2.1 The egee distributed computing infrastructure  (Page 2/3)

To join a Virtual Organisation you need to be able to prove who you are electronically. This is similar to the way that a passport is used to prove your identity when you cross international borders. Within the Grid community this is frequently done through the use of a certificate – generally issued by proving your identity to someone at your local institute. Some organisations allow you to generate a certificate through your existing ability to access your organisation’s own network.

Once you are able to identify yourself you can apply to join a Virtual Organisation. Different Virtual Organisations exist to cover the needs of different communities. A community may have more than one Virtual Organisation within it with each one having different entry criteria and possibly providing access to different resources.

Information

With many thousands of services potentially available to a user, discovering which one to use presents many challenges. The infrastructure is continually changing – services are appearing, disappearing or being upgraded as the sites evolve. Being able to discover, in near real-time, the types of services that are available, the Virtual Organisations that are able to access them, and the characteristics of each service (i.e. the data that it stores or the speed of the processors), and the load on the service, are all information points that can drive which service to select.

The information collected by gLite on the resources within the infrastructure can be presented in many ways. The information can be browsed directly through a web portal, searched manually through command line tools, or programmatically from within an application.

Data

Many of the researchers that use EGEE’s infrastructure do so in order to analyse data stored in files. Frequently these files are stored at locations different from the currently available computational resources. EGEE provides services that allow users to retrieve a file from off-line tape storage onto disk, and to then move that file to the site where the computational resources for that user is going to be available. (This method of data storage and retrieval is normal in high energy physics experiments and frequently used in other communities dealing with large archived data sets such as climate modelling and satellite observation records.)

How does a user locate the file that they need to use on an infrastructure where the location of files is continually changing? File catalogues run by some communities provide a register where a ‘logical’ file name can be mapped to a number of physical replicas. Having files stored in multiple places has many benefits - files are still available even if one of the sites storing the files is temporarily disconnected from the network or the service is down. Software can be written to exploit the distributed location of the files so as to run an application on the computing resources located near to their storage location – thereby reducing the time taken to move the files from their storage location to the compute resources.