Abstract-In this paper, we analyze and experimentally compare state-machine-based and deferred-update (or transactional) replication, both relying on atomic broadcast. We define a model that describes the upper and lower bounds on the execution of concurrent requests by a service replicated using either scheme. The model is parametrized by the degree of parallelism in either scheme, the number of processor cores, and the type of requests. We analytically compared both schemes and a nonreplicated service, considering abcast-and request-executiondominant workloads. To evaluate transactional replication experimentally, we developed Paxos STM-a novel fault-tolerant distributed software transactional memory with programming constructs for transaction creation, abort, and retry. For statemachine-based replication, we used JPaxos. Both systems share the same implementation of atomic broadcast based on the Paxos algorithm. We present the results of performance evaluation of both replication schemes, and a non-replicated (thus prone to failures) service, considering various workloads. The key result of our theoretical and experimental work is that neither system is superior in all cases. We discuss these results in the paper.

Abstract-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.

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In this paper we study the properties of eventually consistent distributed systems that feature arbitrarily complex semantics and mix eventual and strong consistency. These systems execute requests in a highly-available, weakly-consistent fashion, but also enable stronger guarantees through additional inter-replica synchronization mechanisms that require the ability to solve distributed consensus. We use the seminal Bayou system as a case study, and then generalize our findings to a whole class of systems. We show dubious and unintuitive behaviour exhibited by those systems and provide a theoretical framework for reasoning about their correctness. We also state an impossibility result that formally proves the inherent limitation of such systems, namely temporary operation reordering, which admits interim disagreement between replicas on the relative order in which the client requests were executed.

In this paper we propose executive deferred update replication (EDUR), a novel algorithm for multi-primary replication of transactional memory and databases. EDUR streamlines transaction certification (i.e., checking for conflicts between concurrent transactions) with the broadcast protocol, which improves overall performance and scalability compared to deferred update replication based on total order broadcast (TOB). EDUR uses executive order broadcast (EOB), a novel protocol that can be seen as a generalization of TOB. Compared to TOB, EOB features new primitives and properties that enable the application to delegate some work to a leader-a process inherently present in many TOB algorithms that is responsible for coordination of message dissemination. The results of experimental evaluation show significant performance gains when using our approach.

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Abstract-We propose Hybrid Transactional Replication (HTR) a novel replication scheme for 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 formally prove that the higher flexibility of the scheme does not come at the cost of weaker guarantees for clients: HTR satisfies strong consistency guarantees akin to those provided by other popular transactional replication schemes such as Deferred Update Replication. We developed HTR-enabled Paxos STM, an object-based distributed transactional memory system, and evaluated it thoroughly under various workloads. We show the benefits of using a novel oracle, which relies on machine learning techniques for automatic adaptation to changing conditions. In our tests, the ML-based oracle provides up to 50% improvement in throughput when compared to the system running with DU-only or SM-only oracles. Our approach is inspired by a well known algorithm used in the context of the multi-armed bandit problem.