Today, computational scientists mainly execute parallel or distributed applications, and try to scale up to get more results or greater data precision. As a result, they use more and more distributed resources, using local large-scale HPC systems (such as clusters or MPP), grids or even clouds. The difficulty of managing those platforms is their differences in nature, each degree abstracting some of the complexity created by resource distribution. For instance, clusters and MPP systems are located on a single site, composed of different ``partitions'' (e.g., I/O nodes, compute nodes). In grids, ``virtual organizations (VOs)'' are one of the main concepts; since VOs are global and multi-users, they abstract both the complexity of the local r...