613893
Expiration Times for Data Management
This paper describes an approach to incorporating the notion of expiration time into data management based on the relational model. Expiration times indicate when tuples cease to be current in a database. The paper presents a formal data model and a query algebra that handle expiration times transparently and declaratively. In particular, expiration times are exposed to users only on insertion and update, and when triggers fire due to the expiration of a tuple; for queries, they are handled behind the scenes and do not concern the user. Notably, tuples are removed automatically from (materialised) query results as they expire in the (base) relations. For application developers, the benefits of using expiration times are leaner application code, lower transaction volume, smaller databases, and higher consistency for replicated data with lower overhead. Expiration times turn out to be especially useful in open architectures and loosely-coupled systems, which abound on the World Wide Web as well as in mobile networks, be it as Web Services or as ad hoc and intermittent networks of mobile devices.
1348801
Using differential technlques to efficiently support transaction time
We present an architecture for query processing in the relational model extended with transaction time. The architecture integrates standard query optimization and computation techniques with new differential computation techniques. Differential computation computes a query incrementally or decrementally from the cahced and indexed results of previous computations. The use of differential computation techniques is essential in order to provide efficient processing of queries that access very large temporal relations. Alternative query plans are integrated into a state transition network, where the state space includes backlogs of base relations, cached results from previous computations, a cache index, and intermediate results; the transitions include standard relational algebra operators, operators for constructing differential files, operators for differential computation, and combined operators. A rule set is presented to prune away parts of state transition networks that are not promising, and dynamic programming techniques are used to identify the optimal plans from the remaining state transition networks. An extended logical access path serves as a “structuring” index on the cached results and contains, in addition, vital statistics for the query optimization process (including statistics about base relations, backlogs, and queries-previously computed and cached, previously computed, or just previously estimated).
1723263
Multidimensional data modeling for location-based services
Abstract.With the recent and continuing advances in areas such as wireless communications and positioning technologies, mobile, location-based services are becoming possible.Such services deliver location-dependent content to their users. More specifically, these services may capture the movements and requests of their users in multidimensional databases, i.e., data warehouses, and content delivery may be based on the results of complex queries on these data warehouses. Such queries aggregate detailed data in order to find useful patterns, e.g., in the interaction of a particular user with the services.The application of multidimensional technology in this context poses a range of new challenges. The specific challenge addressed here concerns the provision of an appropriate multidimensional data model. In particular, the paper extends an existing multidimensional data model and algebraic query language to accommodate spatial values that exhibit partial containment relationships instead of the total containment relationships normally assumed in multidimensional data models. Partial containment introduces imprecision in aggregation paths. The paper proposes a method for evaluating the imprecision of such paths. The paper also offers transformations of dimension hierarchies with partial containment relationships to simple hierarchies, to which existing precomputation techniques are applicable.
616587
Querying the trajectories of on-line mobile objects
Position data is expected to play a central role in a wide range of mobile computing applications, including advertising, leisure, safety, security, tourist, and traffic applications. Applications such as these are characterized by large quantities of wirelessly Internet-worked, position-aware mobile objects that receive services where the objects' position is essential. The movement of an object is captured via sampling, resulting in a trajectory consisting of a sequence of connected line segments for each moving object. This paper presents a technique for querying these trajectories. The technique uses indices for the processing of spatiotemporal range queries on trajectories. If object movement is constrained by the presence of infrastructure, e.g., lakes, park areas, etc., the technique is capable of exploiting this to reduce the range query, the purpose being to obtain better query performance. Specifically, an algorithm is proposed that segments the original range query based on the infrastructure contained in its range. The applicability and limitations of the proposal are assessed via empirical performance studies with varying datasets and parameter settings.
1655888
Supporting imprecision in multidimensional databases using granularities
Online analytical processing (OLAP) technologies are being used widely, but the lack of effective means of handling data imprecision, which occurs when exact values are not known precisely or are entirely missing, represents a major obstacle in applying these technologies in many domains. The paper develops techniques for handling imprecision that aim to maximally reuse existing OLAP modeling constructs such as dimension hierarchies and granularities. With imprecise data available in the database, queries are tested to determine whether or not they may be answered precisely given the available data; if not, alternative queries unaffected by the imprecision are suggested. When processing queries affected by imprecision, techniques are proposed that take into account the imprecision in the grouping of the data, in the subsequent aggregate computation, and in the presentation of the imprecise result to the user. The approach is capable of exploiting existing OLAP query processing techniques such as pre-aggregation, yielding an effective approach with low computational overhead and that may be implemented using current technology.
45102117
Temporal specialization
The authors explore a variety of temporal relations with specialized relationships between transaction and valid time. An example is a retroactive temporal event relation, where the event must have occurred before it was stored, i.e., the valid time-stamp is restricted to be less than the transaction time-stamp. The authors discuss many useful restrictions, defining a large number of specialized types of temporal relations, and indicate some of their applications. A detailed taxonomy of specialized temporal relations is presented. This taxonomy may be used during database design to specify the particular time semantics of temporal relations.<<ETX>>
8891155
The Claim Tool Kit for ad hoc recognition of peer entities
In ubiquitous/pervasive computing environments, it is envisaged that computing elements--entities--will start interacting in an ad hoc fashion. The peer-to-peer (p2p) paradigm is appealing for such types of interaction especially with JXTA, which supports the development of reusable p2p building blocks, which facilitate implementation on any smart device. However, the inability to rely on a centralised authentication infrastructure, the openness of the environment and the absence of an administrator (it is assumed to be too expensive to have a skilled administrator at hand due to the large number of peers) challenge the use of legacy authentication mechanisms.Supporting spontaneous interactions among previously unknown entities requires dynamic enrolment of strangers and unknown entities. Entity recognition (ER) is a process that is carried out each time an interaction happens between entities in order to dynamically recognise previously met entities.In this paper, we present the Claim Tool Kit (CTK), a Java-based implementation of ER: entities exchange messages, called Claims, and rely on their associated clues to evaluate the level of confidence in recognition.The CTK employs advanced features available with Java, such as JXTA and Java Cryptography and Security Architectures. We show that the CTK needs performance results on these features in order to increase the level of auto-configuration of the CTK. We describe how to obtain performance assessment for some of these new features. Finally, we explain how the CTK can be instrumented to take into account performance assessment. By analysing the evaluation results, the applicability of these advanced Java-based technologies for peer entity recognition is assessed.
2275333
Risk probability estimating based on clustering
Ubiquitous computing environments are highly dynamic, with new unforeseen circumstances and constantly changing environments, which introduces new risks that cannot be assessed through traditional means of risk analysis. Mobile entities in a ubiquitous computing environment require the ability to perform an autonomous assessment of the risk incurred by a specific interaction with another entity in a given context. This assessment will allow a mobile entity to decide whether sufficient evidence exists to mitigate the risk and allow the interaction to proceed. Such evidence might include records of prior experiences, recommendations from a trusted entity or the reputation of the other entity. We propose a dynamic mechanism for estimating the risk probability of a certain interaction in a given environment using hybrid neural networks. We argue that traditional risk assessment models from the insurance industry do not directly apply to ubiquitous computing environments. Instead, we propose a dynamic mechanism for risk assessment, which is based on pattern matching, classification and prediction procedures. This mechanism uses an estimator of risk probability, which is based on the automatic clustering of defining features of the environment and the other entity, which helps avoid subjective judgments as much as possible.
26363914
Trust-Based Route Selection in Dynamic Source Routing
Unlike traditional mobile wireless networks, ad hoc networks do not rely on any fixed infrastructure. Nodes rely on each other to route packets to other mobile nodes or toward stationary nodes that may act as a gateway to a fixed network. Mobile nodes are generally assumed to participate as routers in the mobile wireless network. However, blindly trusting all other nodes to respect the routing protocol exposes the local node to a wide variety of vulnerabilities. Traditional security mechanisms rely on either the authenticated identity of the requesting principal or some form of credentials that authorise the client to perform certain actions. Generally, these mechanisms require some underlying infrastructure, e.g., a public key infrastructure (PKI). However, we cannot assume such infrastructures to be in place in an ad hoc network. In this paper we propose an extension to an existing ad hoc routing protocols, which selects the route based on a local evaluation of the trustworthiness of all known intermediary nodes (routers) on the route to the destination. We have implemented this mechanism in an existing ad hoc routing protocol, and we show how trust can be built from previous experience and how trust can be used to avoid routing packets through unreliable nodes.
206519638
PEM/sup 3/ - the policy enhanced memory management model
Over the past decade, clusters of workstations have become widely accepted as a cost efficient way of obtaining computational power. Moreover, clusters have increasingly been used to support multi-application environment, such as web servers and application servers, and to concurrently support a number of different services. In such an environment, resources become difficult to manage, e.g., it is difficult to adequately support the varying memory usage requirements of each application with a single strategy. In this paper we propose a policy-based model that provides applications with an interface to the underlying system in order to adapt the behavior of system services at runtime. The use of policies is illustrated by presenting the design of a memory management model for distributed shared memory systems, which allows different memory placement policies, while providing the ability to change consistency and coherency protocols at runtime.
11578916
Adaptability in CORBA: the mobile proxy approach
Adaptability is one of the most important challenges in modern distributed systems. It may be defined as the ease with which a software application satisfies the different system constraints and the requirements of users and other applications. Adaptability is needed because distributed systems are inherently open, heterogeneous, and dynamic environments integrating a wide range of platforms, operating systems and applications from a number of different sources. We propose to use mobile proxies to provide adaptability in distributed applications integrated using the CORBA technology. Downloading stubs and skeletons at runtime allows the adaptation of either client or server interfaces as well as the protocol linking the two.