Array CRDTs Using Delta-Mutations

ABSTRACTδ-CRDTs can achieve the best of both worlds compared to operation and state based alternatives, enabling small incremental messages disseminated over unreliable communication channels. The main composable causal δ-CRDT defined to date is an Observed-Remove Map which can embed other causal δ-CRDTs. However, maps on their own are not sufficient to handle most real world scenarios. In this paper we construct nested δ-based Array CRDTs with support for move operation. Arrays and maps can then be combined to represent many useful data types, notably JSON documents, which require (1) maps, (2) arrays, and (3) atoms, and allow arbitrary nesting. To achieve this we define two new algorithms for δ-based array CRDTs, one with remove-wins semantics and the other with observed-remove semantics. While previous solutions exist for operations based CRDTs, we provide a δ-CRDT based solution.