## 1 Introduction  
Modeling and simulation (M&S) is a widely used approach to analyze systems, natural phenomena, and processes [1; 2]. It encompasses the specification of a simulation model describing the addressed system or process from the required perspective, and the consequent implementation of the corresponding executable simulation system, which is run to reproduce the behavior of the system or process under study [3]. M&S approaches are neither tied to any specific application domain nor constrained to the adoption of a given development paradigm. Indeed, they have proven their effectiveness in various application domains. Among others, the development of _complex systems_, such as distributed systems, high-performance computing systems, or cyber-physical systems (CPS) might benefit from the adoption of a M&S approach as it allows analysts to evaluate different design alternatives before starting the actual system implementation. Specifically, innovative approaches, such as those that exploit methods and standards form the model-driven engineering (MDE) field, can be introduced to automate and ease the generation of a simulation model from the design models of the system under study [4; 5; 6]. Moreover, M&S approaches might be effectively adopted to support the development of _digital twins_ (DTs) [7], which have recently gained great popularity among researchers and practitioners in the simulation field. A DT is a dynamic digital representation (e.g., a simulation model) of a physical system, which is referred to as a _physical twin_ (PT). The fundamental aspect of this paradigm is that the DT and the related PT are constantly aligned so that changes in the operational state of the PT are reflected in relevant changes in the DT configuration parameters. In this context, M&S approaches might be introduced to derive a DT from the design model of the addressed system, thus ensuring the initial compliance of the DT with its physical counterpart [8]. In order to leverage the potential benefits of M&S, technologies such as _cloud computing_ and _service-oriented architectures_ (_SOAs_) can be introduced to build and provide M&S applications through the composition of services available in the cloud, according to the emerging _M&S-as-a-service_ (_MSaaS_) paradigm [9; 10]. Thus, according to a MSaaS perspective, an M&S application can be built by integrating and orchestrating existing M&S services, to enhance the interoperability, composability, reusability, and cost-effectiveness of the M&S effort [11]. That is, the delivery of service composition in the cloud is a non-trivial task that requires the proper execution of the following activities:  
1. Starting with the simulation requirements and objectives, a candidate _set of component M&S services_ has to be _identified_, with each component service providing an interface that meets the composite service requirements;
2. A choreography-based or an orchestration-based _composition model_ has to be specified to handle the execution flow of several component services;
3. Each component service has to be _configured and deployed_ onto an execution platform, which, in turn, requires the integration of a set of hardware and software resources, such as computational nodes, containers, applications, networks connections, databases, and middleware. As discussed in Section 4.1, in this work, we use the term _application orchestration_ with reference to the aforementioned activities 1 and 2, while activity 3 is referred to as _infrastructure orchestration_. The development of a MSaaS application becomes even more complex when the needed composition includes M&S services provided by different and heterogeneous cloud implementations in a so-called _MSaaS-federated infrastructure_. In this context, the paper introduces an architecture for TOSCA-based inter-cloud modeling and simulation (ArTIC-MS), a conceptual architecture that aims to address the fair principles (findability, accessibility, interoperability, and reusability) in the development of a MSaaS platform.