Examples

• In a database system, data items appear within specific structures. In most cases these include tables, columns and sometimes relationships. The metadata for a database involves a description of the tables and columns, including their names and the types of data they contain. For example, in a Java application, the JDBC (Java Database Connectivity) resources provide connections with databases. Some of the JDBC methods allow programs to dynamically retrieve information about a database structure, as well as querying the database for data using the metadata information. Library cataloging systems and websites also frequently use metadata to facilitate information processing.
  
  

Purpose

• In general, the purpose of metadata is to facilitate use of the data within a database or other source. In the Java example, the metadata provides the program with the information it needs to effectively query the database and process the results. Similarly, with HTML Web pages, metadata listed within "meta" elements provides the Web browser with information about the page content. This information helps the browser program to effectively present the page to website users. Metadata aids communication between data sources and any programs attempting to use them.
  
  

Content

• The content of metadata depends on the data it is describing. For example, in a database application, the metadata may include an indication of the data type a column uses, as well as a description of the relationship between two database tables. In a Web page, the metadata may contain information about the media types included within the page. This may help the browser to render the content, particularly if browser plug-ins are required. Metadata can therefore be as varied as the potential data sources it describes.
  
  

Issues

• Where metadata allows an application to query a data structure, there is a possibility of error. If a programmer is writing an application to connect to and query a database, but does not know about the database structure, therefore requiring the use of metadata, this implies that there is a lack of understanding between the database and the application. Just as programs need to understand the structures in a data source in order to retrieve data accurately, the structures in the metadata also need to be understood for effective processing.