5-Data+Flow+Diagrams

> The data flow diagram is a modeling tool that allows us to picture a system as a network of functional processes, connected to one another by "pipelines" and "holding tanks of data." A data flow diagram is a graphical representation of an organization or department's flow of data through a system. Data Flow Diagrams are organized into "a series of levels so that each level provides successively more detail about a proportion of the level above it." Context level diagrams are the most basic Data Flow Diagram. "Here's the system and what interacts with it."
 * //__ Data Flow Diagrams __//**
 * The Purpose of this section is to explain: **
 * 1) What are data flow diagrams?
 * 2) Guidelines to design a data flow diagram.
 * 3) What are data flow diagrams used for?
 * 4) Helpful links about data flow diagrams.
 * 1. What are data flow diagrams? **
 * Yourdon Definition: **
 * Our definition: **
 * Data Flow Diagrams can have many levels. **

Level 0 provides slightly more detail and “defines individual processes.”

As the levels increase, the diagrams become more detailed. A data flow diagram can be defined as a Context level which is very basic, showing only the system and the entities that interact with it, to the Level “n” which is a data flow diagram that is detailed until it cannot be detailed any further. An example of Level “n” follows.

In this example, the Payroll Departments process cannot be simplified any further, making it the Level “n” process. (It is called the Level “n” process because the level that is the most simplified will vary depending on the data flow diagram and the process being explained. Level “n” is just a representation of the most simplified level the data flow diagram). To determine what level a data flow diagram is representing, look at the numerical representation on the diagram. In the diagram above, the processes are numbered: 4.1.1, 4.1.2, 4.1.3, and 4.1.4. Take the number represented, ex. 4.1.2 and subtract 1 (in this case, .2) and you will get a two digit number. This is a level two diagram. Likewise, for a diagram that is labeled, 1.1, this is a level one diagram. 2.3.4 is a level two, while 6.5.4.3 is a level three.

1. Choose meaningful names for the processes, flows, stores, and terminators. 2. Number the processes. 3. Redraw the data flow diagram as many times as necessary. 4. Avoid overly complex data flow diagrams. 5. Make sure the data flow diagrams are internally consistent and consisted with any associated data flow diagrams.
 * 2. Guidelines to designing a data flow diagram. **

Components of a data flow diagram.
· Process: shows a part of the system that transforms inputs into outputs (a rectangle with curved sides). · Flow: is used to describe the movement of chunks or packets of information from one part of the system to another part; “data in motion" (an arrow). · Data Stores: used to model a collection of data packets (or data waiting to be used), (a three sided rectangle, a rectangle with one side open).  · Source/Sink (Terminator): external entities that the system communicates with (a rectangle).  Combinations of the four symbols are used to create data flow diagrams. "In a nutshell... All data flows must begin and/or end at a process, because data flows either initiate or result from a process" (Professor Johnson's lecture). (Process Modeling PowerPoint presentation).
 * Rules of data flow diagrams.**
 * Data must be connected to a process.
 * There are no "bi-directional" flows. Data cannot go from a process to a data store and then back to the same process.
 * Data cannot flow directly back into the process that it just left

** 3. What are data flow diagrams used for? **  Data flow diagrams are used for modeling information processing systems, but can also be used to model whole organizations or departments within an organization. Data flow diagrams are used as a tool for business planning and strategic planning.

=Some interesting sites about Data Flow Diagrams, and other helpful information. =

[|Yourdon.com Wiki] [|Yourdon's Blog] [|Learn How to Draw Data Flow Diagrams with SmartDraw] [|A Website Illustrating Data Flow Diagrams] [|You Tube Video on Databases- This is great!]

*Most of the information is derived from Professor Johnson's lectures and the Yourdon reading.