The aim of the Information and Monitoring Service is to deliver a flexible infrastructure that efficiently provides information about the state of grid services and about applications running on the grid. The information system forms the backbone of the grid and consequently is used heavily by other grid services to locate and select suitable resources.
For its information system, EDG uses R-GMA (Relational Grid Monitoring Architecture). R-GMA uses a relational model and HTTP Servlet technology to implement the Grid Monitoring Architecture from the Global Grid Forum. Information generated by R-GMA ``Producers'' are made available to to R-GMA ``Consumers'' as relations (tables).
Services communicate with the R-GMA servlets via an API which has been implemented in Java, C, C++, python and perl. The servlet responds with an XML document that corresponding to an XML schema definition. Related R-GMA documentationhttp://hepunx.rl.ac.uk/edg/wp3/documentation/ can be found on the web.
A number of information providers have been produced by EDG, including site information, computing element, storage element and network monitoring scripts. On the various grid resources, a program called Gin invokes these scripts to obtain the state information of the resource. Gin then parses the output of the script and publishes this information by using R-GMA.
R-GMA queries use SQL. Queries can be executed with the edg-rgma command line tool available on user interface machines. Several interesting queries are:
edg-rgma -c "latest select UniqueID,RunningJobs,TotalJobs from GlueCE" edg-rgma -c "latest select UniqueID,CurrentIOLoad from GlueSE" edg-rgma -c "latest select URI,Type from Service"which return information about the computing elements, storage elements, and services on the grid. These commands use a ``latest'' producer to process the given SQL query. Using the edg-rgma command with no arguments invokes an interactive command line interface. Within the interactive R-GMA shell, help is available.
The information in R-GMA can also be accessed via a browser. The browser interface runs on the Information Catalog (gppic06.gridpp.rl.ac.uk). Pointing a web browser to the R-GMA browser URLhttp://gppic06.gridpp.rl.ac.uk:8080/R-GMA/index.html allows one to view the information schema and perform interactive queries.