## 4 A RAMI-Inspired Reference Architecture  
The _IoTwins architecture_ was designed to support the DT model discussed in Section 3. According to the model specification, the supporting architecture will have to accommodate: (i) the development of digital objects modeling physical industrial entities and industrial processes; (ii) the design and deployment of pipelines for shipping data from the physical entities to the digital objects, back and forth; (iii) the deployment and operation of digital objects in a distributed and heterogeneous computing environment (i.e., the OT-to-Cloud computing continuum). First, due to the unbalance of resources in the continuum, the architecture will have to provide functions to build virtualized and uniform execution environments out of the physical computing power. Second, due to the intrinsic distributed nature of the conceived digital twins, the architecture will provide a solid mechanism and a choice of protocols to support reliable communication among the multiple digital objects that will populate the above-mentioned virtualized environment. Third, a service is needed to orchestrate the life-cycle (creation, configuration, resource provisioning, roll out, running) of all digital objects. Finally, the architecture will have to guarantee the fulfillment of strict security requirements of compute resources included in the OT perimeter (IoT and Edge ones in our scenario) and of real-time requirements imposed by mission-critical industrial applications. The IoTwins architectural model draws inspiration from the Reference Architectural Model Industry 4.0 (RAMI 4.0) [5] developed by the German Electrical and Electronic Manufacturers' Association (ZVEI) to support Industry 4.0 initiatives. We chose to base our architectural model on the one proposed by RAMI since the latter is a well-established and world-wide recognized reference architecture to implement digital transformation processes in industrial settings, and it also already embeds many of the features that we seek to build a DT-based platform upon. RAMI 4.0 provides a unified model that ensures all the stakeholders involved in an 14.0 ecosystem share data and information in an efficient and effective way. The RAMI 4.0 model comprises three \"axes\" named Life Cycle value stream, Hierarchy levels, and Architecture Layers, respectively. Grounding on the IEC62890 standard [25] the Life cycle value stream axis provides a view of the product life cycle from conception to disposal. The foundation of the Hierarchy levels axis are IEC62264 [26] and IEC61512 [27] respectively, which aim to represent different functional levels of a factory. Finally, the Architecture layers axis enables the transformation of industrial assets into their interoperable Digital Twins. This axis support most of IoTwins research objectives. In the following, we report a short description of it. Figure 1: IoTwins: a distributed and hybrid Digital Twins model. The Architecture layers axis defines a framework where the physical world meets the digital one and a strong interconnection among the manufacturing operations is enabled. A layered view of the axis is depicted in the left-end of Figure 2. At the bottom, the _Assets layer_ identifies and describes the real assets in the physical world. It comprises sensors, devices, machine parts, machines, machine groups, etc. The _Integration layer_ describes the digital equivalents of physical assets. This layer is where the transition from the physical world to the cyber space begins. The _Communications layer_ addresses mechanisms for the interoperable exchange of information between digital assets. The _Information layer_ defines data services such as provisioning and integration that can be leveraged to exchange data among functions, services, and components. The _Functional layer_ provides the runtime and modeling environment to build functions and services to support the business. Finally, the _Business layer_ defines organizational and business-related applications, processes, and operations.