Virtual Database
R-GMA enables Grid users to share information in a virtual database. To the ordinary user a virtual database looks much like a real one. It organises its data into tables, and data can be inserted and queried using standard SQL constructs. It even supports indexes and views. However there is no central repository holding the data for each table. A virtual database just consists of a list of table definitions (called a schema), a list of data providers (called a registry), and a set of rules for deciding which data providers to contact for any given query. These rules are fixed by R-GMA, and are encoded into a hidden component called the mediator.
The picture above shows the principal components of R-GMA. Users and services query the virtual database by connecting to consumers. Information is provided to the virtual database by connecting to producers. The model used by R-GMA, where consumers contact the registry to obtain a list of producers able to answer their query, and then contact the producers in order to obtain appropriate information is known as the Grid Monitoring Architecture (GMA), and was first proposed by the Global Grid Forum (GGF) R-GMA is a Relational implementation of GMA.
The grid can support any number of virtual databases, provided each virtual database has a unique name. Usually, a virtual database will be owned by a Virtual Organisation (VO), which represents a collaboration of users and resources on the grid, typically to carry out work in a particular field of research.
Next : Producers