Abstract-Geo-replicated databases often operate under the principle of eventual consistency to offer high-availability with low latency on a simple key/value store abstraction. Recently, some have adopted commutative data types to provide seamless reconciliation for special purpose data types, such as counters. Despite this, the inability to enforce numeric invariants across all replicas still remains a key shortcoming of relying on the limited guarantees of eventual consistency storage.We present a new replicated data type, called bounded counter, which adds support for numeric invariants to eventually consistent geo-replicated databases. We describe how this can be implemented on top of existing cloud stores without modifying them, using Riak as an example. Our approach adapts ideas from escrow transactions to devise a solution that is decentralized, fault-tolerant and fast. Our evaluation shows much lower latency and better scalability than the traditional approach of using strong consistency to enforce numeric invariants, thus alleviating the tension between consistency and availability.
The purpose of this paper is to discuss the limitations imposed by introducing fault-tolerance in a partial replication system which aims to provide causal consistency. The general outcome is that, to provide support for indefinite replica-failure, the notion of partial in partial replication becomes not-really-partial-at-all. We prove that to implement causal consistency when indefinite replica-failures are possible, it is impossible to respect the concept of genuine partial replication-not storing or propagating operations which are on objects a given replica does not replicate locally. In our initial approach to tackle this issue client replicas need only to replicate the operations they depend on which have not yet been marked as durable by the centralised component. We discuss remaining limitations and expected improvements in future work. CCS Concepts: • Computer systems organization → Peerto-peer architectures; Distributed architectures; Availability.
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