IEEE COMMUNICATIONS SURVEYS & TUTORIALS 1 Integration of Blockchain and Cloud of Things: Architecture, Applications and Challenges Dinh C. Nguyen, Student Member, IEEE, Pubudu N. Pathirana, Senior Member, IEEE, Ming Ding, Senior Member, IEEE, and Aruna Seneviratne, Senior Member, IEEE Abstract—The blockchain technology is taking the world by transactions. The blockchain is basically a decentralized, im- storm. Blockchain with its decentralized, transparent and secure mutable and public database. The concept of blockchain is nature has emerged as a disruptive technology for the next gener- based on a peer-to-peer network architecture in which trans- ation of numerous industrial applications. One of them is Cloud of Things enabled by the combination of cloud computing and action information is not controlled by any single centralized Internet of Things. In this context, blockchain provides innovative entity. Transactions stored in a chain of blocks are publicly solutions to address challenges in Cloud of Things in terms accessible to all blockchain network members in a trustworthy of decentralization, data privacy and network security, while manner. Blockchain uses consensus mechanisms and cryptog- Cloud of Things offer elasticity and scalability functionalities raphy to validate the legitimacy of data transactions, which to improve the efficiency of blockchain operations. Therefore, a novel paradigm of blockchain and Cloud of Things integration, guarantees resistance of linked blocks against modifications called BCoT, has been widely regarded as a promising enabler and alterations [7]. In particular, the blockchain technology for a wide range of application scenarios. In this paper, we also boasts the desirable characteristics of decentralization, present a state-of-the-art review on the BCoT integration to accountability, and security which improve service efficiency provide general readers with an overview of the BCoT in and save operational costs. Such exceptional properties pro- various aspects, including background knowledge, motivation, and integrated architecture. Particularly, we also provide an in- mote the usage of applications built on blockchain in recent depth survey of BCoT applications in different use-case domains years. Thus, it makes now the right time to pay attention to such as smart healthcare, smart city, smart transportation and this hot research topic. smart industry. Then, we review the recent BCoT developments On the other side, the revolution in the field of information with the emerging blockchain and cloud platforms, services, and and communication has created a wealth of opportunities for research projects. Finally, some important research challenges and future directions are highlighted to spur further research in advanced technologies, especially Internet of Things (IoT) this promising area. and Cloud computing. IoT has reshaped and transformed our lives with various new industrial, consumer, and commercial Index Terms—Blockchain, cloud computing, Internet of Things, Cloud of Things, security, industrial applications. services and applications [8], [9]. Typically, IoT is a system of physical objects that can be monitored, controlled or interacted with by ubiquitous electronic devices to enable ubiquitous I. INTRODUCTION industrial services, e.g., smart cities, smart industries, etc. Due to the limited resources of IoT devices, they always delegate Recent years have witnessed the explosion of interest in IoT application tasks to Cloud computing, which gives birth blockchain, across a wide span of applications from cryptocur- to the Cloud of Things (CoT) paradigm [10], [11]. The rencies to industries [1], [2]. The rapid development in the CoT provides a flexible, robust cloud computing environment adoption of blockchain as a disruptive technology is paving the for processing and managing IoT services, showing great way for the next generation of financial and industrial service potentials to improve the system performance and efficiency of arXiv:1908.09058v2 [cs.CR] 28 Aug 2020 sectors. Indeed, new research activities on blockchain and its service delivery [12]. However, the conventional CoT infras- applications take place every day, impacting many aspects of tructures tend to be ineffective due to the following challenges. our lives, such as finance [3], energy [4], and government First, the conventional CoT solutions have mainly relied on services [5]. centralized communication models, e.g., central cloud, for IoT From a technical perspective, blockchain is a distributed service operations which make it hard to scale when IoT net- ledger technology that was first used to serve as the public works become more widespread [13]. Moreover, most current digital ledger of cryptocurrency Bitcoin [6] for economic CoT systems mandate trusting a third party, e.g., a cloud provider, for IoT data processing, which raises data privacy *This work was supported in part by the CSIRO Data61, Australia. Dinh C. Nguyen is with School of Engineering, Deakin University, Waurn concerns. Final, the centralized network infrastructure results Ponds, VIC 3216, Australia, and also with the CSIRO Data61, Docklands, in higher communication latency and power consumption for Melbourne, Australia (e-mail: [email protected]). IoT devices due to long data transmission, which hinders the Pubudu N. Pathirana is with School of Engineering, Deakin University, Waurn Ponds, VIC 3216, Australia (email: [email protected]). large-scale deployments of CoT in practical scenarios [14]. Ming Ding is with the CSIRO Data61, Australia (email: In order to achieve a sustainable development of CoT, [email protected]). building a more decentralized ecosystem has been regarded Aruna Seneviratne is with School of Electrical Engineering and Telecom- munications, University of New South Wales (UNSW), NSW, Australia as a future direction to replace centralized computing models (email: [email protected]). used in current applications, as illustrated in Fig. 1. It is IEEE COMMUNICATIONS SURVEYS & TUTORIALS 2 Before 2005 Today 2025 and beyond Cloud servers Edge servers Closed data access with Open data access with Closed data access with Open data access with centralized computing centralized computing decentralized computing decentralized computing Fig. 1: Past, present and future Cloud of Things infrastructure. strongly believed that blockchain will be a strong candidate ments where any IoT users and cloud providers interested to realize the full decentralization of future CoT networks. in the system can participate and collaborate to achieve Particularly, the integration of blockchain and CoT leads to the common goals within the BCoT ecosystem [18]. a novel paradigm called as BCoT. The combination of these On the other hand, CoT can support blockchain platforms with emerging technologies brings great benefits to both worlds and the following key benefits: thus gains sustainable interest in academics and industries. In • Scalable support for blockchain transactions: In large- fact, the blockchain and CoT have a number of complementary scale blockchain applications, the number of transactions connections for practical applications. In the context of cloud in blockchain networks can be enormous. Therefore, it computing, blockchain has been regarded as a service called is highly necessary to provide powerful data process- Blockchain as a Service (BaaS). By providing a decentralized ing services to accelerate transaction execution in order storage architecture using virtual storage nodes, blockchain to enable scalable blockchain services. In this context, can enable completely new cloud storage functions which the cloud can offer on-demand computing resources for are strongly resistant to data modifications. Instead of relying blockchain operations thanks to its elasticity and scalabil- on traditional cloud data centres, blockchain interconnects ity capability [19]. For example, public clouds can offer computer nodes, including virtual machines on cloud and a large-scale network of resources for blockchain service external computers, to build a fully decentralized storage operators in a federated cloud environment. Therefore, system without requiring a central authority. Blockchain also the combination of cloud computing and blockchain can functions as network management services which are closely achieve a high scalability of the integrated system. related to smart contract-based applications. In such scenar- • Fault tolerance: Cloud can help replicate blockchain ios, blockchain acts as a communication layer among cloud data across a network of computing servers which are servers, IoT devices, and end users. Specifically, the adoption interconnected robustly by collaborative clouds [20]. This of blockchain can provide many potential benefits for CoT will minimize the single-failure risks due to the disruption systems as follows. of any cloud node and thus ensure uninterrupted ser- • Decentralization: Blockchain with its decentralized nature vices. Further, the inter-cloud ecosystem can enable the is a promising methodology to effectively solve the bot- blockchain system to operate continuously in the event tleneck and single-point failure issues by eliminating the of a certain cloud server being under attack. requirement for a trusted third party in the CoT network Reviewing the state of the art in the field, we find that [15]. Further, the peer-to-peer architecture of blockchain BCoT attracts enormous interests of research communities