Hybrid Replication: State-Machine-Based and Deferred-Update Replication Schemes Combined

We propose a novel algorithm for hybrid transactional replication (HTR) of highly dependable services. It combines two schemes: a transaction is executed either optimistically by only one service replica in the deferred update mode (DU), or deterministically by all replicas in the state machine mode (SM); the choice is made by an oracle. The DU mode allows for parallelism and thus takes advantage of multicore hardware. In contrast to DU, the SM mode guarantees abort-free execution, so it is suitable for irrevocable operations and transactions generating high contention. For expressiveness, transactions can be discarded or retried on demand. We developed HTR-enabled Paxos STM, an object-based distributed transactional memory system, and evaluated it using several benchmarks: Bank, Distributed STMBench7, and Twitter Clone. We tested our system under various workloads and three oracle types: DU and SM, which execute all transactions in one mode, and Hybrid -- tailored specifically for each benchmark -- which selects a mode for each transaction dynamically based on various parameters. In all our tests, the Hybrid oracle is not worse than DU and SM and outperforms them when the number of replicas grows.