Sharding PoW-based Blockchains with Simple Cross-Shard Transaction Processing

Sharding is a promising solution to address the scalability challenge of blockchains by enabling parallel transaction processing across multiple shards. However, handling diverse cross-shard transactions with low inter-shard dependence remains a challenge. Existing approaches, such as two-phase commit and transaction decomposition, suffer from strong inter-shard dependencies or limited support for complex cross-shard transactions.This paper proposes a hierarchical sharded blockchain architecture with a root shard and multiple working shards, all using Nakamoto consensus. The verification and execution of cross-shard transactions are separated between the root shard and the working shards. The root shard, holding the final authority over the system state, endorses working chains and verifies cross-shard transactions, while working shards focus on executing operations related to their own shard. To ensure security, all operations of cross-shard transactions are executed based on the state used by the root shard during transaction validation. We extend the structure of working block which enables continuous intra-shard transaction processing alongside cross-shard transaction execution, even during root chain forks, thereby reducing processing delays and improving overall throughput.