= QCG Libraries and Cross-cluster communication =
From a development perspective, the !QosCosGrid applications were grouped into two classes: (a) Java applications taking advantage of !ProActive library as the parallelization technology, and (b) applications based on ANSI C or similar codes, which rely on the message passing paradigm. Based on these groups, !QosCosGrid was designed to support two parallel programming and execution environments, namely: QCG-OpenMPI (aiming at C/C++ and Fortran parallel applications developers) and QCG-!ProActive (aiming at Java parallel application developers). 

Additional services were required in order to support the spawning of parallel application processes on co-allocated computational resources. The main reason for this was that standard deployment methodologies used in OpenMPI and !ProActive relied on either RSH/SSH or specific local queuing functionalities. Both are limited to single-cluster runs (e.g., the SSH-based deployment methods are problematic if at least one cluster has worker nodes that have private IP addresses). Those services are called the coordinators and are implemented as Web services.