1 Chapter 4 Documenting Information Systems Yong Choi BPA CSUB.

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Chapter 4Documenting Information

Systems

Yong Choi

BPA

CSUB

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System Concept

• A system exits by taking input from the environment , transforming (processing) input, and release an output

• A system may be decomposed (exploded) into subsystems

• Output of one subsystem may become the input of other subsystems

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System Concept (con’dt)

TheSystem

is aProcess

input output

input

input output

output

Feedack andControl Loop

The System's Environment(constantly changing)

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System Decomposition (General)

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Decomposition Diagram (Hierarchy Chart)

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Example of Decomposition Diagram

SoundStage Entertainment Club

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Enter ServiceRequest

Client

Technician

Enter Work Record

Establish NewClient

Establish New Equip

Resolve ServiceRequest

Management

Establish New WorkType

Modify Work Type

Establish EquipType

«uses»

Enter Misc Charge

Enter ComponentInformation

EstablishComponent Type

Service Subsystem

Mgmt Subsystem

Equipment Subsystem

Check UnresolvedRequests

«uses»

Make ServiceStatistic Inquiry

Generate MonthlyInvoices

Time

Decomposition: Use-Case Diagram

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Separating data and processes

• Characteristic

• System focus

• Design stability

• Data organization

• State of data

Data-orientation

Operation of Data

More stable as data needs change slowly

Designed for enterprise

Controlled

Process-orientation

what, when & how to do

Limited due to change in business processes

Designed for individual applications

unstructured

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Separating data and processes

• Process-oriented approach– An IS development strategy that focuses on how and when data

are moved through and transformed by an IS

– Sequence of steps in the process-oriented approach:• define problem -> identify necessary decisions -> describe

information needs -> determine necessary processing -> specify data needs

• Data-oriented approach– An IS development strategy that focuses on the optimal

organization of data, not on where or how data are used

– Sequence of steps in the data-oriented approach:• define problem -> identify requirements -> develop data model for

each user's data -> integrate all the data models into an overall model

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Process Modeling with DFD

Process Modeling is a technique for organizing and documenting the structure and flow of data through a system’s processes, and the logic, policies, and procedures to be implemented by a system’s processes.

A Data Flow Diagram (DFD) is a graphical modeling tool to depict the flow of data through a system and the work or processing performed by that system.

– What’s the system doing

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Data Flow Diagrams (DFD)

• Key points for modeling– How data moves through the organization– Relationships between various data flows– Storage of data

• There are no FIXED rules about how a DFD should be developed…

• There is no such a DFD call “CORRECT DFD”…• Expert SAs may not need much…..but definitely

need for YOU!!

Developing a DFD

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Simple Data Flow Diagram for Banking

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Process Logic

• DFDs are effective tools for identifying processes, but are not good at showing all the detail logic inside those processes.– Not always easy to know what the lowest logical level is

• However, it is better than:– Flowcharts and Pseudocode (difficult for users to

understand).

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Flowchart

Flowchart Limitations

• Complex logic: Sometimes, the program logic is quite complicated. In that case, flowchart becomes complex and clumsy.

• Alterations and Modifications: If alterations are required the flowchart may require re-drawing completely.

• Reproduction: As the flowchart symbols cannot be typed, reproduction of flowchart becomes a problem.

• The essentials of what is done can easily be lost in the technical details of how it is done.

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Pseudocode• This is the pseudocode for a Game of Monopoly, including one person's move as a

procedure:

Main Procedure Monopoly_Game Hand out each player's initial money. Decide which player goes first. Repeat

Call Procedure Monopoly_Move for next player. Decide if this player must drop out.

Until all players except one have dropped out. Declare the surviving player to be the winner.

Procedure Monopoly_Move Begin one's move. Throw the dice. Move the number of spaces on the board shown on the dice. If the token landed on "Go to Jail,"

then go there immediately. Else if the token landed on "Chance" or "Community Chest,"

then draw a card and follow its instructions. Else

follow the usual rules for the square (buying property, paying rent, collecting $200 for passing "Go", etc.).

End one's move.

http://www.wiley.com/college/busin/icmis/oakman/outline/chap05/misc/monopoly.htm

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Elements of a DFD

External Entity

Data Flow

Process

Data Store

DE MARCO & YOURDON NOTATIONS (Textbook)

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Elements of a DFD

External Entity

Data Flow

Process

Data Store

GANE & SARSON NOTATIONS (more universal)

Pay Bill

AP Clerk

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External Entities

• An External Entity is a provider (source) or receiver (sink) of data and information of the system

• External agents define the “boundary” or “scope” of a system being modeled.

SUPPLIER

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External Entities …

• An external entity (agent) defines a person, organization unit, or other organization that lies outside of the scope of the project but that interacts with the system being studied. – As scope changes, external agents can become processes, and

vice versa.– Almost always one of the following:

• Office, department, division inside the business but outside the system scope.

• An external organization or agency.

• Another business or another information system.

• One of system’s end-users or managers

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Data Stores

• A data store is an inventory of data. – A data store means “data at rest.” – A data flow means “data in motion.”

D1 Accounts Receivable

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Data Stores …

– Almost always one of the following:• Persons (or groups of persons): e.g., customer• Places: e.g, cash register• Objects: e.g., product• Events (about which data is captured): e.g., sales• Concepts (about which data is important): e.g., discount

– Data stores depicted on a DFD store all instances of data entities (depicted on an ERD)

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Data Flows

• A Data Flow represents an input of data to a process, or the output of data from a process.– A data flow may also be used to represent the creation,

reading, deletion, or updating of data in a “data store.”– A composite data flow (packet) is a data flow that consists of

other data flows.

• A Data Flow does not represent a document or a physical good: it represents the exchange of information in the document or about the good

DELIVERY SLIP

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Processes

• A Process is a work or action performed on input data flow to produce an output data flow

• Use a verb to label the action performed by the process (not the name of person or department who does it as in physical DFD)

• A Process must have at least one input data flow and at least one output data flow.

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Pay Bill

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How to find each one…

• External Entities (EE) – people/organizations/things that supply information to or use

information from the system

• Processes (P)– actions/doing words/verbs

• Data Flows (DF)– movement/exchange of information/data between external

entities to processes, and processes to processes

• Data Stores (DS)– store/record information/data

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DFD using Use-Case

• The actor that initiates the event– External entities

• The use cases– Will be handled by a process

• Inputs (or triggers) & outputs and responses– Will become a flow of data

** data store is a result of data flow..thus, need naming

creativity… **

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DFD Naming Guidelines

• External Entity Noun

• Data Flow Names of data

• Process verb phrase

– a system name

– a subsystem name

• Data Store Noun

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Guidelines for Developing DFD

1. Develop a list of business activities

– Similar to business rules of ERD

2. Create the Context Diagram (we know this)

– shows external entities and data flows to and from the

system: conceptual DFD

3. Create level 0 DFD (or Diagram 0 in textbook)

– shows general processes at the highest level

4. Create level 1 DFD (or Child Diagram in textbook)

– shows more detailed processes

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Apply logical sequence of the system..

• A student (EE / DS) sends in an application form (DF) containing their personal details, and their desired course

• The university checks (P) that the course is available. • If the course (DS) is available the student is enrolled (P)

in the course, and the university confirms (P) the enrolment by sending a confirmation letter (DF) that they are registered for the course to the student.

• Or if the course is unavailable the student is sent a rejection letter (DF).

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Level 0 Tips

• Generally move from top to bottom, left to right• Minimize crossed lines• Iterate as needed

– The DFD is often drawn many times before it is finished, even with very experienced systems analysts

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Level 0 and 1 DFD Building Procedure

• Level-0 DFD – Identify what is being done between each input and

its corresponding output– Identify the processes– Identify external data flows between external entities

and processes– Identify internal data flows between processes and

data stores

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Tips for Level 1 and Below

• Level-1 DFD’s– Sub-processes (primitive processes) of Level-0 processes

• Sources for inputs and outputs listed at higher level• List source and destination of data flows to processes

and stores within each DFD• Depth of DFD depends on overall system complexity

– Two processes generally don’t need lower level– More than seven processes become overly complex and difficult

to read

Context diagram for the example

External entity: StudentProcess: Student Administration process applicationData Flows: Application Form, Confirmation/Rejection Letter

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Apply logical sequence of the system..

• A student (EE / DS) sends in an application form (DF) containing their personal details, and their desired course

• The university checks (P) that the course is available. • If the course (DS) is available the student is enrolled (P)

in the course, and the university confirms (P) the enrolment by sending a confirmation letter (DF) that they are registered for the course to the student.

• Or if the course is unavailable the student is sent a rejection letter (DF).

Level-0 DFD

• External entity: Student• Processes: Check course available, Enroll

student, Confirm Registration• Data Flows: Application Form, Course Details,

Course Enrolment Details, Student Details, Confirmation/Rejection Letter

• Data Stores: Courses, Students.• Also see the “Lemonade Stand” example from

the class website

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Rules in DFD Building

• Rule 1 : Use only DFD notations to avoid confusion

• Rule 2 : Use an action VERB to label a process (because a process is an action !!!)

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Rules in DFD Building ...

• Rule 3 : Must be one process associated with each data flow …

M

M

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Detail Rules in DFD Building ...

• Rule 3 : Must be one process associated with each data flow …

M N

M N

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• Rule 3 : Must be one process associated with each data flow.

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• Rule 4 : Shaded corner must appear in ALL occurrences of a duplicated symbol in a same diagram

CUSTOMER

CUSTOMER

D3

D3

Accounts Receivable

Accounts Receivable

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• Rule 5 : No process without output data flow (black hole !!!)

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• Rule 6 : No process without input data flow (miracle !!!)

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• Rule 7 : No need for routing (without transforming) a data flow with a process (non value-added activities !!!)

Info A Info A

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• Rule 8 : Identical input, output data flows for parent and child processes (but the child processes can have their own throughputs)– See the picture in next slide

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M

N

P

1 2

3

M

N

P

Context Diagram

Level-0 Diagram

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• Rule 9 : Data flows cannot split by themselves

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• Rule 9 : Data flows cannot split …

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• Rule 10 : A data packet can combine many data elements being transmitted at the same time to the same destination

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• Rule 11 : Double-headed arrows are forbidden [in- flow (update) and out-flow (extract info) of a data store are different]

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Common Process Errors on DFDs

1 Chapter 4 Documenting Information Systems Yong Choi BPA CSUB.

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