Wall-e and 9 movie
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Watch at least two of the following movies:
• Make a comprehensive reaction paper for the two movies in no less than
300 words each.
• Make a comparative ...
13 years ago
Saturday, March 13, 2010
Data Flow Diagrams (DFDs) model events and processes (i.e. activities which transform data) within a system. DFDs examine how data flows into, out of, and within the system. The DFD Principles are: a system can be decomposed into subsystems, and subsystems can be decomposed into lower level subsystems, and so on; each subsystem represents a process or activity in which data is processed. At the lowest level, processes can no longer be decomposed; each 'process' (and from now on, by 'process' we mean subsystem and activity) in a DFD has the characteristics of a system; just as a system must have input and output (if it is not dead), so a process must have input and output; data enters the system from the environment; data flows between processes within the system; and data is produced as output from the system. The 'Context Diagram ' is an overall, simplified, view of the target system, which contains only one process box and the primary inputs and outputs. The Top or 1st level DFD, describes the whole of the target system. It 'bounds' the system under consideration. Data Flow Diagrams show: the processes within the system; the data stores (files) supporting the system's operation; the information flows within the system; the system boundary; and interactions with external entities.
DFD Notations
Processes, in other methodologies, may be called 'Activities', 'Actions', 'Procedures', 'Subsystems' etc. They may be shown as a circle, an oval, or (typically) a rectangular box. Data are generally shown as arrows coming to, or going from the edge of a process box.
General Data Flow Rules
1. Entities are either 'sources of' or 'sinks' for data input and outputs - i.e. they are the originators or terminators for data flows.
2. Data flows from Entities must flow into Processes
3. Data flows to Entities must come from Processes
4. Processes and Data Stores must have both inputs and outputs (What goes in must come out!)
5. Inputs to Data Stores only come from Processes.
6. Outputs from Data Stores only go to Processes.
The Process Symbol
Processes transform or manipulate data. Each box has a unique number as identifier (top left) and a unique name (an imperative - e.g. 'do this' - statement in the main box area). The top line is used for the location of, or the people responsible for, the process. Processes are 'black boxes' - we don't know what is in them until they are decomposed. Processes transform or manipulate input data to produce output data. Except in rare cases, you can't have one without the other.
Data Flows
Data Flows depict data/information flowing to or from a process. The arrows must either start and/or end at a process box. It is impossible for data to flow from data store to data store except via a process, and external entities are not allowed to access data stores directly. Arrows must be named. Double ended arrows may be used with care.
External Entities
External Entities, also known as 'External sources/recipients, are things (e.g.: people, machines, organizations etc.) which contribute data or information to the system or which receive data/information from it. The name given to an external entity represents a Type not a specific instance of the type. When modeling complex systems, each external entity in a DFD will be given a unique identifier. It is common practice to have duplicates of external entities in order to avoid crossing lines, or just to make a diagram more readable.
Data Stores
Data Stores are some location where data is held temporarily or permanently. In physical DFDs there can be 4 types.
D = computerised Data
M = Manual, e.g. filing cabinet.
T = Transient data file, e.g. temporary program file
T(M) = Transient Manual, e.g. in-tray, mail box.
As with external entities, it is common practice to have duplicates of data stores to make a diagram less cluttered.
REFERENCES:
http://www.cems.uwe.ac.uk/~tdrewry/dfds.htm
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