  The GLEON Research And PRAGMA Lake Expedition -- GRAPLE -- is a collaborative
effort between computer science and lake ecology researchers. It aims to
improve our understanding and predictive capacity of the threats to the water
quality of our freshwater resources, including climate change. This paper
presents GRAPLEr, a distributed computing system used to address the modeling
needs of GRAPLE researchers. GRAPLEr integrates and applies overlay virtual
network, high-throughput computing, and Web service technologies in a novel
way. First, its user-level IP-over-P2P (IPOP) overlay network allows compute
and storage resources distributed across independently-administered
institutions (including private and public clouds) to be aggregated into a
common virtual network, despite the presence of firewalls and network address
translators. Second, resources aggregated by the IPOP virtual network run
unmodified high-throughput computing middleware (HTCondor) to enable large
numbers of model simulations to be executed concurrently across the distributed
computing resources. Third, a Web service interface allows end users to submit
job requests to the system using client libraries that integrate with the R
statistical computing environment. The paper presents the GRAPLEr architecture,
describes its implementation and reports on its performance for batches of
General Lake Model (GLM) simulations across three cloud infrastructures
(University of Florida, CloudLab, and Microsoft Azure).
