# A Survey on IoT-Edge-Cloud Continuum Systems: Status, Challenges, Use Cases, and Open Issues
## 1 Introduction  
The unstoppable proliferation of novel computing and sensing device technologies, and the ever-growing demand for data-intensive applications in the edge and cloud, are driving the next wave of transformation in computing systems architecture [1, 2]. In the same context, there is a vast number of devices that can collect, process, and transmit data to other devices and systems over the Internet or other communications networks. This new concept, known as the Internet of Things (IoT), enables the collection of data from various and diverse sources in the physical world [3]. Leveraging this concept, manydifferent advanced human-centric services and applications can be widely deployed, such as energy management in smart home environments, remote health monitoring, intelligent transportation, etc. [4]. Since most data are created at the edge, and computationally intensive data processing usually takes place in centralized cloud infrastructures, a flexible interconnection of all involved entities is required to bring the edge as close to the cloud as possible and vice versa. Together with the cloud, edge-based computing is pushing the limits of the traditional centralized cloud computing solutions enabling, among other features, efficient data processing and storage as well as low latency for service execution. In this context, multi-access edge computing (MEC), formerly mobile edge computing, is a new architectural concept that enables cloud computing capabilities and an IT service environment at the edge of any network [5; 6]. Located in close proximity to the end users and connected IoT devices, MEC provides extremely low latency and high bandwidth while always enabling applications to leverage cloud capabilities if necessary. The resulting paradigm shift in computing is centered around the dynamic, intelligent, and yet seamless interconnection of IoT devices, edge, and cloud resources in one computing system to form what is known as a continuum [7; 8]. The goal of this synergy is the provision of advanced services and applications to the end users, which is also leveraged by similar advances in the networking field, such as network function virtualization (NFV) [9; 10], which decouples network operations from specific hardware equipment, as well as software defined networking (SDN) [11], which enables a holistic and intelligent network management approach. A continuum, today also referred to as cloud, IoT, edge-to-cloud, or fog-to-cloud continuum, is expected to provide high computational capabilities for data processing at both edge and cloud while inferring and persisting important information for post-mortem and offline analysis. Throughout the rest of this paper, the term continuum will be used to indicate the IoT-edge-cloud (IEC) continuum, unless otherwise stated. The full deployment of such a continuum is expected to leverage the support of latency-critical applications via dynamic task offloading among the IoT nodes and edge or cloud servers. Moreover, data collected directly from all entities of the continuum can be used for optimum resource allocation and scheduling policies [12]. However, there are many technical challenges associated with this new architectural approach:  
* Unlike centrally managed clouds, massively heterogeneous systems in the continuum (including IoT devices, edge devices, and cloud infrastructures) are significantly more complex to manage. Furthermore, distributed data management raises an additional level of complexity by classifying data infrastructures, collecting vast and diverse data volumes, providing transparent data access methods, optimizing the internal data flow, and effectively preserving data collections [13]. * Because of the heterogeneity of the involved devices and associated technologies, hardware and technology-agnostic protocols are important, not only to manipulate a large number of interconnected entities but also to enable scalability which is a key concept in the IEC continuum.