## 2 Background and Related Work
### Related Work  
By leveraging Blockchain and deep reinforcement learning (DRL), a Blockchain-enabled, efficient data collection and sharing framework is proposed to provide a reliable and safe environment for data collection [17]. A distributed DRL-based scheme aims to achieve the maximum data collection and ratio and geographic fairness in the long term, while the Ethereum Blockchain provides a tamper-proof distributed ledger to ensure the security and reliability of data sharing. The simulation results demonstrated that the proposed scheme can prevent against attacks in data collection and sharing. However, the performance adopting Blockchain has not been evaluated, and the storage overhead by directly storing data on the distributed ledger was not discussed. To solve the distrust issues of big data sharing on collaborative edges, a Blockchain-based framework was proposed to ensure efficient and reliable data sharing across resource-limited edge nodes [18]. A green consensus mechanism called proof-of-collaboration (PoC) allows edge devices to mine blocks given their collaboration credits, rather than their computational resources. In addition, this work designed a novel futile transaction filer (FTF) algorithm that offloads transactions from the storage to the cache layer to reduce the response time and storage overhead occupied by the Blockchain. Moreover, the smart-contract-based express transaction (E-TX) can support asynchronous validation, and hollow blocks can significantly reduce the redundancy in block propagation. However, transactionsencapsulating raw data are still directly stored on the distributed ledger, and this brings privacy concerns. With the popularity of the edge-fog-cloud computing paradigm, verifying the integrity of data in use has become a challenging problem. Inspired by the smart contract and Blockchain technology, a real-time index authentication for an event-oriented surveillance video query system is proposed to provide a decentralized video stream security mechanism in the distributed network environment [6]. The hash value of video recordings is stored in the Blockchain as immutable evidence, which is used for the authenticity of raw data in the verification process. The experimental results showed that the entire index authentication process incurs marginal computational overhead for service providers. To solve the issues of the traditional data integrity of cloud servers, a Blockchain-based data integrity verification in P2P cloud storage is proposed, which allows for more open, transparent, and auditable verification of big data [7]. The raw data are divided into several shards, which are stored in private storage, while the digits of the shards construct the hash Merkle trees, which are saved on P2P cloud storage servers for data integrity verification. As the root of a Merkle tree is recorded on the Blockchain before uploading the data, users can verify the integrity of the data without relying on any third party authority. Combining homomorphic verification tags (HVTs) and the data-auditing Blockchain (DAB), a decentralized big data auditing scheme is proposed for smart city environments [19]. Unlike [6;7], the data owners unload their files and HVTs to cloud service providers (CSPs), while all auditing proofs generated by the CSPs are stored into the blocks of the DAB. As all historical auditing proofs cannot be tampered with, data owners or users can verify the data integrity without relying on third party auditors (TPAs). The comparison shows the lower communication and computational overheads incurred in the auditing process. However, the storage overhead of recording auditing proofs on the DAB was not discussed. As a decentralized storage platform that aims to address the issue of file redundancy, the Interplanetary File System (IPFS) has been used to solve the problems of centralized big data storage. A Blockchain-based secure storage and access scheme is proposed to provide the security and efficiency of electronic medical record sharing [20].