDs Best to draw DFDs as if the system has never started and will never stop 4. Iterative Development

6.30 6.30 Analyst should expect to redraw diagram several times before reaching the closest approximation to the system being modeled REMEMBER it is an APPROXIMATION Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Guidelines for Drawing DFDs (continued)

5. Primitive DFDs Lowest logical level of decomposition Decision has to be made when to stop decomposition When do you stop decomposing? When each task is a singular task that can be programmed . 6.31 6.31 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Guidelines for Drawing DFDs (continued) 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 6.32 6.32 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Guidelines for Drawing DFDs (continued) Rules for stopping decomposition: (continued) 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 6.33 6.33

When you believe that there is a separate process for each choice on all lowest-level Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Using DFDs as Analysis Tools Gap Analysis 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 6.34 6.34 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Using DFDs in Business Process Reengineering Example: IBM Credit Credit approval process is required six days before Business Process Reengineering (see Fig 6-12) After Business Reprocess

Engineering, IBM was able to process 100 times the number of transactions in the same amount of time 6.35 6.35 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Logic Modeling Data-flow diagrams do not show the logic inside the processes. They also dont show the sequence

of activities Logic modeling involves representing internal structure and functionality of processes depicted on a DFD Utilizes Decision Tables 6.36 6.36 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Modeling Logic with Decision Tables A matrix representation of the logic of a decision Specifies the possible conditions and the resulting actions Best used for complicated decision

logic 6.37 6.37 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Modeling Logic with Decision Tables Consists of three parts: (continued) Condition stubs Lists condition relevant to decision Action stubs Actions that result for a given set of conditions Rules

Specify which actions are to be followed for a given set of conditions 6.38 6.38 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall Modeling Logic with Decision Tables (continued) Indifferent Condition Condition whose value does not affect which action is taken for two or more rules Standard procedure for creating

decision tables: Name the conditions and values each condition can assume Name all possible actions that can occur List all possible rules Define the actions for each rule (See Figure 6-16) Simplify the decision table (See Figure 6-17) 6.39 6.39 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall

Condition Stubs 6.40 6.40 Action Stubs Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall 6.41 6.41 Copyright 2009 Pearson Education, Inc. Publishing as Prentice Hall DECISION TREES A graphical representation of a decision situation in which decision points (nodes) are connected

together by arcs (one for each alternative decision) and terminate in ovals (the action which is the result of all the decisions made on the path that leads to that oval). Sun Up? Yes No What Day? Go back

to Slee p Slee p2 more Hour s Saturday ay d n Su

Slee p1 more Hour W ee kD ay Wak e up

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