A survey of state-of-the-art sharding blockchains: Models, components, and attack surfaces.

Blockchain has been widely used in various fields, such as health management, finance, the Internet of Things (IoT), identity management, and supply chains. However, the performance of blockchain, including its throughput and transaction processing latency, has hindered its large-scale practical application, which is known as the scalability issue of blockchain. Many on-chain and off-chain solutions have been proposed to solve this problem, such as directed acyclic graph (DAG) blockchain and Rollup schemes. Despite these schemes' ability to enhance the scalability of the blockchain, they compromise on other essential properties. The consensus is that the DAG blockchain can never provide deterministic security for transactions, and Rollup is disputed because it makes a trade-off of decentralization for scalability. Sharding technology is a proposed solution for these limitations, promising to tackle the scalability issue while maintaining security and decentralization at the same time. To date, no survey has been done on sharding design models and their key components and corresponding attack surfaces. To fill this gap, in this survey, we first provide a detailed analysis and comparison of both permissioned and permissionless sharding blockchains. Then, we describe the common building blocks chosen by the above sharding schemes as key components. Finally, we investigate how these components may be attacked and suggest corresponding countermeasures. Several open challenges are also given as future research directions. The survey aims to help researchers, engineers, and educators quickly grasp a consolidated body of knowledge about current sharding blockchain development.