LEC-6 [DFD]

• Data Flow Diagram (DFD)


  A graphical representation of the movement of data between external entities, processes and data stores within a system
• DFD


  Shows how data moves through an information system
• DFD Symbols


  • Process
  • Data Flow
  • Data Store
  • External Entity (Source/Sink)
• Process


  • The work or actions performed on data so that they are transformed, stored, or distributed
  • Process labels should be verb phrases
• Data Flow


  • A path for data to move from one part of the system to another
  • Data in motion
  • Arrows depict the movement of data
  • NO VERBS
• Data Store


  • Used in a DFD to represent data that the system stores
  • Data at rest
  • Labels should be noun phrases
  • NO VERBS
• External Entity (Source/Sink)


  • The origin or destination of data
  • Represents things outside of the system
  • Source - Entity that supplies data to the system
  • Sink - Entity that receives data from the system
  • Labels should be noun phrases
• General DFD Rules


  • Mechanics
  • Outline
  • DFD symbols
  • External entities (sources and sinks)
  • Data Stores
  • Data Flows
  • Processes
  • Types of diagrams
  • Step by step approach
  • Rules
• External Entities


  • People, systems and data stores
  • Reside outside the system, but interact with system
  • Either a) receive info from system, b) trigger system into motion, or c) provide new information to system
  • e.g. Customers, managers
  • Not clerks or other staff who simply move data
• Data Stores


  • Internal to the system
  • Data at rest
  • Include in system if the system processes, transforms the data
  • Store, Add, Delete, Update
  • Every data store on DFD should correspond to an entity on an ERD
  • Can come in many forms: Hanging file folders, Computer-based files, Notebooks
• Data Flows


  • Data in motion, moving from one place to another in the system
  • From external entity (source) to system
  • From system to external entity (sink)
  • From internal symbol to internal symbol, but always either start or end at a process
• Processes


  • Always internal to system
  • Law of conservation of data: 1) Data stays at rest unless moved by a process, 2) Processes cannot consume or create data
  • Must have at least 1 input data flow (to avoid miracles)
  • Must have at least 1 output data flow (to avoid black holes)
  • Should have sufficient inputs to create outputs (to avoid gray holes)
• Logical process models omit any processes that do nothing more than move or route data, thus leaving the data unchanged
• Valid processes include those that

  • Perform computations (e.g., calculate grade point average)
  • Make decisions (determine availability of ordered products)
  • Sort, filter or otherwise summarize data (identify overdue invoices)
  • Organize data into useful information (e.g., generate a report or answer a question)
  • Trigger other processes (e.g., turn on the furnace or instruct a robot)
  • Use stored data (create, read, update or delete a record)
• Context Diagram


  A data flow diagram (DFD) of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system
• Level-0 Diagram


  A data flow diagram (DFD) that represents a system's major processes, data flows and data stores at a high level of detail
• Creating Data Flow Diagrams

  1. Create a preliminary Context Diagram

  2. Identify Use Cases, i.e. the ways in which users most commonly use the system

  3. Create DFD fragments for each use case

  4. Create a Level 0 diagram from fragments

  5. Decompose to Level 1,2,...

  6. Go to step 1 and revise as necessary

  7. Validate DFDs with users
• DFD Rules—General


  • Inputs to a process are always different than outputs
  • Objects always have a unique name
  • In order to keep the diagram uncluttered, you can repeat data stores and sources/sinks on a diagram
• DFD Rules—Symbols


  • Process: No process can have only outputs (a miracle), No process can have only inputs (black hole), A process has a verb phrase label
  • Data Store: Data cannot be moved directly from one store to another, Data cannot move directly from an outside source to a data store, Data cannot move directly from a data store to a data sink, Data store has a noun phrase label
  • Source/Sink: Data cannot move directly from a source to a sink, A source/sink has a noun phrase label
  • Data Flow: A data flow has only one direction of flow between symbols, A fork means that exactly the same data goes from a common location to two or more processes, data stores or sources/sinks, A join means that exactly the same data comes from any two or more different processes, data stores or sources/sinks to a common location, A data flow cannot go directly back to the same process it leaves, A data flow to a data store means update, A data flow from a data store means retrieve or use, A data flow has a noun phrase label
• DFD Rules—Context Diagram


  • One process, numbered 0
  • Sources and sinks (external entities) as squares
  • Main data flows depicted
  • No internal data stores are shown, they are inside the system
  • External data stores are shown as external entities
• How do you tell the difference between an internal and external data store?