Multi-master architecture is desirable for cloud databases in supporting large-scale transaction processing. To enable concurrent transaction execution on multiple computing nodes, we need an efficient transaction commit protocol on the storage layer that ensures ACID as well as consensus among replicas. A leader-based protocol is easy to implement. However, it faces the single-node bottleneck and suffers from high transaction latency in cross-region deployment. While a leaderless protocol can achieve a higher degree of parallelism, it is inefficient in resolving conflicts. This paper proposes the semi-leader protocol, which is a new type of transaction commit protocol for multi-master transaction processing. In a nutshell, the semi-leader protocol is a hybrid protocol that offers separate commit paths for conflicting transactions and non-conflicting transactions. A centralized node, known as the sequencer, is employed to perform precise conflict resolution for conflicting transactions, while non-conflicting transactions can be committed timely in a decentralized manner. Based on the semi-leader protocol, we designed STARRY, a multi-master transaction processing mechanism. Experimental results demonstrate that STARRY is 1.4x and 4.21x as performant as the leaderless and leader-based protocols respectively in throughput. When dealing with high-contention workloads, STARRY can significantly reduce the abort rates.
