Transparent speculation in geo-replicated transactional data stores

摘要

This work presents Speculative Transaction Replication (STR), a protocol that exploits transparent speculation techniques to enhance performance of geo-distributed, partially replicated transactional data stores. In addition, we define a new consistency model, Speculative Snapshot Isolation (SPSI), a variant of Snapshot Isolation (SI), which shelters applications from the subtle anomalies that arise when using speculative transaction processing techniques. STR provides a form of speculation that is fully transparent for programmers (it does not expose the effects of misspeculations to clients). Since the speculation techniques employed by STR satisfy SPSI, they can be leveraged by application programs in a transparent way, without requiring any source-code modification to applications designed to operate using SI. STR combines two key techniques: speculative reads, which allow transactions to observe pre-committed versions, which can reduce the 'effective duration' of pre-commit locks and enhance throughput; Precise Clocks, a novel timestamping mechanism that uses per-item timestamps with physical clocks, which together greatly enhance the probability of successful speculation. We assess STR's performance on up to nine geo-distributed Amazon EC2 data centers, using both synthetic benchmarks as well as realistic benchmarks (TPC-C and RUBiS). Our evaluation shows that STR achieves throughput gains up to 11 × and latency reduction up to 10× (with respect to non-speculative systems that ensure SI) in workloads characterized by low inter-data center contention. Furthermore, thanks to a self-tuning mechanism that dynamically and transparently enables and disables speculation, STR offers robust performance even when faced with unfavorable workloads that suffer from high misspeculation rates.