  Blockchain sharding improves the scalability of blockchain systems by
partitioning the whole blockchain state, nodes, and transaction workloads into
different shards. However, existing blockchain sharding systems generally
suffer from a small number of shards, resulting in limited concurrency. The
main reason is that existing sharding systems require large shard sizes to
ensure security. To enhance the concurrency of blockchain sharding securely, we
propose SpiralShard. The intuition is to allow the existence of some shards
with a larger fraction of malicious nodes (i.e., corrupted shards), thus
reducing shard sizes. SpiralShard can configure more and smaller shards for
higher concurrency at the same network size. To ensure security with the
existence of corrupted shards, we propose the Linked Cross-shard Endorsement
(LCE) protocol. According to our LCE protocol, the blocks of each shard are
sequentially verified and endorsed by a group of shards before being finalized.
As a result, a corrupted shard can eliminate forks with the help of the other
shards. We implement SpiralShard based on Harmony and conduct extensive
evaluations. Experimental results show that, compared with Harmony, SpiralShard
achieves around 19x throughput gain under a large network size with 4,000+
nodes.
