Realizing Infrastructure-as-a-Service (IaaS) cloud requires a control platform to orchestrate cloud resource provisioning, configuration, and decommissioning across a distributed set of diverse physical resources. This orchestration is challenging due to the rapid growth of data centers, high failure rate of commodity hardware and the increasing sophistication of cloud services. This paper presents the design and implementation of TROPIC, a highly available, transactional resource orchestration platform for building IaaS cloud infrastructures. TROPIC’s orchestration procedures that manipulate physical resources are transactional, automatically guaranteeing atomicity, consistency, isolation and durability of cloud operations. Through extensive evaluation of our prototype implementation, we demonstrate that TROPIC can meet production-scale cloud orchestration demands, while maintaining our design goals of safety, robustness, concurrency and high availability