[hal-01109121] Putting the Consistency back into Eventual Consistency

Geo-replicated storage systems are at the core of current Internet services. The designers of the replication protocols for these systems have to choose between either supporting low latency, eventually consistent operations, or supporting strong consistency for ensuring application correctness. We propose an alternative consistency model, explicit consistency, that strengthens eventual consistency with a guarantee to preserve specific invariants defined by the applications. Given these application-specific invariants, a system that supports explicit consistency must identify which oper- ations are unsafe under concurrent execution, and help programmers to select either violation-avoidance or invariant- repair techniques. We show how to achieve the former while allowing most of operations to complete locally, by relying on a reservation system that moves replica coordination off the critical path of operation execution. The latter, in turn, allow operations to execute without restriction, and restore invariants by applying a repair operation to the database state. We present the design and evaluation of Indigo, a middleware that provides Explicit Consistency on top of a causally- consistent data store. Indigo guarantees strong application invariants while providing latency similar to an eventually consistent system.



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