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Distributed data management systems employ data sharding techniques to achieve scalability. Traditional sharding approaches typically operate under the assumption of a trusted environment, where nodes may crash,but do not act adversarially. In untrustworthy environments, however, this assumption is no longer valid. This paper presents Marlin,an adaptive scalable data management system specifically designed for untrustworthy environments. Marlinleverages data sharding to enhance scalability while dynamically redistributing data across clusters to adapt to dynamic workloads. We propose two architectures: a centralized architecture serving as a baseline, which employs hypergraph partitioning within a trusted administrative domain, and a decentralized architecture that eliminates the need for such a trusted domain by managing shards across nodes in a decentralized manner. Both architectures utilize real-time monitoring and adaptive algorithms to dynamically adjust sharding in response to workload characteristics and adversarial conditions. Experimental results show that Marlinmaintain consistent performance under diverse dynamic scenarios in untrustworthy environments by continuously optimizing shard distributions. 

This paper articulates our vision for a learning-based untrustworthy distributed database. We focus on permissioned blockchain systems as an emerging instance of untrustworthy distributed databases and argue that as novel smart contracts, modern hardware, and new cloud platforms arise, future-proof permissioned blockchain systems need to be designed withfull-stack adaptivityin mind. At the application level, a future-proof system must adaptively learn the best-performing transaction processing paradigm and quickly adapt to new hardware and unanticipated workload changes on the fly. Likewise, the Byzantine consensus layer must dynamically adjust itself to the workloads, faulty conditions, and network configuration while maintaining compatibility with the transaction processing paradigm. At the infrastructure level, cloud providers must enable cross-layer adaptation, which identifies performance bottlenecks and possible attacks, and determines at runtime the degree of resource disaggregation that best meets application requirements. Within this vision of the future, our paper outlines several research challenges together with some preliminary approaches. 

This paper presents AdaChain , a learning-based blockchain framework that adaptively chooses the best permissioned blockchain architecture to optimize effective throughput for dynamic transaction workloads. AdaChain addresses the challenge in Blockchain-as-a-Service (BaaS) environments, where a large variety of possible smart contracts are deployed with different workload characteristics. AdaChain supports automatically adapting to an underlying, dynamically changing workload through the use of reinforcement learning. When a promising architecture is identified, AdaChain switches from the current architecture to the promising one at runtime in a secure and correct manner. Experimentally, we show that AdaChain can converge quickly to optimal architectures under changing workloads and significantly outperform fixed architectures in terms of the number of successfully committed transactions, all while incurring low additional overhead.