Delta State Replicated Data Types

TL;DR

This paper introduces Delta State CRDTs ($oldsymbol{ extdelta}$-CRDTs), a new class of replicated data types that combine the small message size of operation-based CRDTs with the robustness of state-based CRDTs over unreliable networks, ensuring efficient and reliable eventual consistency.

Contribution

The paper defines the $oldsymbol{ extdelta}$-CRDT framework, establishes its relation to existing CRDTs, and provides algorithms and specifications for various data types, including novel ones.

Findings

Delta-CRDTs achieve small message sizes similar to operation-based CRDTs.

They ensure eventual convergence and causal consistency over unreliable channels.

Several $ extdelta$-CRDT specifications, including a generic map, are introduced.

Abstract

CRDTs are distributed data types that make eventual consistency of a distributed object possible and non ad-hoc. Specifically, state-based CRDTs ensure convergence through disseminating the entire state, that may be large, and merging it to other replicas; whereas operation-based CRDTs disseminate operations (i.e., small states) assuming an exactly-once reliable dissemination layer. We introduce Delta State Conflict-Free Replicated Data Types ($\delta$-CRDTs) that can achieve the best of both worlds: small messages with an incremental nature, as in operation-based CRDTs, disseminated over unreliable communication channels, as in traditional state-based CRDTs. This is achieved by defining delta mutators to return a delta-state, typically with a much smaller size than the full state, that to be joined with both local and remote states. We introduce the $\delta$-CRDT framework, and we explain it through establishing a correspondence to current state-based CRDTs. In addition, we present an anti-entropy algorithm for eventual convergence, and another one that ensures causal consistency. Finally, we introduce several $\delta$-CRDT specifications of both well-known replicated datatypes and novel datatypes, including a generic map composition.