Systems Analysis and Design: Data Dictionaries

Analyzing Systems Using Data Dictionaries

• A data dictionary is a reference work that collects and coordinates data terms, and confirms their meanings to different people in an organization.
• Data dictionaries provide documentation, eliminate redundancy, validate data flow diagrams, and determine the contents of data stored in files.

The Data Repository

• A data repository is a large collection of project information, including:
  • Information about the data maintained by the system
  • Procedural logic and use cases
  • Screen and report design
  • Data relationships
  • Project requirements and final system deliverables
  • Project management information

Data Dictionary Categories

• Data dictionary entries can be categorized into:
  • Data flows
  • Data structures
  • Elements
  • Data stores

Defining Data Flows

• A data flow description includes:
  • Identification number
  • Unique descriptive name
  • General description of the data flow
  • Source of the data flow
  • Destination of the data flow
  • Type of data flow
  • Name of the data structure describing the elements
  • Volume per unit time
  • Area for further comments and notations

Describing Data Structures

• Data structures are made up of smaller structures and elements, and are described using algebraic notation.
• Algebraic notation includes:
  • Equal sign (=) meaning "is composed of"
  • Plus sign (+) meaning "and"
  • Braces {} for repetitive elements
  • Brackets [] for an either/or situation
  • Parentheses () for an optional element

Data Elements

• A data element description includes:
  • Element ID
  • Name of the element
  • Aliases
  • Short description of the element
  • Element is base or derived
  • Element length
  • Type of data
  • Input and output formats
  • Validation criteria
  • Default value
  • Additional comment or remark area

Data Stores

• A data store description includes:
  • Data store ID
  • Data store name
  • Alias for the table
  • Short description of the data store
  • File type
  • File format
  • Maximum and average number of records on the file
  • File or data set name
  • Data structure
  • Primary and secondary keys
  • Comments

Creating the Data Dictionary

• Data dictionary entries are created after the data flow diagram is completed or as the data flow diagram is being developed.
• A top-down approach is used to create the data dictionary.

Using the Data Dictionary

• The data dictionary can be used to:
  • Create screens, reports, and forms
  • Generate computer language source code
  • Analyze the system design, detecting flaws and areas that need clarification

Creating XML

• XML is used to exchange data between businesses.
• XML addresses the problem of sharing data when users have different computer systems and software or different database management systems.
• The data dictionary is an ideal starting point for developing XML content.
• XML documents may be transformed into different output formats.
• XML is a way to define, sort, filter, and translate data into a universal data language that can be used by anyone.

XML Document Type Definitions (DTDs)

• DTDs are used to determine if the XML document content is valid.
• DTDs may be created using the data dictionary.
• DTD may be used to validate the XML document.

XML Schemas

• XML schemas provide a more precise way to define the content of an XML document.
• XML schemas include:
  • Exact number of times an element may occur
  • Type of data within elements