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Section: New Results

CRDTs, a principled approach to eventual consistency

Participants : Marc Shapiro [correspondent] , Marek Zawirski.

Most well-studied approaches to replica consistency maintain a global total order of operations. This serialisation constitutes a performance and scalability bottleneck, while the CAP theorem imposes a trade-off between consistency and partition-tolerance. An alternative approach, eventual consistency or optimistic replication, is attractive. A replica may execute an operation without synchronising a priori with other replicas. The operation is sent asynchronously to other replicas; every replica eventually applies all updates, but possibly in different orders. This approach ensures that data remains available despite network partitions, and is perceived to scale well and to provide acceptable quality of service. The consensus bottleneck remains but is off the critical path. However, reconciliation is generally complex. There is little theoretical guidance on how to design a correct optimistic system, and ad-hoc approaches have proven brittle and error-prone. We propose a simple, theoretically sound approach to eventual consistency, the concept of a convergent or commutative replicated data type (CRDT), for which some simple mathematical properties ensure eventual consistency. Provably, any CRDT converges to a common state that is equivalent to some sequential execution. A CRDT requires no synchronisation, thus every update can execute immediately, unaffected by network latency, faults, or disconnection. It is extremely scalable and is fault-tolerant, and does not require much mechanism. Previously, only a handful of CRDTs were known. Our current research aims to push the CRDT envelope, to study the principles of CRDTs, and to design a library of useful CRDTs. So far we have designed variations on registers, counters, sets, maps (key-value stores), graphs, and sequences. Potential application areas include computation in delay-tolerant networks, latency tolerance in wide-area networks, disconnected operation, churn-tolerant peer-to-peer computing, and partition-tolerant cloud computing. CRDTs are the main topic of ANR project ConcoRDanT (Section  8.1.3 ). This research is also funded in part by a Google European Doctoral Fellowship.