Saturday, April 4, 2009

Data Flow Diagram

1. Data Flow Diagram

  • Used to perform structured analysis to determine logical requirements
  • A graphical tool, useful for communicating with users, managers, and other IS personnel
  • Useful for analyzing existing as well as proposed systems
  • A relatively simple technique to learn and use
2. Context Diagram

Defines the scope of the system by identifying the system boundary.
  • One process (which represents the entire system)
  • All sources/sinks (external entities)
  • Data flows linking the process to the sources and sinks (external entities)
  • The diagram does not contain any data stores.
  • All external entities are shown on the context diagram as well as major data flow to and from them.
Example of Context Diagram :

3. Zero Diagram
  • Describes the overall processing of the system
  • Show one process for each major processing step or functional requirement
  • Data flows from the context appear on system diagram also (level balancing)
  • Can show a single data store to represent all data in aggregate at this level
  • Can draw duplicate sources, sinks and data stores to increase legibility
Example of Zero Diagram :

4. Detail Diagram
  • Detail Diagram is the explosion of the context level diagram.
  • It should include up to 7 or 9 processes.
  • Processes are numbered with an integer.
  • The major data stores and all external entities are included on Diagram 0.
Numbering Level On DFD :

5. Spesification of Process

Each process in DFD must have the process specification. in top level method used to describe process by using descriptive sentence. At more level detailed that is under process (functional primitive) required the specification structure.Process Specification will become guide to programmer in coding. Method used in the process specification : description process in the story, decision table and decision tree.

6. External Entity
  • Any class of people, an organization, or another system which exists outside the system you are studying.
  • Form the boundaries of the system.
  • The system and external entities exchange data in the form of data flows.
  • Must be named, titles preferred to names of individuals - use a noun
7. Data Flow

  • Data flow shows the data about a person, place, or thing that moves through the system.
  • Names should be a noun that describes the data moving through the system.
  • Arrowhead indicates the flow direction.
  • Use double headed-arrows only when a process is reading data and updating the data on the same table or file.
Other rule:

* Data flow name which turn in at a process may not same by the name of secretary data flow of that process.
* Data flow into or out of data storage doesn't need to give a name if:
o The flow of data simple and easy to understand
o Describes the data flow of all data items
* There may not any data flow of terminal to data storage or on the contrary because terminal isn't part of system, terminal relation with data storage have to pass process.

8. Process

  • Transforms incoming data flows into outgoing dataflows
  • Each process has a unique number and name
  • Name each process using a verb and a noun phrase
  • the name of a process should describe what the process does
  • avoid vague names where possible
  • represent with a bubble or rounded square

9. Data Storage

  • A data store represents a collection of data flows "at rest"
  • Each data store has a unique name
  • The name should describe the contents of the data store
  • A data store may represent many different types of physical locations of data
  • A data store may be a temporary or a permanent repository of data

10. DFD Symbol

11. Data Dictionary

Data dictionary is a reserved space within a database which is used to store information about the database itself. Data dictionary is also called with a system data dictionary is a catalog of facts and data information needs of an information system. In function to help system agent to interpreting application in detail and organization all of data element that utilized by system exactly so user and system analyst have same understanding basic about entry, output, storage and process. In analysis phase, data dictionary used as communication between system analyst with user. in development system phase, data dictionary used to design input, and report database. Data flow in DFD have the character of globally, boldness more detailed can be seen in data dictionary.
Data dictionary load the followings :

  • Name of data current: must note that readers who need further explanation about a flow of data can find it easily
  • Alias: alias or other name of the data can be written when there is
  • Forms of data: used to segment the data dictionary to use when designing the system
  • Flow data: indicates from which data flows and where the data
  • Description: to give an explanation of the meaning of the data flow

12. Balancing In DFD

Data flow into and out of a process must be the same as the flow of data into and out of the details of the process on the level / levels below. Name of the data flow into and out of the process must match the name of the flow of data into and out of the details of the process. Number and the name of an entity outside the process must be equal to the number of names and entities outside of the details of the process.
Things which must be gave attention to DFD owning than one level:

  • There are must input balance and output between one level and level next
  • Balance between level 0 and level 1 seen at input / output of data stream to or from terminal in level 0, while balance between level 1 and level 2 seen at input / output of data stream to/from pertinent process
  • Data flow name, data of storage terminal and every level must be same if its same object

13. Prohibition In DFD
  • The data flow may not directly from one external entity to another external entity without any process.
  • The data flow may not directly from data storage to external entity without any process.
  • The data flow may not directly from data storage to the other data storage without any process.
  • The data flow from one process directly go to another process without any data storage should be avoided.

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