# MSSP: A Blockchain Sharding Protocol Based on Multi-Shard Storage
## 1 Introduction  
The rise of cryptocurrencies such as Bitcoin [1] and Ethereum [2] has brought widespread attention to blockchain technology. It is composed of interconnected blocks, forming an immutable transaction ledger, with each block containing verified transactions and the hash value of the previous block [3]. Currently, blockchain technology has been significantly applied in various fields including supply chain management [4], digital identity authentication [5], digitization of healthcare information [6], and the Internet of Things (IoT) [7, 8]. With the development of blockchain technology, scalability has become a key attribute affecting the widespread application of blockchain technology [9, 10]. Sharding is currently one of the most promising technologies to improve the scalability of blockchain systems [11, 12]. Its basic principle is to divide nodes into multiple shards, with each shard responsible for maintaining an independent blockchain and only processing transactions related to its own stored state [13]. Sharding technology has been widely applied in multiple blockchain projects, such as ELASTICO [14] and Zilliqa [15]. In recent years, this technology has continued to evolve, with optimizations in throughput, latency, consensus protocols, and other aspects [16, 17, 18]. Although sharding technology has significantly improved the performance and scalability of blockchain systems, it still faces numerous challenges in handling cross-shard transactions [19]. Cross-shard transactions involve data synchronization and communication between multiple shards, leading to additional overhead, which reduces system efficiency and increases latency. To address this issue, existing research has proposed various sharding system solutions. For instance, RapidChain [20] and RepChain [21] divide cross-shard transactions into multiple sub-transactions, which are processed separately by the relevant shards. However, this approach may result in a surge in the number of sub-transactions, thereby undermining the advantages of sharding. BrokerChain [22] proposes a broker mechanism that utilizes Broker accounts to handle cross-shard transactions, achieving load balancing. Additionally, Estuary [23] has designed a multi-level state model that introduces state splitting and aggregation mechanisms, making state management more flexible and dynamically adjustable according to demand. This mechanism significantly reduces the number of cross-shard transactions and lowers communication overhead and complexity. Although the aforementioned methods have improved system performance to some extent, they have not completely eliminated the additional communication overhead caused by cross-shard transactions. Therefore, further optimizing the processing mechanisms for cross-shard transactions remains an important direction for future research. This paper proposes a new blockchain sharding protocol based on multi-shard storage. The core idea is to allow some nodes in the blockchain to store multiple shard data, and these nodes can directly process cross-shard transactions, thus improving the efficiency of cross-shard transaction processing. In MSSP, the consensus zone consists of nodes storing the same multiple shards, making shards no longer equivalent to consensus zones. Therefore, the content stored by consensus nodes in the same shard may be inconsistent. This change transforms the data structure of the blockchain in a shard from a chain to a tree, eliminating the ability of traditional blockchains to defend against double-spending attacks [24]. To address this issue, we propose a priority sorting mechanism. It is crucial for all nodes storing relevant account data to be promptly updated after cross-shard transaction processing is completed. To ensure this, we propose a \(P\)-probability return update completion proof mechanism to guarantee that relevant nodes are promptly updated and enhance the security of the blockchain.