Tutorial 8.01100

 

What are Data Dictionaries and Repositories?

 

 

A Data Dictionary is defined as an element in a Database Management System, that defines the basic organization of a database.

A Data Dictionary contains a list of all the files in a Database, the numbers of records in each of the files, plus the names and types of each field. Data Dictionaries do not contain the actual data from the Database, they are primarily used for bookeeping and management reasons. Without a data dictionary a DBMS would not be able to acces the actual Data.

A data dictionary is an entity which is kept hidden from users and only available to Database Administrators. This is done to prevent the actual information to be tampered with or corrupted by individuals not aware of the contents.

 

 

Some of the Basic Elements of a Data Dictionary, include...

 

  • Data element number
    Data element number is used in the technical documents.
  • Data element name (caption)
    Commonly agreed, unique data element name from the application domain. This is the real life name of this data element.
  • Short description
    Description of the element in the application domain.
  • Related data elements
    List of closely related data element names when the relation is important.
  • Field name(s)
    Field names are the names used for this element in computer programs and database schemas. These are the technical names, often limited by the programming languages and systems.
  • Code format
    Data type (characters, numeric, etc.), size and, if needed, special representation. Common programming language notation, input masks, etc. can be used.
  • Null value allowed
    Null or non-existing data value may be or may not be allowed for an element. Element with possible null values needs special considerations in reports and may cause problems, if used as a key.
  • Default value
    Data element may have a default value. Default value may be a variable, like current date and time of the day (DoD).
  • Database table references
    Reference to tables the element is used and the role of the element in each table. Special indication when the data element is the key for the table or a part of the key.
  • Definitions and references needed to understand the meaning of the element
    Short application domain definitions and references to other documents needed to understand the meaning and use of the data element.
  • Source of the data in the element
    Short description in application domain terms, where the data is coming. Rules used in calculations producing the element values are usually written here.

Data Repositories

 

A Data Repository, generically refers to a central place where data is stored and maintained. It works basically in the same way a Data Dictionary the only difference is taht a Data Repository is more inclined to storing metadata about the data and the processing resources that an Information System requieres.

Another definition for a Data repository is: A place where multiple databases or files are located for distribution over a network, or a repository can be a location that is directly accessible to the user without having to travel across a network.

 

 

Basic Elements of a Repository System

 

Most DBMS rely on Data Dictionaries and Repositories to operate properlly, and many CASE tools also come available with applications for creating and mantaining Repositories and Directories, current and readily available.

Data Dictionaries and Repositories are not only present in Database Management Systems or in Database ralated topics, they are a crucial part of the System Development Life Cycle (SDLC) and many other types of software development methodologies. Because organization is so important to this endeavour; information about information can make or brake a project, no matter the size. A well organized designer wheter it is a Database or an Information System designer must always be aware of the information and data he is manipulating and minning.