Harmony is a fast and secure blockchain with key innovations in state sharding and peer-to-peer networking.
Since the publication of the Bitcoin whitepaper in 2008, the concept of blockchain has spread across the world. While decentralized money and applications are becoming well-publicized ideas, design limitations have challenged the core aspiration of Bitcoin. The original Bitcoin blockchain was designed as a peer-to-peer payment system  that allows people to transfer value without intermediaries like banks or payment processors. However, as Bitcoin gained popularity, its performance bottleneck became evident due to its limited throughput of ~7 transactions per second (TPS), and its cost as a payment system became prohibitively expensive. In 2014, Buterin et. al.  proposed a new blockchain infrastructure called Ethereum, which enabled developers to create various kinds of blockchain applications using “smart contracts.” However, Ethereum didn’t solve the scalability problem and, with its ~15 TPS, failed to support high-throughput applications such as gaming or decentralized exchanges. Given Ethereum and Bitcoin’s performance limitations, many blockchain projects proposed various solutions [3,4,5,6,7,8,9,10,24,25] that attempt to increase transaction throughput. Various blockchains [3,4,5,6,24,25] proposed to replace Proof-of-Work (PoW) consensus with Proof-of-Stake (PoS) consensus. Other blockchains like EOS use Delegated Proof of Stake (DPoS), where block proposers are elected by voting rather than by an on-chain algorithmic process. Projects like IOTA replaced the chain-of-blocks data structure with a DAG (Directed Acyclic Graph) data structure, which breaks the limitation of sequential processing of transactions. However, these proposed solutions cannot make significant performance gains without sacrificing other critical aspects, such as security and decentralization. The scalability solution that both preserves security and decentralization is sharding, which creates multiple groups (i.e. shards) of validators and lets them process transactions concurrently. As a result, the total transaction throughput increases linearly as the number of shards grows. Zilliqa  was the first public blockchain that proposed to address the scalability problem with sharding. However, Zilliqa's sharding approach falls short in two ways. First, it does not divide the storage of blockchain data (state sharding). This prevents machines with limited resources from participating in the network, thus curtailing decentralization. Second, Zilliqa’s sharding process is susceptible to a single-shard takeover attack due to its reliance on PoW as its randomness generation mechanism. We introduce Harmony, the next generation sharding-based blockchain that is fully scalable, provably secure, and energy efficient. Harmony addresses the problems of existing blockchains by combining the best research results and engineering practice in an optimally tuned system.