Analysis – 003
Goals:
Process modeling (Data flow diagrams) Recognize and describe abstractions Identify the four elements of a DFD Construct a DFD
Decompose a DFD
Modeling the System
Process models: diagrams how data flows through the system Data model: diagrams the relationships between data files
Object model: diagrams the relationships between enterprise objects Systems models: diagram system functions, hardware, and software
DFD and the Six CIS components
Recall that classifying CIS into six components helps us know which parts to focus on when performing a given function. The Process Model (DFD) includes (a) people, (b) procedures, and (c) data from the Six CIS components [leaving hardware, networks, and software aside for now.]
Abstractions and CIS
Abstraction is a simplified description, view, or model of an object or system of objects. In a CIS: the process, data, object, and system models are abstractions of the computer infor-mation system.
DFD – Data Flow Diagram
DFD is a picture of what people and procedures do to transform data into info DFD has four elements:
1. External entity: the original of data (source) or the receiver of information (sink) 2. Process: a series of steps that manipulate data
3. Data store: place to keep the data for later use
4. Data flow: describes the data and information elements passing between external entities, processes, and data stores.
[know these]
Graphing the DFD – Symbol Sets
Many symbol sets available! Gane-Sarson DFD Symbol Set: [3-3, chart7]
Data flows must originate or terminate at a process
Data stores must have at least one entry and one exit data flow Processes must have at least one entry and one exit data flow
Bottom-Up DFD Creation
Develop a narrative of the system Underline the action words
Develop a sequential list of the action terms Eliminate tasks that do not transform the data Identify cohesive tasks
Fit all the remaining tasks to a cohesive task Develop an IPO chart for each cohesive task Example: Part 1
“Sometimes the office manager cannot determine whether the request is reasonable. When this happens, the request is forwarded to the owner, who makes this determination, con-tacts the customer if necessary, and either adjusts the request or rejects it outright. All requests are returned to the office manager for scheduling and filing.”
Example: Part 2
Action Word Task Number Task Description
Submits none not identified
Is forwarded none not identified
Contacts #1 evaluate special requests
Adjusts #1 evaluate special requests Rejects #1 evaluate special requests
Are returned none not identified
Consults #2 schedule charter
Is reserved #2 schedule charter
Checks #2 schedule charter
Find #2 schedule charter
Are entered #2 schedule charter
Contacts #3 complete charter papers
Finalize #3 complete charter papers
Prepares #3 complete charter papers
Signs #3 complete charter papers
Example: Part 3
Input Processing Output
Request contacts notation
Adjusts Rejects
Example: Part 4
Process Data Flow Process Data Flow Data Store Intake request evaluate special note charter requests
Requests
Top-Down DFD creation
Get oral answers to the question “What is the first task of this system?” Continue with “What happens next?”
Keep asking what happens next until the response is either “We do it all over again” or “We’re finished.”
Context Diagrams
Show a single process
Connected to the external entities Establishes the system boundaries. Example: A Time Card System
Level Decomposition
Single process in the context diagram is composed of a collection of well-defined activities called cohesive tasks.
Task definition begins by locating the events within a process. Events identify the end of one task and the beginning of another.
Once a task is identified, the analyst can identify the inputs and outputs. Example: Level Decomposition
•
Issue Access Cards INPUTS:–
class rosters (Source: Data Processing)–
access cards for registered students (Source: Data Processing)–
student registration (Source: Data Processing)–
signed non-piracy agreement (Source: Student)–
name on class rosters (Data Store: Roster)•
Issue Access Cards OUTPUTS:–
lab handbook (Sink: Student)–
card numbers already issued (Sink: Data Processing)–
checkoff on class rosters (Data Store: Roster)–
new lab account (Data Store: Time) nonpiracy agreement (Data Store: Agreements)Logical and Physical DFDs
Logical DFDs remove all reference to the implementation specifics of the system Physical DFDs specify the real world objects that are used to make the system work During the analysis phase:
Develop the physical DFD Abstract the logical DFD During the design phase:
Develop the logical DFD Develop the physical DFD
Class Project Discussion
The analysis phase has started. Now you must understand more fully the existing system before a replacement system can be designed. The process model is the first abstraction to develop. Example: Existing System Context Diagram
Example: Existing System First-Level DFD
Class Project: Process Modeling
Prepare the existing system context diagram and first level DFD Review the project management sections of Appendices A and B Develop the first draft of your week-by-week project budget Develop the first draft of your week-by-week project status
Prepare a cover letter to transmit the budget and status reports to your client.
Recap
DFD models the processes of an information system
DFDs consist of processes, external entities, data stores, and data flows
DFD modeling begins with the context diagram, which depicts the entire system in a single process symbol surrounded by external entities.
The first-level DFD (aka level-1) show the major functional components of the system as processes, connected by data flows to external entities, data stores, and other processes.
understand-ing of the system.
[task] IPO
Narrative: xxx Task: [task] Inputs:
The item (Source: source name)
Another item, and more (Data Store: staff)
Outputs:
Paycheck (Data Store: staff) Mailing labels (Sink: staff) Newsletter (Sink: staff)
