**4.1 Global Scheduler**

As mentioned before the Master Node holds the brain of the grid that is represented by the Global Scheduler. It is a software module developed in Java responsible for the distribution of smaller parts of the input data (blocks) to each Worker Nodes. It communicates with other nodes thanks to grid services provided by the Globus Toolkit. The files are sent by the scheduler in order to balance the execution on each node: this is achieved by checking WNs availability and the number of files to sent. In order to know the overall status of the grid, the GS queries the database and transfers file only to nodes that have communicate their status within a specific time period. It is worth noting that this monitoring system that periodically push data into the database instead of gather information from each machine, allows to reduce time and computational wastes Gradwell (2003).
