analysis dfd 003

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Systems Analysis Process Modeling (DFD) 1 of 10

Analysis 003

Goals:

Process modeling (Data flow diagrams)

Recognize and describe abstractions

Identify the four elements of a DFD

Construct a DFDDecompose 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 componentsRecall 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 abstractionsof the computer infor-

mation system.

DFD Data Flow Diagram

DFD is a picture of what people and procedures do to transformdata into info

DFD hasfour 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]

How to Construct the DFD some rules


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Data flows must originate or terminate at a process

Data stores must have at least one entry and one exit data f low

Processes must have at least one entry and one exit data f low

Bottom-Up DFD Creation

Develop a narrative of the systemUnderline 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 requestsAre 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


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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

Were finished.

Context Diagrams

Show a single process

Connected to the external entities

Establishes the system boundaries.

Example: A Time Card System

Example: Revised Time Card System


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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:

access cards (Sink: Student)


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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

TKSystem First-Level DFD


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Data Flow Walk-Thru


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TK System DFD Levels


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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


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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 systemDFDs 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 componentsof the system as

processes, connected by data flows to external entities, data stores, and other processes.

As needed, youll create successively more detailed levels to promote complete understand-


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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)

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