Structuring system requirements: process modeling

1
Structuring system requirements process
modeling
Chapter 8
2
Three views for structuring data requirements

• Process view
• the sequence of data movement and handling
  operations within the system (chapter 8) ?
  process modelling
• Logic and timing view
• the rules by which data are transformed and
  manipulated and an indication of what triggers
  data transformation (chapter 9 12)? Logic
  modelling
• Data view
• the inherent structure of data independent of how
  or where it is processed (chapter 10)? Data
  modelling

3
Key Definitions

• A process model is a formal way of representing
  how a business operates
• Data flow diagramming shows business processes
  and the data that flows between them
• Logical process models describe processes without
  suggesting how they are conducted
• Physical models include information about how the
  processes are implemented

4
Objectives

• Understand the logical of processes modeling
  through studying examples such as data flow
  diagrams (DFD)
• Learn basic mechanisms of drawing and revising
  data flow diagram (DFD)
• Use flow diagrams as tools to support the
  analysis of information

5
Modelling for three main reasons

• Facilitate understanding a system
• Process modelling involves graphical
  representation of function and processes which
  capture, manipulate, store and distribute data
  between a system and its environment, and between
  component within a system
• It supports communication with a systems
  analysts, end-users, and other roles

6
Reading a DFD
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DFD Elements
8
DFD Shapes from Visio
Visio 2000
Visio 5.x
9
DFD Practical Example
Launched Dec. 11, 1998, the Climate Orbiter
plunged too steeply into the Martian atmosphere
Sept. 23, 1999, and either burned up or crashed.
In an initial failure report released Oct. 15,
2000 the review board blamed the navigation error
on a communications foul-up between NASA's Jet
Propulsion Laboratory and prime contractor
Lockheed Martin.
10
Data Flow Diagram (DFD)

• DFD is one of several notations that are called
  structured analysis technique
• It is a graphical tool that allows analyst-users
  to depict the flow of data in an IS
• It concentrates on the movement of data between
  processes
• DFD evolves four symbols for process modelling
• Data flows
• Data stores
• Process
• Source / sink

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Data flows

• They are groups of data that move and flow
  through a system from one place to another.
• Include names of all individual elements of data
  moving
• Data flow is represented by arrow
• E.g. results of grading mark

Current score
Final score
Final grades
12
Data stores

• They refer to data at rest that may represent one
  or many physical locations.
• Each data store has a label and number (number of
  files location)
• Data store is represented by a rectangle
• E.g. data about students (paper-based or floppy
  disk)

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Processes

• They are the action performed on data so that
  they are transformed (manual or automatic),
  stored or distributed
• E.g. calculate class mark
• Each process includes a name process number
• It is represented by rectangle with rounded
  corners

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Sources / sinks

• They are the origin (sources) and/or destinations
  (sink) of the data, i.e. that supplies or receive
  data
• They refer to external entities (or outside) to a
  system boundaries of the system boundaries
• Each source/ sink has a name that states what the
  external agent is, e.g. students
• A source (sink) is represented by square

Teacher
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Example of Data Flow Diagram
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Items not considered in DFD

• Interaction between source (sink)
• Actions of source (sink) are black box
• How to redesign or control the source (sink)
• How to provide source (sink) direct access to
  data,

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Clarification of the concept source (sink)

• Any organisation unit or other organisation which
  send or receive data (information) from the
  system you are analysing (e.g. supplier)
• A person inside or outside the business supported
  by the system (e.g. customer)
• Another system with which the system, you are
  analysing, exchanges data (information)

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Context diagram of DFD
Final grades
Student
Registers office
Test scores
Final grades
Current score
Final score
Teacher
Context diagram is an overview of an
organisational system that show the system
boundaries, external entities that interact with
the system, and the major information flows
between the entities and the system
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Level-0 diagram

• DFD could have many levels diagrams level-0,
  level-1, etc.
• Level-0 diagram is a data flow diagram that
  represents a systems major processes, data
  flows, and data stores at a high level of detail.
• Level-n A DFD that is the result of n nested
  decomposition of a series of sub processes from a
  process on a level 0 diagram
• Example of level-0 diagram

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Context diagram of MacDonald food ordering system
Customer
Kitchen
Restaurant manager
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Level-0 diagram

• Exercise in the classroom
• Generate the level-0 diagram by detailing the
  main process
• Food ordering system (process 0) could be split
  into three sub processes
• Receive and transform customer food order
• Update good sold file
• Update inventory file
• Produce management report

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Rules of DFD / decomposition of process 4.0

• Functional decomposition is an iterative
  processes of breaking the description of a system
  down into finer and finer details which creates a
  set of charts in which one process in a given
  chart is explained in greater details on another
  chart

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Rules of DFD / balancing

• Balancing is the conservation of inputs and
  outputs to a DFD process when that process is
  decomposed to lower levels

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Which of the two diagrams is balanced or
unbalanced?
A
B
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A DFD Fragment Example
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A Second DFD Fragment Example
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Four types of DFD used in system development

• Current physical
• Processes label include the name of people and
  technology used to process data
• Data flow and data store include the name of
  actual physical media in which data flow or in
  which data are stored (file location, PCs, etc.)
• Current logical
• Physical model is reduced to only essential part,
  i.e. reduced to data processes that transform
  them (what and how)
• New logical
• Models is the same as current logical model if
  users were happy about current functionality of
  the system (there were errors during
  implementation)
• Others, new logical models include new functions
  according to users wishes
• New physical
• Reflect decisions about which functions to
  automate or to leave manually

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Output/deliverable of process modeling
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Using DFD in the analysis process guideline

• Correctness
• Completeness
• Inconsistency
• Timing
• Primitive DFD

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Correctness and Completeness

• Correctness it refers to drawing a correct DFD
  (see rules e.g. balancing, functional
  decomposition)
• Completeness it refers to whether you have
  included in your DFDs all the components
  necessary for the system you are modeling.
• Examples of completeness
• There are entities that are not connected to any
  thing else
• There is a process with only input and without
  output

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Example of completeness where is the error?
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Consistency

• Consistency it refers to whether or not the
  depiction of the system shown in at one level of
  a nested set of DFD is compatible with the
  depiction of the system shown in at other level
• Example of inconsistencies
• Existing of a level-1 diagram with no high level
  -0 diagram
• Existing of a data flow at level-0 diagram but
  not at level-1 diagram

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Example of inconsistencies where is the error?
(already seen)
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Other guidelines

• DFD does not show timing, draw diagrams as if the
  system has never started and will never stop
• Iterative development DFD recognize that system
  requirement determination structuring are
  interacting
• Primitive DFD it refers to the lowest level of
  DFD decomposition. When does a DFD decomposing
  stop?
• Very important idea there is no good or bad
  modelling!!!
• Drawing DFD is facilitated by CASE upper tools
  (generating processes and diagrams)
• DFD can be used as an analysing tool to check
  correctness, inconsistencies and completeness
• DFD can be used as a tool to support Business
  Process Re-engineering (BPR)
• Example of CASE tool, developed in SIMNET, used
  to generate processes (different from DFD)
• It consists to generate processes manually and
  the system converts the process into a coding
  system

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Context Diagram

• Shows the context into which the business process
  fits
• Shows the overall business process as just one
  process
• Shows all the outside entities that receive
  information from or contribute information to the
  system

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Relationship Among DFD levels
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Decomposition Diagram
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Level 0 Diagram

• Shows all the processes that comprise the overall
  system
• Shows how information moves from and to each
  process
• Adds data stores

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Level 1 Diagrams

• Shows all the processes that comprise a single
  process on the level 0 diagram
• Shows how information moves from and to each of
  these processes
• Shows in more detail the content of higher level
  process
• Level 1 diagrams may not be needed for all level
  0 processes

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Level 2 Diagrams

• Shows all processes that comprise a single
  process on the level 1 diagram
• Shows how information moves from and to each of
  these processes
• Level 2 diagrams may not be needed for all level
  1 processes
• Correctly numbering each process helps the user
  understand where the process fits into the
  overall system

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Data Flow Splits and Joins

• A data flow split shows where a flow is broken
  into its component parts for use in separate
  processes
• Data flow splits need not be mutually exclusive
  nor use all the data from the parent flow
• As we move to lower levels we become more precise
  about the data flows
• A data flow join shows where components are
  merged to describe a more comprehensive flow

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Alternative Data Flows

• Where a process can produce different data given
  different conditions
• We show both data flows and use the process
  description to explain why they are alternatives
• Tip -- alternative data flows often accompany
  processes with IF statements

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Your Turn

• At this point in the process it is easy to lose
  track of the big picture.
• Describe the difference between data flows, data
  stores, and processes.
• Describe in your own words the relationship
  between the DFD and the ultimate new application
  being developed.