Sharding is a popular way to achieve scalability in blockchain protocols, increasing their throughput by partitioning the set of transaction validators into a number of smaller committees, splitting the workload. Existing approaches for blockchain sharding, however, do not scale well when concurrent transactions alter the same replicated state component—a common scenario in Ethereum-style smart contracts.

We propose a novel approach for efficiently sharding such transactions. It is based on a folklore idea: state-manipulating atomic operations that commute can be processed in parallel, with their cumulative result defined deterministically, while executing non-commuting operations requires one to own the state they alter. We present CoSplit—a static program analysis tool that soundly infers ownership and commutativity summaries for smart contracts and translates those summaries to sharding signatures that are used by the blockchain protocol to maximise parallelism. Our evaluation shows that using CoSplit introduces negligible overhead to the transaction validation cost, while the inferred signatures allow the system to achieve a significant increase in transaction processing throughput for real-world smart contracts.