OmniLedger: A Secure, Scale-Out, Decentralized Ledger via Sharding

Designing a secure permissionless distributed ledger (blockchain) that performs on par with centralized payment processors, such as Visa, is a challenging task. Most existing distributed ledgers are unable to scale-out, i.e., to grow their total processing capacity with the number of validators; and those that do, compromise security or decentralization. We present OmniLedger, a novel scale-out distributed ledger that preserves longterm security under permissionless operation. It ensures security and correctness by using a bias-resistant public-randomness protocol for choosing large, statistically representative shards that process transactions, and by introducing an efficient crossshard commit protocol that atomically handles transactions affecting multiple shards. OmniLedger also optimizes performance via parallel intra-shard transaction processing, ledger pruning via collectively-signed state blocks, and low-latency “trust-butverify” validation for low-value transactions. An evaluation of our experimental prototype shows that OmniLedger’s throughput scales linearly in the number of active validators, supporting Visa-level workloads and beyond, while confirming typical transactions in under two seconds

Increasing the Resilience of Atomic Commit, at No Additional Cost

This paper presents a new atomic commitment protocol, Enhanced Three Phase Commit (E3PC ), that always allows a quorum in the system to make progress. Previously suggested quorum-based protocols (e.g. the quorum-based Three Phase Commit (3PC) [Ske82]) allow a quorum to make progress in case of one failure. If failures cascade, however, and the quorum in the system is "lost" (i.e. at a given time no quorum component exists, e.g. because of a total crash), a quorum can later become connected and still remain blocked. With our protocol, a connected quorum never blocks. E3PC is based on the quorumbased 3PC [Ske82], and it does not require more time or communication than 3PC. The principles demonstrated in this paper can be used to increase the resilience of a variety of distributed services, e.g. replicated database systems, by ensuring that a quorum will always be able to make progress. 1 Introduction Reliability and availability of loosely coupled distributed database systems is beco..

How Fast can a Distributed Transaction Commit?

The atomic commit problem lies at the heart of distributed database systems. The problem consists for a set of processes (database nodes) to agree on whether to commit or abort a transaction (agreement property). The commit decision can only be taken if all processes are initially willing to commit the transaction, and this decision must be taken if all processes are willing to commit and there is no failure (validity property). An atomic commit protocol is said to be non-blocking if every correct process (a database node that does not fail) eventually reaches a decision (commit or abort) even if there are failures elsewhere in the distributed database system (termination property). Surprisingly, despite the importance of the atomic commit problem, little is known about its complexity. In this paper, we present, for the first time, a systematic study on the time and message complexity of the problem. We measure complexity in the executions that are considered the most frequent in practice, i.e., failure-free, with all processes willing to commit. In other words, we measure how fast a transaction can commit. Through our systematic study, we close many open questions like the complexity of synchronous non-blocking atomic commit. We also present optimal protocols which may be of independent interest. In particular, we present an effective protocol which solves what we call indulgent atomic commit that tolerates practical distributed database systems which are synchronous ``most of the time''

The Complexity of Reliable and Secure Distributed Transactions

The use of transactions in distributed systems dates back to the 70's. The last decade has also seen the proliferation of transactional systems. In the existing transactional systems, many protocols employ a centralized approach in executing a distributed transaction where one single process coordinates the participants of a transaction. The centralized approach is usually straightforward and efficient in the failure-free setting, yet the coordinator then turns to be a single point of failure, undermining reliability/security in the failure-prone setting, or even be a performance bottleneck in practice. In this dissertation, we explore the complexity of decentralized solutions for reliable and secure distributed transactions, which do not use a distinguished coordinator or use the coordinator as little as possible. We show that for some problems in reliable distributed transactions, there are decentralized solutions that perform as efficiently as the classical centralized one, while for some others, we determine the complexity limitations by proving lower and upper bounds to have a better understanding of the state-of-the-art solutions. We first study the complexity on two aspects of reliable transactions: atomicity and consistency. More specifically, we do a systematic study on the time and message complexity of non-blocking atomic commit of a distributed transaction, and investigate intrinsic limitations of causally consistent transactions. Our study of distributed transaction commit focuses on the complexity of the most frequent executions in practice, i.e., failure-free, and willing to commit. Through our systematic study, we close many open questions like the complexity of synchronous non-blocking atomic commit. We also present an effective protocol which solves what we call indulgent atomic commit that tolerates practical distributed database systems which are synchronous "most of the time", and can perform as efficiently as the two-phase commit protocol widely used in distributed database systems. Our investigation of causal transactions focuses on the limitations of read-only transactions, which are considered the most frequent in practice. We consider "fast" read-only transactions where operations are executed within one round-trip message exchange between a client seeking an object and the server storing it (in which no process can be a coordinator). We show two impossibility results regarding "fast" read-only transactions. By our impossibility results, when read-only transactions are "fast", they have to be "visible", i.e., they induce inherent updates on the servers. We also present a "fast" read-only transaction protocol that is "visible" as an upper bound on the complexity of inherent updates. We then study the complexity of secure transactions in the model of secure multiparty computation: even in the face of malicious parties, no party obtains the computation result unless all other parties obtain the same result. As it is impossible to achieve without any trusted party, we focus on optimism where if all parties are honest, they can obtain the computation result without resorting to a trusted third party, and the complexity of every optimistic execution where all parties are honest. We prove a tight lower bound on the message complexity by relating the number of messages to the length of the permutation sequence in combinatorics, a necessary pattern for messages in every optimistic execution

Virtual infrastructure for wireless ad hoc networks

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (v. 2, p. 585-591) and index.One of the most significant challenges introduced by ad hoc networks is coping with the unpredictable deployment, uncertain reliability, and erratic communication exhibited by emerging wireless networks and devices. The goal of this thesis is to develop a set of algorithms that address these challenges and simplify the design of algorithms for ad hoc networks. In the first part of this thesis, I introduce the idea of virtual infrastructure, an abstraction that provides reliable and predictable components in an unreliable and unpredictable environment. This part assumes reliable communication, focusing primarily on the problems created by unpredictable motion and fault-prone devices. I introduce several types of virtual infrastructure, and present new algorithms based on the replicated-state-machine paradigm to implement these infrastructural components. In the second part of this thesis, I focus on the problem of developing virtual infrastructure for more realistic networks, in particular coping with the problem of unreliable communication. I introduce a new framework for modeling wireless networks based on the ability to detect collisions. I then present a new algorithm for implementing replicated state machines in wireless networks, and show how to use replicated state machines to implement virtual infrastructure even in an environment with unreliable communication.by Seth Gilbert.Ph.D