Process Modeling

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Process Modeling DFD
Chao-Hsien Chu, Ph.D. School of Information
Sciences and Technology The Pennsylvania State
University
IDEF
DFD
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Enterprise Modeling
Organization Modeling
Network Modeling
Requirement Analysis
Data Modeling
Process Modeling
Normalization
Process Analysis
Data Management
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Building Blocks of Process
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What is DFD?

• DFD, stands for data flow diagram, is a graphical
  technique that depicts information flow and the
  transforms that are applied as data moves from
  input to output.
• DFD is also known as data flow graph or a bubble
  chart.
• A DFD can be seen as a method of organizing data
  from its raw state.

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Characteristics of DFD

• Graphic
• Partitioned
• Hierarchic
• Multidimensional
• Emphasize flow of data
• Viewpoint of data and process

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Facts About DFDs

• Not all models use control information in their
  DFD.
• Some models describe manual as well as computer
  activities in their DFD.
• DFDs go from left to right, up and down or other
  directions.
• Notations (symbols) in DFDs differ heavily.
• A minimum or a lot of explaining text around the
  symbols in a DFD.

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Symbols for DFD
Yourdon- Constantine
Game- Sarson
External Interactor
External Entity Source or destination of
data Process Action on data Data Store Storage
of data Data Flow Data Transfer
Process name
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Data Flow Diagram Example

• An employment system
• An applicant submits an application form, which
  is reviewed by the personnel section and filed. A
  request for reference letter is sent to the
  references named by the applicant. After one-week
  a summary of reference reports received is
  prepared and a decision is made whether or not to
  interview. Unsuccessful applicants are sent a
  standard rejection letter. An interview is
  scheduled with likely applicants, and references
  are stored with the applicant details. Applicants
  are interviewed and a decision is made on who to
  hire. Unsuccessful interviewees are sent a
  commiseration letter, while the successful
  applicant is sent an employment contract

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Data Flow Diagram Example
An applicant submits an application form, which
is reviewed by the personnel section and filed. A
request for reference letter is sent to the
references named by the applicant.
Job Applications
Application Form
Receive Review Application
Applicant
Referees
Request for reference
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Data Flow Diagram Example
After one-week a summary of reference reports
received is prepared and a decision is made
whether or not to interview. Unsuccessful
applicants are sent a standard rejection letter.
Job Applications
Check References Summarise
Referees
reference data
Rejection or Interview Advice
Applicant
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External Entity

• External entity represents the sources and
  destination of data created by the system.
• External entity represents the immediate
  interface of the system with the external world.
• When an external source of data is also a
  destination for data, a loop or occurrence number
  may be used.
• In case the destination or use of data created by
  the process are not known, the flow simply points
  outside the system. Similarly, data flows may
  originate from nowhere.

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Process Boxes

• Each processes box in a DFD describes an action
  on data.
• The Identifier. A number indicating the sequence
  of the process.
• The Action. A verb specifying the action on which
  it is performed on the data.
• The Actor or Place. A noun indicating who
  performs the action or where it is performed.

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Data Flow Arrows

• Data flow arrows link all the process boxes and
  data stores in DFDs.
• Data flows should be labeled, except in case the
  data flows into and out of simple files.
• DFDs show only the flow of data, not materials.
• A DFD depicts information flow without explicit
  representation of procedural logic (e.g.,
  conditions or loops).

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Data Store Rectangles

• Data stores can be manual files or computer
  files. The type of file is not indicated.
• Only in case the data store is altered the flow
  is not indicated. A simple access is not
  indicated.
• A data store is never the direct recipient of
  unprocessed data from external sources or from
  other data stores nor is data from a data store
  ever directly delivered to an external sources.
  There must be a process step in between.

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Examples of Data Stores
A data item stored in the data store is read
by a process
Read
Read/ Write
W rite
A data item is created or deleted or updated in
the data store by a process
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Rules for Constructing DFDs

• Overall
• Know the purpose of the DFD. It determines the
  level of detail to be included in the diagram.
• Organize the DFD so that the main sequence of
  actions reads left to right and top to down.
• Very complex or detailed DFDs should be leveled.
• Processes
• Identify all manual and computer processes.
• Label each process symbol with an active verb and
  the data involved.
• A process is required for all data
  transformations and transfers.
• Do not indicate hardware or where a process is
  manual or computerized.

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Rules for Constructing DFDs

• Data Flows
• Identify all data flows for each process, except
  simple record retrievals.
• Label data flows on each arrow.
• Data Stores
• Data not indicate file types for data stores.
• Draw data flows into data stores only if the data
  store will be changed.
• External Entities
• Indicate external sources and destination of data
  when known.
• Number each occurrence of repeated external
  entities.
• Do not indicate actors or places as entity
  squares when the process is internal to the
  system.

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DFD Not Allowed Flows
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DFD Not Allowed Flows
If part of our system
If not part of our flow ignore
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Data Flows

• Only one direction of flow between processes

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Data Flows

• Joins forks allowed only if exactly the same
  data

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Data Flows

• Cannot go directly back to the process it leaves

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Data Flows

• Data which moves together should be shown in a
  single data flow

invoice payment
itemised calls
Pay Invoice
Telephone Company
invoice
invoice payment
Pay Invoice
Telephone Company
itemised calls And invoice
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DFD Rules
Incorrect
Correct
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DFD Rules
Correct
Incorrect
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DFD Rules
Incorrect
Correct
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Naming

• Use process name as a qualifier

Edited Invoice
Edit Invoice
Verify Invoice
Invoice
Verified Invoice
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Naming

• To avoid clutter external agents can be
  duplicated indicated by a corner line

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Modeling Procedure

• Determine requirements/purposes.
• Divide activities.
• Model separate activities
• Construct preliminary context diagram.
• Construct preliminary level 0 diagrams.
• Deepen into preliminary level n diagrams

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Creating a Data Flow Diagram

• The level 0 should depict the software/system as
  a single bubble
• Primary input and output should be carefully
  noted
• Refinement should begin by isolating candidate
  processes, data items and stores to be
  represented at the next level

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Creating a Data Flow Diagram

• All arrows and bubbles should be labelled with
  meaningful names
• Information flow continuity must be maintained
  from level to level
• One bubble at a time should be refined

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Producing DFDs

• Pretend you are looking at the system from above
• Record where you go and what happens to you as
  you sit on each data flow in turn, record each
  process as you as the data flow is being
  transformed
• Start with an high-level summary
• Build on this detail
• Do not try and draw the final
• product first time round
• refine and redraw
• Check it

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Hierarchical DFDs

• DFDs are hierarchically structured
• The top-level is referred to as the context
  diagram (or fundamental system model or the level
  zero design)
• the purpose of the context diagram is to define
  external to the system interfaces and identify
  system boundaries

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Hierarchical DFDs

• The system of interest is usually contained in a
  single process bubble
• Subsequent DFD levels will show increasing
  system detail
• label clearly all DFD symbols
• Stop when
• each process is a single decision or operation
• each data store has data for a single entity
• when every data flow does not need to be split to
  handle different flows

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Decomposition Conventions

• Ensure no information is lost
• balancing
• hierarchical numbering
• add data stores at lower levels, if they are
  internal to the higher level process, not at
  context diagram
• external agents are introduced at level-0 never
  at owner levels

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Level 0 DFD
Origin 1
Destination 1
a
z
System
b
Origin 2
Destination 2
r
c
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Level 1 DFD
a
1.1
1.3
d
z
b
e
1.5
g
h
r
i
1.2
1.4
f
c
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Level 2 DFD
f
2.3
2.2
2.1
c
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Usages of DFDs

• Requirements Analysis. DFDs has been used to
  transform users requirements to processes,
  entities, and data stores.
• Process Modeling.
• Systems Implementation.
• Understanding
• Communication
• Improvement

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Thank You? Any Question?
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Balancing

• The conservation of input and output flows
  through different levels

A
A
C
D
B
B
E
C
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Data-flow models

• Show the processing steps as data flows through
  a system
• Intrinsic part of many analysis methods
• Simple and intuitive notation that customers can
  understand
• Show end-to-end processing of data

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Data-flow diagrams

• may be used to show processing at different
  levels of abstraction from fairly abstract to
  fairly detailed
• May also be used for architectural description
  showing data interchange between the sub-systems
  making up the system
• Not a good way to describe system interfaces

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Data Flow Diagrams

• They show the overall data flow through a system
  and they do NOT show
• control
• order
• time
• errors
• It is primarily a systems analysis tool used to
  draw the basic procedural components and the data
  that pass among them

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DFDs

• Physical
• describes operations of existing system, however
  badly the existing system is performing.
• This is essential in the fact finding stage of
  the life cycle
• Logical
• shows the essential processes and data
  interfaces, without reference to design or
  implementation
• Can be Current and NEW
• current physical or logical
• new logical

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DFD Elements

• Sources and sinks (or external entities or
  terminators)
• a producer or consumer of information that
  resides outside the bounds of the system to be
  modelled
• can be a person, another system, a program,
  hardware,
• usually a singular noun is used to name the
  source or sink

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DFD Elements

• Processes (or functions or data transformations)
• a transformer of information that resides within
  the bounds of the system to be modelled
• it receives the data and changes it in some way
• if not decomposed usually a verb and noun
  together describe the activity
• if decomposed then a verb is used to name the
  process

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DFD Elements

• Data flows (directional)
• represent the flow of data item of a collection
  of data items from one node to another
• a node can be a process, a source, or a data
  store
• can be input or output information
• the arrowhead indicates the direction of data
  flow
• all arrows should be labelled (a data flow into
  or out of a data store may be nameless in which
  case it is assumed to carry the entire contents
  of one record in the store)
• usually with a singular noun

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DFD Elements

• Data stores
• a repository of data that is to be stored for use
  by one or more processes
• usually named using a plural noun (the singular
  describes the individual data items in the store)

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Naming
Receive Data
Store
Needed data
information

• Too general
• process names should define a specific action
• data stores should store only a single structure