Performance and Fault Tolerance Trade-offs in Sharded Permissioned Blockchains

Blockchain has become a promising technology in distributed systems in recent years, but scalability remains a major problem. The traditional approach to scalability, namely sharding, does not solve the problem easily because the process of interleaving blocks stored in different shards to create a unified master ledger introduces overhead. This paper examines two techniques for interleaving the shards of permissioned blockchains, which we refer to as strong temporal coupling and weak temporal coupling. We implement these techniques in a prototype system with a Bitcoin-like transaction structure, using the EPaxos consensus protocol for transaction ordering. Our experimental results show that strong coupling can achieve lower latency as compared to weak coupling but same level of peak throughput. However, strong coupling requires all shards to grow at the same rate, and cannot tolerate any shard failure. In contrast, the higher latency of weak coupling is because of the consensus strategy it uses to order the blocks. However, if shard failure occurs, weak coupling can still make progress without stalling the whole system.