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.

Due to severe performance penalties associated with synchronous replication, there is an increasing interest in asynchronous replica management protocols in which database transactions are executed locally, and the effects of these transactions are incorporated asynchronously on remote database copies. However, the asynchronous protocols currently in use either do not guarantee consistency and serializability as needed by transactional semantics or they impose restrictions on placement of data and on which data objects can be updated. In this paper we investigate an epidemic update protocol that guarantees consistency and serializability in spite of a write-anywhere capability. We conducted experiments on a detailed simulation of a distributed, replicated database to evaluate this protocol. Our results establish that this epidemic approach is indeed a viable alternative to traditional eager update protocols for a distributed database environment where consistency and full seri...

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 pair-wise 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 currencies allows voting 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 voting schemes, while allocating all currency to a single server emulates a primary-copy scheme. We present simulation results showing both schemes, as well as the performance advantages of using currency proxies to temporarily reallocate currency during planned disconnections. Furthermore, we discuss an initial design of the underlying replicated-object system and present a basic API. 1.

We study deterministic gossiping in synchronous systems with dynamic crash failures. Each processor is initialized with an input value called rumor. In the standard gossip problem, the goal of every processor is to learn all the rumors. When processors may crash, then this goal needs to be revised, since it is possible, at a point in an execution, that certain rumors are known only to processors that have already crashed. We define gossiping to be completed, for a system with crashes, when every processor knows either the rumor of processor v or that v has already crashed, for any processor v. We design gossiping algorithms that are efficient with respect to both time and communication. Let t < n be the number of failures, where n is the number of processors. If n − t = Ω(n/polylog n), then one of our algorithms completes gossiping in O(log 2 t) time and with O(n polylog n) messages. We develop an algorithm that performs gossiping with O(n 1.77) messages and in O(log 2 n) time, in any execution in which at least one processor remains non-faulty. We show a trade-off between time and communication in gossiping algorithms: if the number of messages is at most O(n polylog n), then the time has to be at least Ω ( log n. By way of application, we show that if n − t = Ω(n), then log(n log n)−log t consensus can be solved in O(t) time and with O(n log 2 t) messages.

Optimistic replication algorithms allow replica contents to be stale but in a controlled way. By propagating updates in background and allowing any replica to be read most of the time, they become more available and more efficient than traditional replication algorithms using unreliable network media and inexpensive computers. The use of optimistic replication has recently grown explosively due to the proliferation of the Internet and mobile computing devices, but its systematic study has begun only recently. We survey optimistic replication algorithms in this report. In particular, we focus on the update propagation mechanisms invented for optimistic replication and study how these mechanisms affect the functional characteristics of the systems, including data consistency guarantees and performance.

We present the notion of a distributed database made up entirely of mobile components. Since disconnections will be frequent in such an environment, we develop a disconnection and reconnection procedure to allow normal processing on the connected components. We briefly discuss a protocol based on epidemic communication to support such a system while ensuring one-copy serializability. 1

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.

As mobility permeates into todays computing and communication arena, we envision application infrastructures that will increasingly rely on mobile technologies. Traditional database applications and information service applications will need to integrate mobile entities: people and computers. In this paper, we develop a distributed database framework for mobile environments. A key requirement in such an environment is to support frequent connection and disconnection of database sites. We present algorithms that implement this framework in an asynchronous system.

An increasing number of applications, such as electronic mail servers, web servers, and personal information managers, handle large amounts of homogeneous data. This data can be effectively represented as records and manipulated through simple operations, e.g., record reading, writing, and searching. Unfortunately, modern storage systems are inappropriate for the needs of these applications. On one side, file systems store only unstructured data (byte strings) with very limited reliability guarantees. On the other side, relational databases store structured data and provide both concurrency control and transactions; but relational databases are often too slow, complex, and difficult to manage for many applications. This paper presents a transactional record store that directly addresses the needs of modern applications. The store combines the simplicity and manageability of the file system interface with a select few features for managing record-oriented data. We describe the principles gui...

Abstract – e-Commerce services have become a promising and profitable application of the Internet. In order to keep them growing, solutions must be found to deal with unreliable connections and high latencies, among other problems. The best solutions to such problems tend to depend on the distribution of the service over the network, placing servers in multiple locations, closer to customers. If placement of servers is effective it tends to reduce delays and traffic-related costs. In this paper we discuss the distribution of e-commerce services by introducing a traffic-aware cost model and evaluating it using an actual log from an e-tailer. The results show that the model yields good placement solutions, which perform better than simpler ad-hoc solutions. I