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This paper addresses the problem of designing an efficient implementation of a basic atomic read-write data structure over an asynchronous message-passing system. In particular, we consider time-efficient implementations of this abstraction in the case of a single writer, multiple readers (also called a SWMR atomic register) and S servers: the writer, the readers, and t out of the S servers may fail by crashing. Previous implementations tolerate the failure of any minority of servers (i.e., t < S/2) and require one communication round-trip for every write, and two round-trips for every read.We investigate the possibility of fast implementations, namely, implementations that complete both reads and writes in one round-trip. We show that, interestingly, the existence of a fast implementation depends on the maximum number of readers considered. More precisely, we show that a fast implementation is possible if and only if the number of readers is less that S t − 2. We also show that a fast implementation is impossible in a multiple writers setting when t ≥ 1.Our results draw sharp lines between the time-complexity of regular and atomic register implementations, as well as between single-writer and multi-writer implementations. The results lead also to revisit, in a message-passing context, the folklore theorem that "atomic reads must write". Abstract Devises]: Models of Computation-relations among models * This work is partially supported by the Swiss National Science Foundation (project number 2100-66768.01/1).

We study erasure-coded atomic register implementations in an asynchronous crash-recovery model. Erasure coding provides a cheap and space-efficient way to tolerate failures in a distributed system. This paper presents ORCAS, Optimistic eRasure-Coded Atomic Storage, which consists of two separate implementations, ORCAS-A and ORCAS-B. In terms of storage space used, ORCAS-A is more efficient in systems where we expect large number of concurrent writes, whereas, ORCAS-B is more suitable if not many writes are invoked concurrently. Compared to replication based implementations, both ORCAS implementations significantly save on the storage space. The implementations are optimistic in the sense that the used storage is lower in synchronous periods, which are considered common in practice, as compared to asynchronous periods. Indirectly, we show that tolerating asynchronous periods does not increase storage overhead during synchronous periods.

Total order broadcast is a fundamental communication primitive that plays a central role in bringing cheap software-based high availability to a wide range of services. This paper studies the practical performance of such a primitive on a cluster of homogeneous machines.We present LCR, the first throughput optimal uniform total order broadcast protocol. LCR is based on a ring topology. It only relies on point-to-point inter-process communication and has a linear latency with respect to the number of processes. LCR is also fair in the sense that each process has an equal opportunity of having its messages delivered by all processes.We benchmark a C implementation of LCR against Spread and JGroups, two of the most widely used group communication packages. LCR provides higher throughput than the alternatives, over a large number of scenarios.

Abstract. We study efficient and robust implementations of an atomic read-write data structure over an asynchronous distributed message-passing system made of reader and writer processes, as well as a number of servers implementing the data structure. We determine the exact conditions under which every read and write involves one round of communication with the servers. These conditions relate the number of readers to the tolerated number of faulty servers and the nature of these failures.