Grid architecture and functionality as seen by the users from their UI (User Interface) are described using the following simple sketch. 1
In the figure, the Grid is represented as a truncated pyramid with the RB (Resource Brocker) and Information Services on top and the computing units at floor level. The UI is shown as en external element, as it should really be.
The heart or better the brain of the system is the top area were the lightnings represent the communication and allocation activity of the system. Intercommunication, resource query, reservation and allocation are represented as signals that travel through brokers and information databases.
The UI is the user entry point to the system. User accounts are assigned at UI level, while grid access is granted by user certificates. User prepare their jobs and collect the related output in their UI interactive sessions.
When the user submits a job, the job is sent to the RB that checks job requirements and, matching them with the informations about node resources, chooses an appropriate CE (Computing Element) for job execution. The job and all related input files are sent to the CE which is charged of job scheduling through its queue manager. The best matching WN (Worker Node) is selected for job execution and jobs start running as soon as possible.
When the job terminates at the executing WN, the RB is aknowledged, the termination status id published and the user can collect the job output. A more detailed description of grid job processing is given in Section 7.