Efficient state-based CRDTs by decomposition

Eventual consistency is a relaxed consistency model used in large-scale distributed systems that seek better availability when consistency can be delayed. CRDTs are distributed data types that make eventual consistency of a distributed object possible and non ad-hoc. Specifically, state-based CRDTs achieve this through shipping the entire replica state that is, eventually, merged to other replicas ensuring convergence. This imposes a large communication overhead when the replica size or the number of replicas gets larger. In this work, we introduce a decomposable version of state-based CRDTs, called Delta State-based CRDTs (Δ-CRDT). A Δ-CRDT is viewed as a join of multiple fine-grained CRDTs of the same type, called deltas (Δ). The deltas are produced by applying Δ-mutators, on a replica state, which are modified versions of the original CRDT mutators. This makes it possible to ship small deltas (or batches) instead of shipping the entire state. The challenges are to make the join of deltas equivalent to the join of the entire object in classical state-based CRDTs, and to find a way to derive the Δ-mutators. We address this challenge in this work, and we explore the minimal requirements that a communication algorithm must offer according to the guarantees provided by the underlying messaging middleware.