A programmable overlay network goes beyond existing Grid applications,
as it enables nodes to actively participate in a distributed
computation as opposed to simply share their information and
computational resources. In this talk we will discuss the challenges
involved in programming such overlay network. These are not limited to
the decentralized and distributed nature of the overlay but also
involve the difficulty of orchestrating the data and computational
services provided by different and heterogeneous nodes. To this end,
we claim that workflow languages can provide appropriate high level
abstractions for specifying the flow linking the services provided by
the nodes of the overlay. In particular, we will present JOpera, an
advanced workflow tool for modeling and efficiently executing large
scale distributed computations that is being extended to overlay
networks in the context of the AEOLUS EU project. JOpera provides an
integrated environment built on the Eclipse platform to support the
whole life cycle of such overlay programs: development, execution,
monitoring and debugging. A short demo will be given, showing the main
features of JOpera's visual composition language and emphasizing the
extensibility of JOpera's plugin-based architecture. Among many
others, JOpera's growing user community includes the Swiss National
Supercomputing Center, the Swiss Bioinformatics Institute, NCSA and
the Rosen Center for Advanced Computing (at Purdue University).
JOpera can be freely downloaded from www.jopera.org.