OptiShard: An Optimized and Secured Hierarchical Blockchain Architecture

Abstract

Blockchain has become an emerging decentralized computing technology for transaction-based systems due to its peer-to-peer consensus protocol over an open network consisting of untrusted parties. Bitcoin and other major alt-coins based on monolithic blockchain architecture exhibit significant performance overhead which in turn make them highly non-scalable. Imposing hierarchy in blockchain can improve performance, however it adds on additional security and fault-tolerance measures necessary for correctness of transaction validation. This paper presents a hierarchical blockchain architecture named OptiShard, which addresses the issues of performance, fault-tolerance, and security in the presence of faulty and malicious nodes. The hierarchy comes as a result of dividing the network nodes into multiple disjoint shards. Majority of transactions are distributed among these shards in non-overlapped fashion (i.e., one-to-one mapping of transactions to shards). The model of OptiShard provides a theoretical measure to determine optimal shard size based on two parameters: performance and correctness of transaction validation in the presence of malicious or faulty nodes. The theoretical measure provides guaranteed majority of good shards by allowing to choose the right shard size and forms the basis of network and workload sharding protocols discussed in this paper. OptiShard also provides a mechanism for identifying faulty shards through the overlapping of a small fraction of transactions across all the shards so that all faulty transactions can be discarded. Experimental results exhibit the impact of sharding the network on performance and conform to the theoretical results.