Fundamentasl of DFD
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The document describes data flow diagrams (DFDs), which illustrate how data moves through a system, detailing inputs, outputs, data stores, and processes. It elaborates on different DFD levels (0, 1, and 2), explaining the abstraction, external entities, and requirements for each level, including the relationships and interactions between various components. The example of a railway reservation system is used to demonstrate these concepts, highlighting the interactions between external entities, processes, and data flow.
Fundamentasl of DFD
- 1. Prepared by – Kumaresh Chandra Baruri Software Engineer
- 2. DFD includes- 1. Data inputs and outputs. 2. Data stores. 3. Various process and subprocesses the data moves through. 4. Standardized symbols and notations to describe various entities and their relationships. Data Flow Diagram(DFD) represents how data objects are transformed when they flow through a process or system.
- 3. External Entity Process An external system that sends or receives data, communicating with the system being developed. Like – person, device, sensor etc. The process changes the data, produces an output in terms of area determination, display graph etc. It represents route that data takes between the external entities, processes and data stores. Data store means files or repositories that hold information for later use, such as a database table or a membership form.
- 4. 1. Each process should have at least one input and one output. Proce ss Input Output 2. Data store should have at least one flow in and one flow out. Flow In Flow Out 3. Data can not flow between two entities or two data stores. Data flow can occur from one data store to many process.
- 5. 4. Data stored in a system must go through a process. Proce ss Input Data 5. All the processes in DFD must go to another process or data store . Proce ss Input Data Proce ss #01 Proce ss #02 OR
- 6. External Entity Process 1. External entity to process External Entity Process 2. Process to external entity Process 3. Process to data store and back Process Process 4. Process to process
- 7. External Entity External Entity 1. External entity to external entity External Entity 2. External entity to store External Entity 3. Data store to external entity 4. Data store to data store Data flow is not allowed on the following 4 cases
- 8. There are different levels of DFD like- 0 – Level 1 – Level 2 – Level Apart from the above, level 3, 4 also can be used but to avoid complexities, up to 2 is used generally.
- 9. An abstraction view. This is also known as a context diagram. The full system is shown as a single process with its relationship to external entities. System as a single bubble with input and output data indicated by incoming/outgoing arrows. Meaningful label of the line is used to make more informative. The above diagram shows a railway reservation system where two external entities – Passenger and Admin interact with the system. External Entity Process External Entity Reservation Cancellation Reserve/cancel info Up/down Train info
- 10. Things that need to consider in Level 0 is – This layers shows only the external outlook of the system. Full system is shown as a single process. No need to show data store in this level.
- 11. Level 0 context diagram is decomposed into multiple bubbles/processes. Main functions of the system is highlighted. The high-level process of 0-level DFD broken-down into subprocesses here. Reservation Process Enquiry Ticket Reservation Process Report to Admin Up storage Down storage Passenger Reservation Storage Admin Request Data Request Response Store Request Query for info Response Request Store Cross checking
- 12. Things that need to consider in Level 1 is – It provides a more detailed view of the Context Level Diagram or Level 0. The main functions carried out by the system are highlighted as we break into its sub-processes. Generally, 6 or 7 processes are included in in level 1. But more can be used. Same external entity of Level 0 need to use the same entities.
- 13. Railway department Reservation Enquiry Query Confirmation Cancellation Searching Ticket generation Waiting Report Reservation Passenger Reservation Waiting Waiting Cancellation Ticket generation Report Reservation 2-level DFD goes one step deeper into parts of 1-level DFD Request Confirm Info Data Make report Record Cancellation request Request Response Request Request Request for data Ticket
- 14. Things that need to consider in Level 2 is – Each and every individual process is split into many processes and sub-processes received from Level 1. Same external entity need to use the same entities.