Chapter 4 Documenting Information Systems

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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 (condt)
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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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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 -gt identify necessary decisions
  -gt describe information needs -gt determine
  necessary processing -gt 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 -gt identify requirements -gt
  develop data model for each user's data -gt
  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 systems processes, and the logic,
  policies, and procedures to be implemented by a
  systems 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.
• Whats 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!!

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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
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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.

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Elements of a DFD
DE MARCO YOURDON NOTATIONS (Textbook)
External Entity
Data Flow
Process
Data Store
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Elements of a DFD
GANE SARSON NOTATIONS (more universal)
External Entity
Data Flow
3
Pay Bill
Process
AP Clerk
Data Store
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External Entities
SUPPLIER

• 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.

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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 systems end-users or managers

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Data Stores
D1
Accounts Receivable

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

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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
DELIVERY SLIP

• 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

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Processes
1
Pay Bill

• 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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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

• Develop a list of business activities
• Similar to business rules of ERD
• Create the Context Diagram (we know this)
• shows external entities and data flows to and
  from the system conceptual DFD
• Create level 0 DFD (or Diagram 0 in textbook)
• shows general processes at the highest level
• 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 DFDs
• 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 dont need lower level
• More than seven processes become overly complex
  and difficult to read

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Context diagram for the example
External entity Student Process Student
Administration process application Data 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).

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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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Detail Rules in DFD Building ...

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

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Detail Rules in DFD Building ...

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

Accounts Receivable
CUSTOMER
D3
CUSTOMER
D3
Accounts Receivable
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Detail Rules in DFD Building ...

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

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Detail Rules in DFD Building ...

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

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Detail Rules in DFD Building ...

• 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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Detail Rules in DFD Building ...

• 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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Detail Rules in DFD Building ...
M
Context Diagram
P
N
M
1
2
P
3
N
Level-0 Diagram
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Detail Rules in DFD Building ...

• Rule 9 Data flows cannot split by themselves

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Detail Rules in DFD Building ...

• Rule 9 Data flows cannot split

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Detail Rules in DFD Building ...

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

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Detail Rules in DFD Building ...

• 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