In wide area computing systems, it is often desirable to create remote read-only copies (replicas) of files. Replication can be used to reduce access latency, improve data locality, and/or increase robustness, scalability and performance for distributed applications. We define a replica location service (RLS) as a system that maintains and provides access to information about the physical locations of copies. An RLS typically functions as one component of a data grid architecture. This paper makes the following contributions. First, we characterize RLS requirements. Next, we describe a parameterized architectural framework, which we name Giggle (for GIGa-scale Global Location Engine), within which a wide range of RLSs can be defined. We define several concrete instantiations of this framework with different performance characteristics. Finally, we present initial performance results for an RLS prototype, demonstrating that RLS systems can be constructed that meet performance goals.

Data replication is an increasingly important topic as databases are more and more deployed over clusters of workstations. One of the challenges in database replication is to introduce replication without severely affecting performance. Because of this difficulty, current database products use lazy replication, which is very efficient but can compromise consistency. As an alternative, eager replication guarantees consistency but most existing protocols have a prohibitive cost. In order to clarify the current state of the art and open up new avenues for research, this paper analyses existing eager techniques using three key parameters. In our analysis, we distinguish eight classes of eager replication protocols and, for each category, discuss its requirements, capabilities, and cost. The contribution lies in showing when eager replication is feasible and in spelling out the different aspects a database replication protocol must account for.

The goal of this work is to support replicated network services that accept updates to numerical records from multiple wide-area locations. Given the

A Replica Location Service (RLS) allows registration and discovery of data replicas. In earlier work, we proposed an RLS framework and described the performance and scalability of an RLS implementation in Globus Toolkit Version 3.0. In this paper, we present a Peer-to-Peer Replica Location Service (P-RLS) with properties of self-organization, fault-tolerance and improved scalability. P-RLS uses the Chord algorithm to self-organize PRLS servers and exploits the Chord overlay network to replicate P-RLS mappings adaptively. Our performance measurements demonstrate that update and query latencies increase at a logarithmic rate with the size of the P-RLS network, while the overhead of maintaining the P-RLS network is reasonable. Our simulation results for adaptive replication demonstrate that as the number of replicas per mapping increases, the mappings are more evenly distributed among P-RLS nodes. We introduce a predecessor replication scheme and show it reduces query hotspots of popular mappings by distributing queries among nodes.

An ultimate goal for modern Internet services is the development of scalable, high-performance, highly-available and fault-tolerant systems. Replication is an important approach to achieve this goal. However, replication introduces the issue of consistency among replicas, which is further complicated by network partitions. Generally, higher consistency levels result in lower system availability in the presence of network partitions. Thus, there is a fundamental tradeoff between consistency and availability in building replicated Internet services. In this paper, we argue that Internet services can benefit from dynamically choosing availability/consistency tradeoffs. With three consistency metrics, Unseen Writes, Uncommitted Writes and Staleness, we show how consistency can be meaningfully quantified for many Internet services. We present the design of the TACT (Tunable Availability and Consistency Tradeoffs) toolkit that allows Internet services to flexibly and dynamically choose thei...

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.

Abstract. A new field in distributed computing, called Ambient Intelligence, has emerged as a consequence of the increasing availability of wireless devices and the mobile networks they induce. Developing software for mobile networks is extremely hard in conventional programming languages because the network is dynamically demarcated. This leads us to postulate a suite of characteristics of future Ambient-Oriented Programming languages. A simple reflective programming language, called AmbientTalk, that meets the characteristics is presented. It is validated by implementing a collection of high level language features that are used in the implementation of an ambient messenger application. 1

A new field in distributed computing, called Ambient Intelligence, has emerged as a consequence of the increasing availability of wireless devices and the mobile networks they induce. Developing software for such mobile networks is extremely hard in conventional programming languages because the network is dynamically defined. This hardware phenomenon leads us to postulate a suite of characteristics of future Ambient-Oriented Programming languages. A simple reflective programming language kernel, called AmbientTalk, that meets these characteristics is subsequently presented. The power of the reflective kernel is illustrated by using it to conceive a collection of high level tentative ambient-oriented programming language features.

Abstract: In distributed and mobile environments, the connections among the hosts on which a software system is running are often unstable. As a result of connectivity losses, the overall availability of the system decreases. The distribution of software components onto hardware nodes (i.e., deployment architecture) may be ill-suited for the given target hardware environment and may need to be altered to improve the software system’s availability. The critical difficulty in achieving this task lies in the fact that determining a software system’s deployment that will maximize its availability is an exponentially complex problem. In this paper, we present an automated, flexible, software architecturebased solution for disconnected operation that increases the availability of the system during disconnection. We provide a fast approximative solution for the

Abstract. Supporting mobile collaborative work over wide areas is challenging due to the limitations and unreliability of wide area wireless networks. However, variations in patterns of collaboration require different levels of timeliness and synchrony, and place different demands on groupware and its supporting technologies. In this paper, we argue that groupware supported by wide area mobile networks strongly favors loosely-coupled work, where workers are autonomous and require a reduced level of communication. We examine the relationship between loosely-coupled group characteristics and wide area mobile groupware by considering one particular loosely coupled group—teams of home care workers. Over a two-year period, we analyzed home care work practices, and designed and field tested MoHoC, a mobile groupware application to support home care work. From this experience, we identified four characteristics of loosely-coupled groups that enable workers to accommodate the uncertainty of wide area mobile groupware: autonomy and the partitioning of work, clear ownership of data and artifacts, asynchronous awareness, and explicit asynchronous communication. 1