Simplifying Multilayer and Multitenant Support in OpenStack: The SACHER Use Case The majority of cloud computing deployment environments follow the typical service delivery models, i.e., Software as a Service (SaaS), Platform as a Service (PaaS) and the Infrastructure as a Service (IaaS), that provide specific functionalities to users depending on the service delivery layer and clear resource isolation among different tenants. However, in certain scenarios, such as the Cultural heritage one, it is often required to provide SaaS, IaaS and PaaS crossfunctionality support to users and crosstenant resource visibility among isolated tenants. In the CH scenario, for example, restorers and professionals may often need to connect and to share temporarily data among different tenants or to work with old customized software requiring the possibility to exploit specific management functionalities that are typically available not at the SaaS but at the IaaS or PaaS layers. This paper proposes a middleware called Registry that can provide additional functionalities to different user categories by joining the SaaS with the IaaS layer and that offers also multitenancy operations making private data available to certain project ondemand and enabling, in this way, crosstenant data sharing. The Registry has been designed, developed and tested within the context of the SACHER project that provides a cloudbased infrastructure for the management of the cultural data lifecycle.

Dynamic Provisioning of Service Composition in a MultiTenant SaaS Environment Multitenant service composition has become a common delivery model for business processes in cloud computing. To dynamically support the workload tenant variation, elasticity holds the promise of ensuring the quality of service (QoS) of the business process by providing the involved service instances at a low cost. However, integrating both of multitenancy and elasticity during service composition is a key problem for serving multiple tenants from a single process instance. Nowadays, existing approaches in the field of cloud service composition, although numerous, still fall short since they cannot adequately address issues related to supporting the scalability of the composed service and adapting it to the workload fluctuation. In this paper, we propose a holistic approach which makes the dynamic multitenant services matching and manages their elasticity in distributed business processes. This approach is based on a generic service pattern that integrates multitenancy property and handles elasticity at the process and service levels. Furthermore, we present elastic composition algorithms to compose multitenant cloud services and perform their elasticity through the proposed service pattern. The evaluation of our approach, compared to the baseline approach, proves that the latency taken to provide an elastic multitenant service composition and detect its SLA (Service Level Agreements) violation are reasonably short. We also show that the CPU overhead of using our approach is negligible. Furthermore, experimental results demonstrate the merits of our approach in terms of minimizing the memory consumption through the deployed service instances.