# Blockchain Integration in the Era of Industrial Metaverse
## 1 Introduction  
Blockchain technology can be defined as a growing list of records, which are known as blocks. The innovation of blockchain technology, beyond the creation and maintenance of a continuous list of records, is the improvement of digital transaction security and anonymity based on the implementation of cryptography algorithms. For a peer to insert a new block into the blockchain, that specific block must be followed by a cryptographic hash (i.e., a unique code) that is connected to the preceding block. Each new block consists of two key parts, namely the block header and the block body. The structure of the block header includes the version number, a timestamp, a target hash bit of the current block, a nonce, the hash value of the previous block, and a Merkel root. The detailed data of transactions are stored in the block body. More specifically, additional attributes, such as a timestamp, and transaction data are also included in the new block, thus ensuring the continuity of the information. Consequently, it becomes apparent that the blockchain is by definition resistant to modification of its data. Timestamping and hashing processes are required before a new block is added to the chain; thus, the data of the blockchain can be traced back and are transparent to everyone who participates in the blockchain. Essentially, the blockchain can also be considered a distributed, shared database. In Figure 1, the operating principle of the blockchain for the addition of new blocks is described. One of the most important processes that take place in the blockchain is \"mining.\" The term \"mining\" is loosely defined by many as the process of creating, i.e., digitally generating, new cryptocurrency coins. However, it is a much more involving process that is used for the validation of digital transactions and thus the creation of new blocks in the chain, i.e., the digital and distributed ledger. One of the most important processes in the blockchain is the admission of the required rights to the miners in order to add a new block and consequently to create the latest version of the blockchain. Therefore, whenever a new block is added to the chain, a new link is created starting from the initial block, the so-called \"genesis block,\" creating a continuous history of transactions. The mining operation is accompanied by the consensus agreement process, which is investigated in the following paragraphs. The consensus process is required since the digital ledger lacks a centralized authority. Consequently, the miners are rewarded for their work securing the network with the provision of new digital coins (cryptocurrency). The infrastructure of the blockchain can be realized as a six-layer framework (Figure 2), namely (i) application, (ii) contract, (iii) incentive, (iv) consensus, (v) network, and (vi) data layers. More specifically, in the application layer, a plethora of application scenarios are included. Then, the contract layer compiles the necessary algorithmic backend, e.g., code scripts, algorithms, and smart contracts. The incentive layer is used in conjunction with the consensus layer to ensure the reward of each node for the work accomplished on the network. The latter can be realized by the issuance of tokens to the miners after the completion of their task. Among the most important layers is the consensus layer, which encapsulates the consensus mechanism. The consensus mechanism is important in the current blockchain implementations (i.e., permissioned blockchains) since it provides the necessary framework for all the participants of the blockchain (i.e., the nodes) to achieve agreement. It is stressed that two types of blockchain exist considering the admittance of new nodes: permissioned and permissionless blockchain [1; 2; 3]. The network layer consists of the networking functionalities required for the communication between the nodes, the data exchange framework, and the data verification framework. Finally, the  
Figure 1: The operating principle of blockchain.