CShard: Blockchain Sharding via Repairable Fountain Codes and the Paradigm for Sharding Design

Sharding is an important solution to improve the scalability of blockchain. The basic idea of blockchain sharding is to separate transactions among multiple disjoint shards processing in parallel to maximize system performance. The current sharding protocols mainly rely on node rotation (namely, node allocation and migration) randomly among shards periodically to ensure security, which is often considered the most challenging when developing a sharding system. However, (1) if a shard or multiple shards are corrupted, the sharding system will not be available anymore; (2) to avoid shard corruption, the demand of large size of each shard limits the throughput performance of sharding protocols. To solve (1), we introduce a blockchain sharding protocol with scale-out transaction processing capacity called CShard. The main idea of CShard is to use repairable fountain codes (RFCs), an information coding method with the locality feature, to innovate sharding design. By adjusting encoding parameters, topological associations among shards are constructed, which are then utilized to define the verification logic for transactions. The blocks of corrupted shard(s) can be recovered through decoding by its corresponding shard group(s), and the sharding system is still secure and available. Our approach utilizes encoding techniques to build a general architecture of a sharding system, establishing a paradigm of “encoding as verification” and showcases new horizons in the field of blockchain sharding. To solve (2), we propose the ghost reporter mechanism that gives all nodes chances to verify a transaction by submitting reports in the sharding network. The mechanism brings two direct benefits. Firstly, it provides the way to detect corrupted shards and recover the blocks by RFCs; the second is to make the number of nodes in a single shard smaller, which solves the limitation of existing sharding schemes that usually require a larger number of nodes in a single shard to ensure security. In principle, this mechanism can also be applicable to the known and even unknown sharding protocols for its generality.