This paper addresses the performance of distributed database systems. Specifically, we present an algorithm for dynamic replication of an object in distributed systems. The algorithm is adaptive in the sense that it changes the replication scheme of the object (i.e. the set of processors at which the object is replicated), as changes occur in the read-write pattern of the object (i.e. the number of reads and writes issued by each processor). The algorithm continuously moves the replication scheme towards an optimal one. We show that the algorithm can be combined with the concurrency control and recovery mechanisms of a distributed database management system. The performance of the algorithm is analyzed theoretically and experimentally. On the way we provide a lower bound on the performance of any dynamic replication algorithm.

This paper presents an algorithm that emulates atomic read/write shared objects in a dynamic network setting. To ensure availability and fault-tolerance, the objects are replicated. To ensure atomicity, reads and writes are performed using quorum configurations, each of which consists of a set of members plus sets of read-quorums and write-quorums. The algorithm is reconfigurable: the quorum configurations may change during computation, and such changes do not cause violations of atomicity. Any quorum configuration may be installed at any time. The algorithm tolerates processor stopping failure and message loss. The algorithm performs three major tasks, all concurrently: reading and writing objects, introducing new configurations, and "garbage-collecting" obsolete configurations.

In systems based on the client-server model, a single server may serve many clients and the heavy load on the server may cause the response time to be adversely affected. In such circumstances, replicating data or servers may improve performance. Replication may also improve the availability of information when processors crash or the network partitions. Existing replication methods are often needlessly expensive. They sometimes use pointto -point communication when multicast communication is available; they typically pay the full price of end-to-end acknowledgments for all of the participants for every update; they may claim locks, and therefore, may be vulnerable to faults that can unnecessarily block the system for long periods of time. This thesis presents a new architecture and algorithms for replication over a partitioned network. The architecture is structured into two layers: a replication server and a group communication layer. Each of the replication servers maintains a priva...

We describe a new replicated-object protocol designed for use in mobile and weakly-connected environments. The protocol differs from previous protocols in combining epidemic information propagation with voting, and in using fixed per-object currencies for voting. The advantage of epidemic protocols is that data movement only requires pairwise communication. Hence, there is no need for a majority quorum to be available and simultaneously connected at any single time. The protocols increase availability by using voting, rather than primary copy or primary commit schemes. Finally, the use of per-object voting currencies allows votes to take place in an entirely decentralized fashion, without any server having complete knowledge of group membership. We show that currency allocation can be used to implement diverse policies. For example, uniform currency distributions emulate traditional dynamic voting schemes, while allocating all currency to a single server emulates a primary-copy scheme...

This paper presents in detail an efficient and provably correct algorithm for database replication over partitionable networks. Our algorithm avoids the need for end-toend acknowledgments for each action while supporting network partitions and merges and allowing dynamic instantiation of new replicas. One round of end-to-end acknowledgments is required only upon a membership change event such as a network partition. New actions may be introduced to the system at any point, not only while in a primary component. We show how performance can be further improved for applications that allow relaxation of consistency requirements. We provide experimental results that demonstrate the efficiency of our approach.

Quorum systems serve as a basic tool providing a uniform and reliable way to achieve coordination in a distributed system. They are useful for distributed and replicated databases, name servers, mutual exclusion, and distributed access control and signatures. Traditionally, two basic methods have been used to evaluate quorum systems: the analytical approach, and simulation. This paper proposes a third, empirical approach. We collected 6 months' worth of connectivity and operability data of a system consisting of 14 real computers using a wide area group communication protocol. The system spanned two geographic sites and three different Internet segments. We developed a mechanism that merges the local views into a unified history of the events that took place, ordered according to an imaginary global clock. We then developed a tool called the Generic Quorum-system Evaluator (GQE), which evaluates the behavior of any given quorum system over the unified, real-life history. We compared fo...

We present a new architecture and algorithm for distributed replicated database systems. The replication algorithm operates in the presence of message omission faults, processor crashes and recoveries, and network partitions and remerges. The architecture exploits a group communication service to minimize communication costs and to eliminate forced disk writes in the critical path, while preserving complete and consistent operation. End-to-end agreement is required only after a change in the membership of the connected servers, rather than on a per action basis. The updates are globally ordered and, if the system has partitioned, they are applied to the database when they become known to the primary component of the partitioned system. An application may, however, read data and initiate updates at any time, even in a component that is not the primary component. This approach renders replication more efficient and more scalable and, therefore, applicable to many more systems.

Distributed applications often use quorums in order to guarantee consistency. With emerging world-wide communication technology, many new applications (e.g. conferencing applications and interactive games) wish to allow users to freely join and leave, without restarting the entire system. The dynamic voting paradigm allows such systems to define quorums adaptively, accounting for the changes in the set of participants. Furthermore, dynamic voting was proven to be the most available paradigm for maintaining quorums in unreliable networks. However, the subtleties of implementing dynamic voting were not well understood, in fact many of the suggested protocols may lead to inconsistencies in case of failures. Other protocols severely limit the availability in case failures occur during the protocol. In this paper we present a robust and efficient dynamic voting protocol for unreliable asynchronous networks. The protocol consistently maintains the primary component in a distributed system. O...

Abstract—We suggest a method of controlling the access to a secure database via quorum systems. A quorum system is a collection of sets (quorums) every two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We propose a separation between access servers, which are protected and trustworthy, but may be outdated, and the data servers, which may all be compromised. The main paradigm is that only the servers in a complete quorum can collectively grant (or revoke) access permission. The method we suggest ensures that, after authorization is revoked, a cheating user Alice will not be able to access the data even if many access servers still consider her authorized and even if the complete raw database is available to her. The method has a low overhead in terms of communication and computation. It can also be converted into a distributed system for issuing secure signatures. An important building block in our method is the use of secret sharing schemes that realize the access structures of quorum systems. We provide several efficient constructions of such schemes which may be of interest in their own right. Index Terms—Quorum systems, replication, secret sharing, security, cryptography.

... this article, we analyze several quorum types in order to better understand their behavior in practice. The results obtained challenge many of the assumptions behind quorum based replication. Our evaluation indicates that the conventional read-one/write-all-available approach is the best choice for a large range of applications requiring data replication. We believe this is an important result for anybody developing code for computing clusters as the read-one/write-all-available strategy is much simpler to implement and more flexible than quorum-based approaches. In this article, we show that, in addition, it is also the best choice using a number of other selection criteria