Francisco Moura

Mobile Transaction Management in Mobisnap

In this paper we describe a transaction management system designed to face the inherent characteristics of mobile environments. Mobile clients cache subsets of the database state and allow disconnected users to perform transactions independently. Transactions are specified as mobile transactional programs that are propagated and executed in the server, thus allowing the validation of transactions based on application-specific semantics. In the proposed model (as in others previously presented in literature) the final result of a transaction is only determined when the transaction is processed in the central server. Users may be notified of the results of their transactions using system support (even when they are no longer using the same application or even the same computer). Additionally, the system implements a reservation mechanism in order to guarantee the results of transactions performed in disconnected computers.

Using structural characteristics for autonomous operation

The majority of current mobile computing systems operate either in conjunction with a central network by some form of weak connectivity or tend to operate in total isolation and perform sporadic synchronization with a backup or a central network. These configurations miss an additional and very useful pattern of operation --- mobile to mobile interaction. Recent mobile devices have the capacity for direct communication among them, but this option is essentially neglected by the application software.In order to address this pattern of operation we believe that there is a need to support re-usable peer-to-peer synchronization mechanisms that both respects data ownership and enables some level of state reconciliation.Naming this operation pattern as autonomous operation, we can observe that this pattern is already found on many legacy applications deployed in distributed systems. For example, personal information managers, Mail/News readers and Web browsers, often store persistent state in local files, but tacitly assume a single copy. Noticing that these separate copies are in fact replicas of a distributed entity, leads to the creation of semantically knowledgeable file synchronizers that strive to restore an unified state from these replicas.Evolution from static distributed systems to mobile platforms raises a demand for applications that, not only are adapted to user mobility but, take advantage of it. It is clear that despite continuous improvements on connectivity support for mobile environments, the cost and coverage limits still imply a major share of disconnected operation. When connectivity does exist it usually interposes wide area networks between communication peers, when one party is on the road, leading to lower channel quality. On the other hand, user mobility is likely to conduce to, normally unforeseen, physical proximity of the users mobile computer with other mobile or fixed systems. This occurrence is likely to increase as the installed population of mobile devices increases.In this work we show that without imposing restrictions on availability, which is a crucial factor for personal applications, it is possible to enable some data sharing among autonomous mobile applications. This sharing would take advantage of any pairwise encounters of replica holders.To determine the level of sharing that is compatible with permanent availability, we model general purpose data types that provide the necessary reconciliation guarantees. These guarantees are obtained by placing restrictions on the allowed behavior in order to avoid the occurrence of conflicting concurrent operations that would prevent reconciliations. Among other uses, these data types should help to identify sharable segments of data on classes of applications that traditionally support no sharing at all, and identify which parts of the state can be effectively shared.In the next section we present some examples of sharable data that motivates the modeling of a more generic and higher level description. This description is presented in the third section together with a framework of convergent components. Section four builds on this framework and gives a general presentation of a Java implementation for a component hierarchy. Before presenting the conclusions we show how these tools where used to build a merger for pairs of bookmark files, giving some insight on how to combine the components to create concrete applications.

Concurrency annotations in C

This paper describes CA/C++, Concurrency Annotations in C++, a language extension that regu lates method invocations from multiple threads of execution in a shared-memory multiprocessor system . This system provides threads as an orthogonal element to the language, allowing them to travel throug h more than one object . Statically type-ckecked synchronous and asynchronous method invocations ar e supported, with return values from asynchronous invocations accessed through first class future-like ob jects . Method invocations are regulated with synchronization code defined in a separate class hierarchy , allowing separate definition and inheritance of synchronization mechanisms . Each method is protected b y an access flag that can be switched in pre and post-actions, and by a predicate . Both must evaluate to tru e in order to enable a thread to animate the method code . Flags and method predicates are independentl y redefinable along the inheritance chain, thus avoiding the inheritance anomaly .

Improving causality logging in mobile computing networks

Vector clocks, or their compressed representations, have played a central role in the detection of causal dependencies between events in a distributed system. When adapting these techniques to a mobile network, bounding the vector clock size to the number of mobile nodes does not provide a satisfactory approach. This paper builds on previous techniques for efficient causality logging in mobile networks and presents a lighter logging mechanism. The technique is based on a particular partial order that is generated by the interleaving of events on mobile hosts that are mediated by the same support station.

Evaluating certification protocols in the partial database state machine

Partial replication is an alluring technique to ensure the reliability of very large and geographically distributed databases while, at the same time, offering good performance. By correctly exploiting access locality most transactions become confined to a small subset of the database replicas thus reducing processing, storage access and communication overhead associated with replication. The advantages of partial replication have however to be weighted against the added complexity that is required to manage it. In fact, if the chosen replica configuration prevents the local execution of transactions or if the overhead of consistency protocols offsets the savings of locality, potential gains cannot be realized. These issues are heavily dependent on the application used for evaluation and render simplistic benchmarks useless. In this paper, we present a detailed analysis of partial database state machine (PDBSM) replication by comparing alternative partial replication protocols with full replication. This is done using a realistic scenario based on a detailed network simulator and access patterns from an industry standard database benchmark. The results obtained allow us to identify the best configuration for typical online transaction processing applications.

A mobile-agent based architecture for virtual enterprises

Emerging technologies that allow a two-way communication between companies, or among companies and their customers, are changing the rules of the market, facilitating the emergence of virtual entities that have to be supported by some kind of platform. Making it is the main objective of this work, materialized in a Mobile Agent Architecture (MAA) which supports the emerging world of the m-Commerce (mobile-Commerce): the MAgnUM architecture. The MAA’s development was based on the principles of process and knowledge abstraction, compositionality, reuse, formal semantics, formal evaluation, and security.