This paper describes the use of a distributed cloud computing system for
high-throughput computing (HTC) scientific applications. The distributed cloud
computing system is composed of a number of separate
Infrastructure-as-a-Service (IaaS) clouds that are utilized in a unified
infrastructure. The distributed cloud has been in production-quality operation
for two years with approximately 500,000 completed jobs where a typical
workload has 500 simultaneous embarrassingly-parallel jobs that run for
approximately 12 hours. We review the design and implementation of the system
which is based on pre-existing components and a number of custom components. We
discuss the operation of the system, and describe our plans for the expansion
to more sites and increased computing capacity
