# Efficient Load Balancing for Blockchain-Based Healthcare System in Smart Cities
## 1 Introduction  
In recent years, the concept of smart cities has emerged as a new paradigm in improving the quality of life. By employing modern information and communication technologies, smart cities are capable of catering to efficient energy management, intelligent transportation facilities, waste management, enhanced health monitoring, and and efficient resource management [1; 2; 3; 4]. The data received from different IoT devices used in various smart city applications are stored in the cloud. Applying artificial intelligence, machine learning, and modern data analytic techniques to these cloud data are envisioned to play a vital role in enhancing the economic growth of a country by the optimal utilization of available resources [5]. With a rapidly growing number of ubiquitously connected IoT devices in different applications of smart cities, the data produced by these devices are growing exponentially [6; 7]. To handle such a large amount of data, recently, fog computing has emergedas a new paradigm. Fog nodes consist of single or multiple data centers located at the edge of user networks without routing over the internet backbone [8; 9; 10; 11; 12]. Consequently, the sensor devices belonging to the same IoT network can transmit their data at faster rates for quick processing. Fog computing is a better choice for delay-sensitive applications, such as emergency response in multiple smart cities applications, such as healthcare services [13; 14]. The healthcare system is one of the prime applications in smart cities [15; 16; 17]. In the healthcare system, the vital signs data of different humanly planted sensors are required to be examined by remotely placed medical physicians for online monitoring. In case the vital signs of the patient increase their threshold limits, necessary action is urgently required. To ensure the valid delivery of patients' data, a secure data-delivery mechanism, such as blockchain, is required. Moreover, the timely delivery of healthcare services to patients is also a critical challenge [18; 19; 20]. Blockchain, also known as a system of proofs, has been emerging rapidly in many applications over the last few years. Blockchain is a secure method of handling data that results in immutable and decentralized maintenance of records in the form of blocks. The data are stored in the blocks along with the secure hash functions. The blocks are chained together by using the data of the previous block to compute the hash function of the current block as shown in Figure 1.  
Immutability is one of the key features in blockchain technology that allows blockchain ledgers to keep their time-stamped transaction data permanent and unalterable. Furthermore, immutability in blockchain provides the fast, efficient, and cost-effective audit of transaction data. This secure data transaction in blockchain attracts multiple supply chain applications, such as agri-food supply [21], medical equipment demand and supply [22], and perishable food products in the supply chain [23]. Immutability is achieved through cryptography hashes by applying the secure hash algorithm (SHA-256). Miners in blockchain technology are responsible for generating these hash values for each block in blockchain systems [24]. There are four different types of blockchain, such as public, private, hybrid, and consortium blockchain. These have been categorized as permissionless, permissioned, and both. A public blockchain is permissionless and decentralized, which allows anyone to join the blockchain network with equal rights. It is mainly used in cryptocurrency. Conversely, a private blockchain is permissioned and partially decentralized, which restricts network access to certain nodes with varying rights limitations. A hybrid blockchain is a mixture of permissionless and permissioned blockchains. It is centrally controlled by a single organization like a permissioned blockchain; however, to perform certain transaction validations, it offers a level of oversight similar to the public blockchain.