CUB, a Consensus Unit-based Storage Scheme for Blockchain System

Recently, Blockchain becomes a hot research topic due to the success of Blockchain in many applications, such as cryptocurrency, smart contract, digital assets, distributed cloud storage and so on. The power of Blockchain is that it can achieve the consensus of an ordered set of transactions among nodes which do not trust each other, even with the existence of malicious nodes. However, compared to traditional databases, the current Blockchain technology still cannot handle a massive number of transactions, which is caused by many factors, such as the consensus protocol, structure of the blocks and storage challenge. Among them, the high storage requirement is a key factor that prevents the wide usage of Blockchain on various devices such as mobile phones or low-end PCs. In this paper, to address the storage challenge, we introduce a novel concept called Consensus Unit (CU), which organizes different nodes into one unit and lets them to store at least one copy of Blockchain data in the system together. Based on this idea, we further define the Blocks Assignment Optimization (BAO) problem which determines the optimal assignment of blocks such that the storage space is fully used and the query cost is minimized. We prove that the BAO problem is NP-hard. Thus, we propose three efficient heuristic algorithms to solve the static assignment problem. Furthermore, we present solutions to address the dynamic scenarios when new blocks arrive and nodes join or depart from the CU. To verify the effectiveness of CU, we have conducted extensive experiments on synthetic data and BLOCKBENCH [1]. The results have confirmed the superiority of CU in saving the storage and maintaining the system throughput.