Information systems are used for service discovery, publishing accounting data, and publishing other stuff like site location or contacts.
Typically, each node or entity on the grid will have an entry in the information system, and may have sub-entries, according to the schema and publishing requirements. So storage elements and compute elements for example publish information; historically they would publish their own information which would be aggregated by a site or grid-wide information system.
There are also specialised information systems such as GOCDB which contain information for system administrators; the purpose of this page is, however, to document information systems for the users in general (and for other services).
The LHC Grid uses the LDAP protocol as the main information system. This system is based upon a hierarchy of LDAP proxies that publish and query LDAP servers and then republish this information. These services are often called BDII after one of the main implementations.
For a long time, information systems published, er, information in GLUE 1.3; however, sites are required (as of January 2012? or something) to publish GLUE2.
Unlike the GLUE 1.3 infrastructure, GLUE 2 can be published as:
GLUE 2 provides the schema for publishing information, but guidance on how to populate the information systems with information structured according to the schema is still needed. This guidance was provided by EGI in this document.
Additional Links for BDII information: