The logical characteristics of current systems data stores, including name, description, aliases, contents, and organization, identifies processes where the data are used and where immediate access to information required, Serves as the basis for identifying database requirements during system design.
Uses of Data Dictionary:
- To manage the details in large systems.
- To communicate a common meaning for all system elements.
- To Document the features of the system.
- To facilitate analysis of the details in order to evaluate characteristics and determine where system changes should be made.
DATAFLOW DIAGRAMS (DFD)
The data flow diagram is used for classifying system requirements to major transformation that will become programs in system design. This is starting point of the design phase that functionally decomposes the required specifications down to the lower level of details. It consists of a series of bubbles joined together by lines.
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Bubbles: Represent the data transformations.
Lines: Represents the logic flow of data.
Data can trigger events and can be processed to useful information. System analysis recognizes the central goal of data in organizations. This dataflow analysis tells a great deal about organization objectives are accomplished.
Dataflow analysis studies the use of data in each activity. It documents this finding in DFD's. Dataflow analysis give the activities of a system from the viewpoint of data where it originates how they are used or hanged or where they go, including the stops along the way from their destination. The components of dataflow strategy span both requirements determination and system's design. The first part is called dataflow analysis.
As the name suggests, we didn't use the dataflow analysis tools exclusively for the analysis stage but also in the designing phase with documentation.
Notations Used In Data Flow Diagrams
The logical dataflow diagrams can be drawn using four simple notations or special symbols or icons and the annotation that associates them with a specific system. Since the choice of notation we follow, does not affect impede or catalyze the system process; we used three symbols from YOURDON notation and one from Gain and Sarson notation as specified below.
- Process: Describes how input data is converted to output data
- Data Store: Describes the repositories of data in a system
- Data Flow: Describes the data flowing between process, Data
- stores and external entities.
- Sources: An external entity causing the origin of data.
- Sink: An external entity, which consumes the data.
The top-level diagram is often called a “context diagram”. It contains a single process, but it plays a very important role in studying the current system. The context diagram defines the system that will be studied in the sense that it determines the boundaries. Anything that is not inside the process identified in the context diagram will not be part of the system study. It represents the entire software element as a single bubble with input and output data indicated by incoming and outgoing arrows respectively.
Levels of DFD:
The complexity of the business system means that it id responsible to represent the operation of any system of single data flow diagram. At the top level, an over view or the different systems in an organization is shown by the way of context analysis diagram, when exploded into
- DFD. They are represented by
- LEVEL-0: System Input/Output
- LEVEL-1: Subsystem Level Data Flow Functional
- LEVEL-2: File level Detail Data flow
The input and output data shown be consistent from one level to the next.
LEVEL-0: System Input/Output
A level-0 DFD describes the system wide boundaries, dealing inputs to and outputs from the system and major processes.
LEVEL-1: Subsystem Level Data Flow Functional
A level -1 DFD described the level of details within the system, details the data flows between subsystems, which makeup the whole.
LEVEL-2: File level Detail Data flow
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Types of data flow diagrams
DFDs are two types
Structured analysis states that the current system should be first understand correctly. The physical DFD is the model of the current system and is used to ensure that the current system has been clearly understood. Physical DFDs shows actual devices, departments, people etc., involved in the current system
Logical DFDs are the model of the proposed system. They clearly should show the requirements on which the new system should be built. Later during design activity this is taken as the basis for drawing the system's structure charts.