A Grid contains many services distributed over many sites. Some services are centralised, some services are optionally available at a site and others are (for practical reasons) always present at a site. Storage is a service that all sites provide. This is because using storage places a burdon on network bandwidth and site-to-site bandwidth is a precious resource.

If resources are provided locally, then a computing job will wish to use these services preferentially. To allow for this, there must be a method for establishing which services exist locally. The WLCG Information System is this mechanism. It uses LDAP servers to store site-local information and for this information to be aggregated to a LDAP server with a view of the entire Grid.

These LDAP server may be queried using standard LDAP tools. However, for the queries to be meaningful, everyone must publish data with the same understanding of what the different attributes mean. If some information (with a specific label) is published by many different sites, the information is meaningful only if all sites agree on what the information represents.

The GLUE Schema is the definition of how the resources within a site, and the site itself, are described. It describes the objects and the attributes for these objects. By following the GLUE Schema, information published into WLCG Information System becomes meaningful to everyone within the Grid.

The Information Provider is a component that publishes information. It queries the resource it is publishing information about, and must follow the GLUE Schema so the information is meaningful. All information published in the WLCG Information System is supplied by an information provider.

The dCache information provider provides information for the local (to the resource) LDAP server (called a BDII server). This resource-level BDII is queried by a site-level BDII. The site-level BDII is itself queried by a top-level BDII, which provides a central point for querying the information system.