Delta state replicated data types

Conflict-free Replicated Data Types (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. We introduce Delta State Conflict-Free Replicated Data Types (delta-CRDT) that can achieve the best of both operation-based and state-based CRDTs: 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. (C) 2017 Elsevier Inc. All rights reserved.The work presented was partially supported by EU FP7 SyncFree project (609551), EU H2020 LightKone project (732505), and SMILES line in project TEC4Growth (NORTE-01-0145-FEDER-000020)

Efficient Renaming in Sequence CRDTs

International audienceTo achieve high availability, large-scale distributed systems have to replicate data and to minimise coordination between nodes. For these purposes, literature and industry increasingly adopt Conflict-free Replicated Data Types (CRDTs) to design such systems. Conflict-free Replicated Data Types (CRDTs) are new specifications of existing data types, e.g., Set or Sequence. While CRDTs have the same behaviour as previous specifications in sequential executions, they actually shine in distributed settings as they natively support concurrent updates. To this end, CRDTs embed in their specification conflict resolution mechanisms. These mechanisms usually rely on identifiers attached to elements of the data structure to resolve conflicts in a deterministic and coordination-free manner. Identifiers have to comply with several constraints, such as being unique or belonging to a dense total order. These constraints may hinder the identifier size from being bounded. Identifiers hence tend to grow as the system progresses, which increases the overhead of CRDTs over time and leads to performance issues. To address this issue, we propose a novel Sequence CRDT which embeds a renaming mechanism. It enables nodes to reassign shorter identifiers to elements in an uncoordinated manner. Experimental results demonstrate that this mechanism decreases the overhead of the replicated data structure and eventually minimises it

CRDTs in highly volatile environments

Publisher Copyright: © 2022 The Author(s)The implementation of collaborative applications in highly volatile environments, such as the ones composed of mobile devices, requires low coordination mechanisms. The replication without coordination semantics of Conflict-Free Replicated Data Types (CRDTs) makes them a natural solution for these execution contexts. However, the current CRDT models require each replica to know all other replicas beforehand or to discover them on-the-fly. Such solutions are not compatible with the dynamic ingress and egress of nodes in volatile environments. To cope with this limitation, we propose the Publish/Subscribe Conflict-Free Replicated Data Type (PS-CRDT) model that combines CRDTs with the publish/subscribe interaction model, and, with that, enable the spatial and temporal decoupling of update propagation. We implemented PS-CRDTs in Thyme, a reactive storage system for mobile edge computing. Our experimental results show that PS-CRDTs require less communication than other CRDT-based solutions in volatile environments.publishersversionpublishe

Approaches to Conflict-free Replicated Data Types

Conflict-free Replicated Data Types (CRDTs) allow optimistic replication in a principled way. Different replicas can proceed independently, being available even under network partitions, and always converging deterministically: replicas that have received the same updates will have equivalent state, even if received in different orders. After a historical tour of the evolution from sequential data types to CRDTs, we present in detail the two main approaches to CRDTs, operation-based and state-based, including two important variations, the pure operation-based and the delta-state based. Intended as a tutorial for prospective CRDT researchers and designers, it provides solid coverage of the essential concepts, clarifying some misconceptions which frequently occur, but also presents some novel insights gained from considerable experience in designing both specific CRDTs and approaches to CRDTs.Comment: 36 page

On Conflict-Free Replicated Data Types and Equivocation in Byzantine Setups

We explore the property of equivocation tolerance for Conflict-Free Replicated Data Types (CRDTs). We show that a subclass of CRDTs is equivocation-tolerant and can thereby cope with any number of Byzantine faults: Without equivocation detection, prevention or remediation, they still fulfill strong eventual consistency (SEC). We also conjecture that there is only one operation-based CRDT design supporting non-commutative operations that fulfills SEC in Byzantine environments with any number of faults

Towards Improved Support for Conflict-Free Replicated Data Types

Conflict-free Replicated Data Types (CRDTs) are distributed data types which ensure Strong Eventual Consistency (SEL), and also has properties such as commutativity and idempotence. There are many variations of CRDTs, and the ones we will study are state-based delta CRDTs. State-based CRDTs is a variation where the CRDT instances synchronize by sending their state to each other. An improvement to this is delta CRDTs which is yet another variation where only the difference of mutations is disseminated. We will explore some of the existing CRDTs, but also present some new CRDT designs and their implementations. One of the new CRDTs presented is Causal Length Set (CLSet), which is a simple, yet effective state-based delta CRDT. It does not use dots as causal context as many other CRDTs; it merely uses a single integer. With this minimalist CRDT design, we can achieve both great performance and a small memory footprint. Another CRDT presented is Multiple Value Map (MVMap), a CRDT which focuses on convenience as opposed to performance

VeriFx: Correct Replicated Data Types for the Masses (Artifact)

Our related article presents our novel verification language, called VeriFx. We used VeriFx to implement and verify 51 Conflict-Free Replicated Data Types (CRDTs) and 9 Operational Transformation (OT) functions. This artifact bundles the implementation of the various CRDTs and OT functions described in the article. The artifact also contains a Docker file that can be used to reproduce the verification results (Table 1 and 2 in the article). In addition, the artifact can also be used to run custom VeriFx programs and verify their correctness