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The distributed ledger or blockchain technologies originated from the Bitcoin have been rapidly widespread in recent years. However, it also gives incentive to malicious users who would like to break the system or take advantage of it (steal money, hide some information stored in the ledger, isolate a particular node from the rest of the network, and so forth). Thus, research focusing on overcoming potential attacks to distributed ledgers is required. In this article, we focus on attacks that damage underlying networks of distributed ledgers. Underlying networks offer useful communication primitives such as an atomic broadcast, however, such attacks may degrade the property of the primitives and make distributed ledgers relying on the primitives no longer work. Hence we should design algorithms to make the distributed ledgers still work even when some attacks degrade the primitives. As the first study for such situations, we consider a problem to implement distributed ledgers tolerating the degradation of an underlying atomic broadcast service that distributed ledgers are relying on. We consider the case where the uniform agreement property of the atomic broadcast is degraded, and propose new algorithms that could ensure to reach eventual consistency despite degraded atomic broadcast. [ABSTRACT FROM AUTHOR]

Eventual consistency is a consistency model that favors liveness over safety. It is often used in large-scale distributed systems where models ensuring a stronger safety incur performance that are too low to be deemed practical. Eventual consistency tends to be uniformly applied within a system, but we argue a demand exists for differentiated eventual consistency, e.g. in blockchain systems. We propose update-query consistency with primaries and secondaries (UPS) to address this demand. UPS is a novel consistency mechanism that works in pair with our novel two-phase epidemic broadcast protocol gossip primary-secondary (GPS) to offer differentiated eventual consistency and delivery speed. We propose two complementary analyses of the broadcast protocol: a continuous analysis and a discrete analysis based on compartmental models used in epidemiology. Additionally, we propose the formal definition of a scalable consistency metric to measure the consistency trade-off at runtime. We evaluate UPS in two simulated worldwide settings: a one-million-node network and a network emulating that of the Ethereum blockchain. In both settings, UPS reduces inconsistencies experienced by a majority of the nodes and reduces the average message latency for the remaining nodes. [ABSTRACT FROM AUTHOR]

Commutative Replicated Data Types (CRDTs) are a promising new class of data structures for large-scale shared mutable content in applications that only require eventual consistency. The WithOut Operational Transforms (WOOT) framework is the first CRDT for collaborative text editing introduced by Oster et al. (In: Conference on Computer Supported Cooperative Work (CSCW). ACM, New York, pp 259–268, 2006a). Its eventual consistency property was verified only for a bounded model to date. While the consistency of many other previously published CRDTs had been shown immediately with their publication, the property for WOOT remained open for 14 years. We use a novel approach identifying a previously unknown sort-key based protocol that simulates the WOOT framework to show its consistency. We formalize the proof using the Isabelle/HOL proof assistant to machine-check its correctness. [ABSTRACT FROM AUTHOR]

Matrix is a new kind of decentralized, topic-based publish-subscribe middleware for communication and data storage that is getting particularly popular as a basis for secure instant messaging. By comparison with traditional decentralized communication systems, Matrix replaces pure message passing with a replicated data structure. This data structure, which we extract and call the Matrix Event Graph (MEG), depicts the causal history of messages. We show that this MEG represents an interesting and important replicated data type for decentralized applications that are based on causal histories of publish-subscribe events: First, we prove that the MEG is a Conflict-Free Replicated Data Type for causal histories and, thus, provides Strong Eventual Consistency (SEC). With SEC being among the best known achievable trade-offs in the scope of the well-known CAP theorem, the MEG provides a powerful consistency guarantee while being available during network partition. Second, we discuss the implications of byzantine attackers on the data type's properties. We note that the MEG, as it does not strive for consensus or strong consistency, can cope with n > f environments with n participants, of which f are byzantine. Furthermore, we analyze scalability: Using Markov chains, we study the number of forward extremities of the MEG over time and observe an almost optimal evolution. We conjecture that this property is inherent to the underlying spatially inhomogeneous random walk. With the properties shown, a MEG represents a promising element in the set of data structures for decentralized applications, but with distinct trade-offs compared to traditional blockchains and distributed ledger technologies.

Current blockchain technologies provide very limited interoperability. Restrictions with regard to asset transfers and data exchange between different blockchains reduce the usability and comfort of users, and hinder novel developments within the blockchain space. As a first step towards blockchain interoperability, we propose the DeXTT cross-blockchain transfer protocol, which can be used to record a token transfer on any number of blockchains simultaneously in a decentralized manner. We provide a reference implementation using Solidity, and evaluate its performance. We show logarithmic scalability of DeXTT with respect to the number of participating nodes, and analyze cost requirements of the transferred tokens.

Collaborative text editing systems allow users to concurrently edit a shared document, inserting and deleting elements (e.g., characters or lines). There are a number of protocols for collaborative text editing, but so far there has been no abstract, high-level specification of their desired behavior, which is decoupled from their actual implementation. Several of these protocols have been shown not to satisfy even basic expectations. This paper provides a precise specification of a replicated abstract list object, which models the core functionality of replicated systems for collaborative text editing. We define a strong list specification, which we prove is implemented by an existing protocol, as well as a weak list specification, which admits additional protocol behaviors. A major factor determining the efficiency and practical feasibility of a collaborative text editing protocol is the space overhead of the metadata that the protocol must maintain to ensure correctness. We show that for a large class of list protocols, implementing either the strong or the weak list specification requires a metadata overhead that is at least linear in the number of elements deleted from the list. The class of protocols to which this lower bound applies includes all list protocols that we are aware of, in particular CRDT and OT protocols, and we show that one of these protocols almost matches the bound. The result holds for peer-to-peer protocols, even if the network guarantees causal atomic broadcast. The result also holds for the metadata cost at the clients in client/server protocols. 1 [ABSTRACT FROM AUTHOR]

In its classical form, a consistent replicated service requires all replicas to witness the same evolution of the service state. If we consider an asynchronous message-passing environment in which processes might fail by crashing, and assume that a majority of processes are correct, then the necessary and sufficient information about failures for implementing a general state machine replication scheme ensuring consistency is captured by the Ω failure detector. This paper shows that in such a message-passing environment, Ω is also the weakest failure detector to implement an eventually consistent replicated service, where replicas are expected to agree on the evolution of the service state only after some (a priori unknown) time. In fact, we show that Ω is the weakest to implement eventual consistency in any message-passing environment, i.e., under any assumption on when and where failures might occur. Ensuring (strong) consistency in any environment requires, in addition to Ω , the quorum failure detector Σ . Our paper thus captures, for the first time, an exact computational difference between building a replicated state machine that ensures consistency and one that only ensures eventual consistency. [ABSTRACT FROM AUTHOR]

Eventual Consistency (EC) model is adopted by numerous large-scale distributed systems. To ensure performance and scalability, this model allows any replica to accept updates without remote synchronization. Nowadays, many EC algorithms are developed to control the behavior of the replicated data in the face of concurrent updates. Among them, those using a central server to order the updates, while others support the decentralization. In this paper, we focus on decentralized EC algorithms. Suitability of such algorithms under users and devices constraints such as execution time, memory requirements, messages size and quality of the result remains to be investigated under different conditions. Evaluate such algorithms in different context and under different parameters require a framework. In this paper, we propose a generic framework designed to evaluate diferent decentralized EC algorithms, in diferent context by controlling diferent parameters. Our framework provides a generic simulator that generates a runnable data following diferent parameters.

Counters are an important abstraction in distributed computing, and play a central role in large scale geo-replicated systems, counting events such as web page impressions or social network "likes". Classic distributed counters, strongly consistent via linearisability or sequential consistency, cannot be made both available and partition-tolerant, due to the CAP Theorem, being unsuitable to large scale scenarios. This paper defines Eventually Consistent Distributed Counters (ECDCs) and presents an implementation of the concept, Handoff Counters, that is scalable and works over unreliable networks. By giving up the total operation ordering in classic distributed counters, ECDC implementations can be made AP in the CAP design space, while retaining the essence of counting. Handoff Counters are the first Conflict-free Replicated Data Type (CRDT) based mechanism that overcomes the identity explosion problem in naive CRDTs, such as G-Counters (where state size is linear in the number of independent actors that ever incremented the counter), by managing identities towards avoiding global propagation and garbage collecting temporary entries. The approach used in Handoff Counters is not restricted to counters, being more generally applicable to other data types with associative and commutative operations. [ABSTRACT FROM AUTHOR]

In this paper we formally study the guarantees provided by highly available, eventually consistent replicated systems in an environment in which machine failures and network splits are possible. Our analysis accounts for possible replica recovery after a crash, and clients that are (1) stateless or stateful, (2) sticky (always connect to a concrete set of replicas) or mobile, and (3) which can timeout before receiving a response to the sent request. We show why the approaches to prove protocol correctness prevalent in the literature, which do not take into account replica or network crashes, may lead to incorrect conclusions regarding the guarantees offered by the protocol. We adapt the existing formal correctness criteria, such as basic eventual consistency , to the considered environment by defining the family of failure-aware consistency guarantees. We formally identify a set of undesired phenomena (in particular phantom operations) observed by the clients, which, as we prove, are unavoidable in highly available systems in which unrecoverable replica crashes are possible. We also introduce context preservation , a new client-side requirement for eventually consistent systems that expose concurrency to the client, i.e., allow clients to use, e.g., multi-value registers or observed-remove sets. Context preservation is incomparable with classic session guarantees. • A formal framework that explicitly considers hardware failures is provided. • Highly available systems cannot achieve liveness guarantees when failures occur. • Undesired phenomena observable by the clients are unavoidable. • New correctness criteria relaxing liveness requirements are proposed. • Novel session guarantees compatible with high availability are proposed. [ABSTRACT FROM AUTHOR]

Eventual consistency is demanded nowadays in geo-replicated services that need to be highly scalable and available. According to the CAP constraints, when network partitions may arise, a distributed service should choose between be- ing strongly consistent or being highly available. Since scalable services should be available, a relaxed consistency (while the network is partitioned) is the preferred choice. Eventual consistency is not a common data-centric consistency model, but only a state convergence condition to be added to a relaxed consistency model. There are still several aspects of eventual consistency that have not been analysed in depth in previous works: 1. which are the oldest replication proposals providing eventual consistency, 2. which replica consistency models provide the best basis for building eventually consistent services, 3. which mechanisms should be considered for implementing an eventually consistent service, and 4. which are the best com- binations of those mechanisms for achieving different concrete goals. This paper provides some notes on these important topics. [ABSTRACT FROM AUTHOR]

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 ( δ -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 δ - 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 δ -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 δ -CRDT specifications of both well-known replicated datatypes and novel datatypes, including a generic map composition. [ABSTRACT FROM AUTHOR]