Cloud computing has become a common technology for provisioning infrastructure services on-demand. Modern Cloud platforms can provide cloud-based applications, software, deployment platforms, or general infrastructure services that may include both computational and storage resources. However existing Cloud provisioning models are based on proprietary solutions and don't allow the combination of services from different providers and/or user legacy application that usually are present in user home organizations or campus networks. This paper introduces GEM Bus (GEANT Multi-domain Bus), a service-oriented middleware platform that allows flexible services composition, and their on-demand provisioning and deployment to create new specialized task-oriented services and applications. GEM Bus is built upon state-of-the-art Enterprise Service Bus (ESB) technologies and extend them with new functionalities that allow dynamic component services deployment, composition and management. The current paper discusses the general case for integration of Service-Oriented Architecture (SOA) principles and technologies with the provision and deployment mechanisms of Cloud-based platforms to support on-demand infrastructure services provisioning. It describes the Composable Services Architecture (CSA) that provides a general framework for GEM Bus services design and operation. The paper also presents the current GEM Bus implementation status and discusses how it can be applied as a general SOA platform for Cloud-based service provisioning. Finally, it discusses the practical use case of the federated network monitoring service that can be used as integration component in creating/building GEM Bus based Cloud infrastructure services.