Hybris: Consistency hardening in robust hybrid cloud storage

We present Hybris key-value store, the first robust hybrid cloud storage system. Hybris robustly replicates metadata on trusted private premises (private cloud), separately from data which is replicated across multiple untrusted public clouds. Hybris introduces a technique we call consistency hardening which consists in leveraging strong metadata consistency to guarantee to Hybris applications strong data consistency (linearizability) without entailing any modifications to weakly (e.g., eventually) consistent public clouds, which actually store data. Moreover, Hybris efficiently and robustly tolerates up to f potentially malicious clouds. Namely, in the common case, Hybris writes replicate data across f + 1 clouds, whereas reads involve a single cloud. In the worst case, f additional clouds are used. We evaluate Hybris using a series of micro and macrobenchmarks and show that Hybris significantly outperforms comparable multi-cloud storage systems and approaches the performance of bare-bone commodity public cloud storage.