Abstract
Recently, enormous interest has been shown by both academia and industry around concepts and techniques related to connecting heterogeneous IoT devices. It is now considered a rapidly evolving technology with billions of IoT devices expected to be deployed in the upcoming years around the globe. These devices must be maintained, managed, traced, and secured in a timely and flexible manner. Previously, the centralized approaches constituted mainstream solutions to handle the ever-increasing number of connected IoT devices. However, these approaches may be inadequate to handle devices at a massive scale. Blockchain as a distributed approach that presents a promising solution to tackle the concerns of IoT devices connectivity. However, current Blockchain platforms face several scalability issues to accommodate diverse IoT devices without losing efficiency. This paper performs a comprehensive analysis of the recent blockchain-based scalability solutions applied to the Internet of Things domain. We propose an evaluation framework of scalability in IoT environments, encompassing critical criteria like throughput, latency, and block size. Moreover, we conduct an assessment of the notable scalability solutions and conclude the results by highlighting six overarching scalability issues of blockchain-based solutions in IoT that ought to be resolved by the industry and research community.