Applications and People in Organisations

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Applications and People in Organisations

• Introduction to Process Modelling

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Data Flow Diagrams DFDs

• Diagrammatic representation of
• how data enters/leaves the system
• what changes the data
• where data is stored
• Allow
• structure
• scope
• checks for completeness/consistency
• basis for program specification

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DFD elements

• Data flow
• Process (data transformation)
• External entity (source or sink)
• Data store

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Components of a DFD

• External Entity
• all data entering the system is provided by
  external entities. All data leaving the system
  goes to external entities.
• Processes
• Process Name Verb Noun, eg. Identify order
• Process ID is numerical
• Current Physical DFDs show location
• Data Stores
• eg.Customer Records
• Can differentiate between manual, Digital and
  Temporary

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Definition of Terms

• Data Flow
• Between External Entities and the system
• Between processes and datastores within the
  system
• may represent physical items, eg. goods delivery
  note

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Data Flows

• Physical DFDs
• show real world flows, e.g. forms, telephone
  calls.
• Logical DFDs
• show data items required by a process (or Its
  output independent of Its real-world form

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DFD symbols

• Data flow
• Shows flows of data
• Give each flow a meaningful name

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Entity in process modelling

• Something of significance to the system
• External Entities
• those which communicate with the defined system
• providers and receivers of data

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External entity
Supplier
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Process

• A process is an activity with a defined objective
• A process may be composite
• and may be decomposed
• processes may be generalised to form higher level
  processes
• Processes act upon (transform) data
• modification, creation

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Decomposition/levelling

• Processes should be broken down to a necessary or
  reasonable level of detail
• Simple processes may not require decomposition
• Numeric prefixing is used to denote groups of
  processes
• 2
• 2.1
• 2.1.1
• 2.1.2
• Dataflows should be consistent between levels of
• the DFD
• DFDs can be checked for consistency with the
  other views

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Process
Sales Office
1.2.1
verify order

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Data Store

• A repository of data
• source or receiver of data
• used by dataflows and processes

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Data store
D 1
stock
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Construction of DFDs

• Normally start at Level 0 (Context Diagram)
• Context Diagram is a high level view of the
  system
• A Level 1 DFD shows the main processes and
  associated stores
• Each LI process can be decomposed
• Customer
• Customer details Register new Customer details

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Guidelines For Drawing DFDs

• Read the description of the system through
  several times
• Identify the inputs and outputs to the system
• Create a table with columns for external
  entities, and whether the external entity is the
  source or destination for the data (or both)
• Consider the data flows entering and leaving the
  system.

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Guidelines For Drawing DFDs (2)

• Draw the context diagram (level 0 DFD) with the
  external entities and dataflows to and from the
  system.

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Context Diagram (Level 0 DFD)
Purchase details
Invoice details
Catering System
Customer
Customer
payment details
Order confirmation
reports
Coding details
Management
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Guidelines For Drawing DFDs

• Draw the first external entity that seems to
  start the processing off.
• Draw the first process box to the right of it.
  Identify and name the process.
• Draw the data flow and name it.
• Draw data flows from the process to data stores
  (files) and name them.

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Guidelines For Drawing DFDs

• Link to the next process
• Draw any internal flows between processes
• Remember, data cannot flow
• from store to store
• from external entity to external entity
• from external entity to store
• Check consistency

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CASE tools for DFDs

• SELECT
• available in k25/k26/k27
• check additional availability with ITS helpdesk

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DFDs in SSADM

• Current
• current physical
• current logical
• For the proposed system
• business systems options drawn using DFDs

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Advantages of DFDs

• Pictorial representation
• communication
• discussion
• modification
• verification
• Shows different levels of a system
• Top level shows the complete system
• Allows decomposition to show further detail
• Each area may be identified in relative isolation
• DFDs allow a top down approach using structured
  decomposition
• A system is too complex to consider in one
  chunk
• concept of divide and conquer - processes are
  broken down into sub-processes