Pierre Sutra

P-Store: Genuine Partial Replication in Wide Area Networks

Partial replication is a way to increase the scalability of replicated systems: updates only need to be applied to a subset of the systems sites, thus allowing replicas to handle independent parts of the workload in parallel. In this paper, we propose P-Store, a partially replicated key-value store for wide area networks. In P-Store, each transaction T optimistically executes on one or more sites and is then certified to guarantee serializability of the execution. The certification protocol is genuine, it only involves sites that replicate data items read or written by T, and incorporates a mechanism to minimize a convoy effect. P-Store makes a thrifty use of an atomic multicast service to guarantee correctness: no messages need to be multicast during Ts execution and a single message is multicast to certify T. In case T is global, that is, Ts execution is distributed at different geographical locations, an extra vote phase is required. Our approach may offer better scalability than previously proposed solutions that either require multiple atomic multicast messages to execute T or are non-genuine. Experimental evaluations reveal that the convoy effect plays an important role even when one percent of the transactions are global. We also compare the scalability of our approach to a fully replicated solution when the proportion of global transactions and the number of sites vary.

A comparison of optimistic approaches to collaborative editing of Wiki pages

Wikis, a popular tool for sharing knowledge, are basically collaborative editing systems. However, existing Wiki systems offer limited support for co-operative authoring, and they do not scale well, because they are based on a centralised architecture. This paper compares the well-known centralised MediaWiki system with several peer-to-peer approaches to editing of wiki pages: an operational transformation approach (MOT2), a commutativity-oriented approach (WOOTO) and a conflict resolution approach (ACF). We evaluate and compare them, according to a number of qualitative and quantitative metrics.

Fault-Tolerant Partial Replication in Large-Scale Database Systems

We investigate a decentralised approach to committing transactions in a replicated database, under partial replication. Previous protocols either reexecute transactions entirely and/or compute a total order of transactions. In contrast, ours applies update values, and generate a partial order between mutually conflicting transactions only. Transactions execute faster, and distributed databases commit in small committees. Both effects contribute to preserve scalability as the number of databases and transactions increase. Our algorithm ensures serializability, and is live and safe in spite of faults.

Fast Genuine Generalized Consensus

Consensus (agreeing on a sequence of commands) is central to the operation and performance of distributed systems. A well-known solution to consensus is Fast Paxos. In a recent paper, Lamport enhances Fast Paxos by leveraging the commutativity of concurrent commands. The new primitive, called Generalized Paxos, reduces the collision rate, and thus the latency of Fast Paxos. However if a collision occurs, Generalized Paxos needs four communication steps to recover, which is slower than Fast Paxos. This paper presents FGGC, a novel consensus algorithm that reduces recovery delay when a collision occurs to one. FGGC tolerates f

Genuine versus Non-Genuine Atomic Multicast Protocols for Wide Area Networks: An Empirical Study

We study atomic multicast, a fundamental abstraction for building fault-tolerant systems. We suppose a system composed of data centers, or groups, that host many processes connected through high-end local links; a few groups exist, interconnected through high-latency communication links. A recent paper showed that no multicast protocol can deliver messages addressed to multiple groups in one inter-group delay and be genuine, i.e., to deliver a message m, only the addressees of m are involved in the protocol.We propose a non-genuine multicast protocol that may deliver messages addressed to multiple groups in one inter-group delay. Experimental comparisons against a latency-optimal genuine protocol show that the non-genuine protocol offers better performance in almost all considered scenarios. We also identify a convoy effect in multicast algorithms that may delay the delivery of local messages, i.e., messages addressed to a single group, by as much as the latency of global messages, i.e., messages addressed to multiple groups, and propose techniques to minimize this effect. To complete our study, we evaluate a latency-optimal protocol that tolerates disasters, i.e., group crashes.

The space complexity of transactional interactive reads

Transactional Web Applications need to perform fast interactive reads while ensuring reasonable isolation guarantees. This paper studies the problem of taking consistent snapshots for transactions with interactive reads. We introduce four levels of freshness, and solutions to guarantee them. We also explore trade-offs between the space complexity and the freshness levels.

Decentralised commitment for optimistic semantic replication

We study large-scale distributed cooperative systems that use optimistic replication. We represent a system as a graph of actions (operations) connected by edges that reify semantic constraints between actions. Constraint types include conflict, execution order, dependence, and atomicity. The local state is some schedule that conforms to the constraints; because of conflicts, client state is only tentative. For consistency, site schedules should converge; we designed a decentralised, asynchronous commitment protocol. Each client makes a proposal, reflecting its tentative and/or preferred schedules. Our protocol distributes the proposals, which it decomposes into semantically-meaningful units called candidates, and runs an election between comparable candidates. A candidate wins when it receives a majority or a plurality. The protocol is fully asynchronous: each site executes its tentative schedule independently, and determines locally when a candidate has won an election. The committed schedule is as close as possible to the preferences expressed by clients.