Geo-Replication: Fast If Possible, Consistent If Necessary.

Geo-replicated storage systems are at the core of current Internet services. Unfortunately, there exists a fundamental tension between consistency and performance for offering scalable geo-replication. Weakening consistency semantics leads to less coordination and consequently a good user experience, but it may introduce anomalies such as state divergence and invariant violation. In contrast, maintaining stronger consistency precludes anomalies but requires more coordination. This paper discusses two main contributions to address this tension. First, RedBlue Consistency enables blue operations to be fast (and weakly consistent) while the remaining red operations are strongly consistent (and slow). We identify sufficient conditions for determining when operations can be blue or must be red. Second, Explicit Consistency further increases the space of operations that can be fast by restricting the concurrent execution of only the operations that can break application-defined invariants. We further show how to allow operations to complete locally in the common case, by relying on a reservation system that moves coordination off the critical path of operation execution.