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.

The emergence of powerful portable computers, along with advances in wireless communication technologies, has made mobile computing a reality. Among the applications that are finding their way to the market of mobile computingthose that involve data managementhold a prominent position. In the past few years, there has been a tremendous surge of research in the area of data management in mobile computing. This research has produced interesting results in areas such as data dissemination over limited bandwith channels, location-dependent querying of data, and advanced interfaces for mobile computers. This paper is an effort to survey these techniques and to classify this research in a few broad areas. Index Terms---Mobile computing, databases, data dissemination, bandwith, location-dependent queries, interfaces, transaction management. ------------------------------ ###p### ------------------------------ 1INTRODUCTION OBILE computing has become a reality thanks to the convergen...

To deal with the frequent, foreseeable and variable disconnections that occur in a mobile environment, we introduce a flexible, two-level consistency model. Semantically related or closely located data are grouped together to form a cluster. While all data inside a cluster are mutually consistent, degrees of inconsistency are allowed among data at different clusters. To take advantage of the predictability of disconnections, and to accommodate mobility, the cluster configuration is dynamic. We allow transactions to exhibit certain degrees of tolerance for inconsistencies by introducing strict and weak operations. Weak operations are operations that can beexecuted under weaker consistency requirements. We define correctness criteria for schedules that involve weak operations and compare them with traditional serializability criteria. Finally, we argue that our model is appropriate for a variety of other environments including very large databases and multidatabases.

Advances in computer and telecommunication technologies have made mobile computing a reality. However, greater mobility implies a more tenuous network connection and a higher rate of disconnection. In order to tolerate disconnections as well as to reduce the delays and cost of wireless communication, it is necessary to support autonomous mobile operations on data shared by stationary hosts. This would allow the part of a computation executing on a mobile host to continue executing while the mobile host is not connected to the network. In this paper, we examine whether object semantics can be exploited to facilitate autonomous and disconnected operation in mobile database applications. We define the class of fragmentable objects which may be split among a number of sites, operated upon independently at each site, and then recombined in a semantically consistent fashion. A number of objects with such characteristics are presented and an implementation of fragmentable stacks is shown and ...

Mobile computing introduces a new form of distributed computation in which communication is most often intermittent, low-bandwidth, or expensive, thus providing only weak connectivity. In this paper, we present a replication scheme tailored for such environments. Bounded inconsistency is defined by allowing controlled deviation among copies located at weakly connected sites. A dual database interface is proposed that in addition to read and write operations with the usual semantics supports weak read and write operations. In contrast to the usual read and write operations that read consistent values and perform permanent updates, weak operations access only local and potentially inconsistent copies and perform updates that are only conditionally committed. Exploiting weak operations supports disconnected operation, since mobile clients can employ them to continue operate even while disconnected. The extended database interface coupled with bounded inconsistency offers a flexible mechan...

We present a replication schema appropriate for distributed environments where connectivity is partial, weak, and variant such as in mobile information systems. The schema is based on augmenting the database interface with operations with weaker consistency guarantees. A implementation of the schema is presented by distinguishing copies into quasi and core and protocols for enforcing the schema are introduced. Then, some practical examples of its applicability are described. The performance of the weak consistency schema is evaluated for various networking conditions by using an analytical model developed for this purpose. 1 Introduction Advances in telecommunications and in the development of portable computers have provided for wireless communications that permit users to actively participate in distributed computing even while moving. The resulting distributed environment is subject to restrictions imposed by the nature of the wireless medium [8, 10, 19, 20]. Mobile computing is su...

Database systems, wireless communication technologies and advanced applications have given rise to new and challenging problems which have to be dealt with in multidatabase systems (MDS). This paper addresses a classification of various replica control protocols as an integral part of MDS with an emphasis on workstation mobility issues. The paper proposes and discusses modifications that have to be made to replicated database management strategies in order to address the effect of mobility on the existing replica control protocols. In a mobile environment replication will be an essential technique providing reliable distributed computing and data availability to MDS users. It is argued that the existing algorithms have to be tuned for a mobile environment and it is discussed what this may entail. As an illustration, a variation of the primary copy algorithm, called "virtual primary copy" method is proposed and discussed and it is shown that it is well suited for the mobile environment....

This paper presents the replication framework of Deno, an object replication system specifically designed for mobile and weakly-connected environments. Deno uses weighted voting for availability and pair-wise, epidemic information flow for flexibility. This combination allows the protocols to operate with less than full connectivity, to easily adapt to changes in group membership, and to make few assumptions about the underlying network topology. Deno has been implemented and runs on top of Linux and Win32 platforms. We use the Deno prototype to characterize the performance of two versions of Deno's protocol. The first version enables globally serializable execution of update transactions. The second supports a weaker consistency level that still guarantees transactionally-consistent access to replicated data. We demonstrate that the incremental cost of providing global serializability is low, and that speculative dissemination of updates can significantly improve commit performance.

Replication is an essential utility in mobile environments. Mobile units not connected to fixed networks are left with their own data resources which may or may not contain the necessary data. This paper describes benefits and problems using replication in mobile infrastructures. We compare different replication approaches for fixed and mobile systems. We show that traditional protocols are inadequate for the requirements for mobile environments, especially by accessing multimedia data. The objectives of replication, high availability, permanent consistency and low communication costs, are incompatible. Consequently, replication protocols must find suitable compromises. However, rather than creating a general approach for all possible mobile environments, we tailor suitable replication mechanisms for special mobile environments and data types. This approach allows for configured replication based on various environmental and data characteristics. keywords: mobile information ...

This paper discusses the currency management mechanisms used in Deno, a replicated object storage system designed for use in mobile and weakly-connected environments. Deno primarily differs from previous work in implementing an asynchronous weighted-voting scheme via epidemic information flow, and in committing updates in an entirely decentralized fashion, without requiring any server to have complete knowledge of system membership.