Blockchain networks offer decentralization, transparency, and immutability for managing critical data but encounter scalability problems as the number of network members and transaction issuers grows. Sharding is considered a promising solution to enhance blockchain scalability. However, most existing blockchain sharding techniques prioritize performance at the cost of availability (e.g., a failure in a few servers holding a shard leads to data unavailability). In this paper, we propose PyloChain, a hierarchical sharded blockchain that balances availability and performance. PyloChain consists of multiple lower-level local chains and one higher-level main chain. Each local chain speculatively executes local transactions to achieve high parallelism across multiple local chains. The main chain leverages a directed-acyclic-graph (DAG)-based mempool to guarantee local block availability and to enable efficient Byzantine Fault Tolerance (BFT) consensus to execute global (or cross-shard) transactions within a collocated sharding. PyloChain speculatively executes local transactions across multiple local chains to achieve high parallelism. In order to reduce the number of aborted local transactions, PyloChain applies a simple scheduling technique to handle global transactions in the main chain. PyloChain provides a fine-grained auditing mechanism to mitigate faulty higher-level members by externalizing main chain operations to lower-level local members. We implemented and evaluated PyloChain, demonstrating its performance scalability with 1.49x higher throughput and 2.63x faster latency compared to the state-of-the-art balanced hierarchical sharded blockchain.

翻译：区块链网络为管理关键数据提供了去中心化、透明性和不可篡改性，但随着网络成员和交易发起者数量的增长，其可扩展性问题日益凸显。分片技术被认为是提升区块链可扩展性的一种有前景的解决方案。然而，现有的大多数区块链分片技术往往以牺牲可用性为代价来优先考虑性能（例如，持有分片的少数服务器发生故障会导致数据不可用）。本文提出了PyloChain，一种平衡可用性与性能的分层分片区块链。PyloChain由多个较低层级的本地链和一个较高层级的主链组成。每条本地链通过推测性执行本地交易，实现在多个本地链间的高度并行化。主链利用基于有向无环图（DAG）的内存池来保证本地区块的可用性，并支持高效的拜占庭容错（BFT）共识，以在共址分片架构内执行全局（或跨分片）交易。PyloChain通过在多个本地链上推测性执行本地交易来实现高并行度。为了减少被中止的本地交易数量，PyloChain在主链中采用了一种简单的调度技术来处理全局交易。PyloChain还提供了一种细粒度的审计机制，通过将主链操作外部化到较低层级的本地成员来减轻高层级成员的故障影响。我们实现并评估了PyloChain，结果表明其性能具有良好的可扩展性，与当前最先进的平衡分层分片区块链相比，吞吐量提高了1.49倍，延迟降低了2.63倍。