Putting the Consistency back into Eventual Consistency
Apr 2015
Geo-replicated storage systems are at the core of current Internet
services. The designers of the replication protocols
used by these systems must choose between either supporting
low-latency, eventually-consistent operations, or ensuring
strong consistency to ease 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 identifies which operations
would be unsafe under concurrent execution, and allows
programmers to select either violation-avoidance or
invariant-repair techniques. We show how to achieve the former,
while allowing operations to complete locally in the
common case, by relying on a reservation system that moves
coordination off the critical path of operation execution. The
latter, in turn, allows 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 similar latency
to an eventually-consistent system in the common case.
