Developing scientific applications with loosely-coupled sub-tasks

The Simple API for Grid Applications (SAGA) can be used to develop a range of applications which are in turn composed of multiple sub-tasks. In particular SAGA is an effective tool for coordinating and orchestrating the many sub-tasks of such applications, whilst keeping the application agnostic to the details of the infrastructure used. Although developed primarily in the context of distributed applications, SAGA provides an equally valid approach for applications with many sub-tasks on single high-end supercomputers, such as emerging peta-scale computers. Specifically, in this paper we describe how SAGA has been used to develop applications from two types of applications: the first with loosely-coupled homogeneous sub-tasks and, applications with loosely-coupled heterogeneous sub-tasks. We also analyse and contrast the coupling and scheduling requirements of the sub-tasks for these two applications. We find that applications with multiple sub-tasks often have dynamic characteristics, and thus require support for both infrastructure- independent programming models and agile execution models. Hence attention must be paid to the practical deployment challenges along with the theoretical advances in the development of infrastructure-independent applications.