Cristina De Castro

Schema versioning for multitemporal relational databases

Abstract In order to follow the evolution of application needs, a database management system is easily expected to undergo changes involving database structure after implementation. Schema evolution concerns the ability of maintaining extant data in response to changes in database structure. Schema versioning enables the use of extensional data through multiple schema interface as created by a history of schema changes. However, schema versioning has been considered only to a limited extent in current literature. Also in the field of temporal databases, whereas a great deal of work has been done concerning temporal versioning of extensional data, a thorough investigation of schema versioning potentialities has not yet been made. In this paper we consider schema versioning in a broader perspective and introduce new design options whose distinct semantic properties and functionalities will be discussed. First of all, we consider solutions for schema versioning along transaction time but also along valid time . Moreover, the support of schema versioning implies operations both at intensional and extensional level. Two distinct design solutions (namely single- and multi-pool ) are presented for the management of extensional data in a system supporting schema versioning. Finally, a further distinction is introduced to define synchronous and asynchronous management of versioned data and schemata. The proposed solutions differ in their semantics and in the possible operations they support. The mechanisms for the selection of data through a schema version are in many cases strictly related to the particular schema versioning solution adopted, that also affects the data definition and manipulation language at user-interface level. In particular, we show how the temporal language TSQL2, originally designed to support basic functionalities of transaction-time schema versioning, can accordingly be extended. ©1997 Elsevier Science Ltd

On Schema Versioning in Temporal Databases

The support of schema versioning has been considered only to a limited extent in the literature on temporal databases. In particular, solutions were proposed so far for the management along transaction-time of schema versions, to be used as different interfaces on the same temporal data.

Cellular aided vehicular named data networking

With the recent new paradigm for the future Internet named data networking (NDN), contents are searched by names and not by address, and any node storing a content can also act as a source of information. Such a paradigm appears to be of particular interest for vehicular ad hoc networks (VANETs), to extend internet services to vehicles and support new mobile applications. In VANETs, devices not only use, but also generate contents; devices are generally not constrained in power or memory, thus easily carrying large amount of information; in addition, high mobility continuously creates new opportunities for data exchange. In such scenario, the TCP/IP networking paradigm shows some performance limitations and NDN appears as a way to improve reliability and efficiency of content distribution. However, when node density is limited, the exclusive use of short range communications might reduce the effectiveness of NDN. Motivated by this consideration, we propose the use of cellular networks to carry the signaling part of NDN, and the exploitation of short range wireless communications for content distribution. We will show, through simulations performed in an urban scenario with different vehicle densities, that moving the signaling part to cellular networks significantly improves the performance of NDN compared to the case when only short range communications are used.

Semantic Interoperability of Multitemporal Relational Databases

Interoperability is concerned with the interaction of databases of different kinds. In this paper we consider semantic interoperability between relational database systems of different temporal types (snapshot, transaction-time, valid-time or bitemporal), and between relations of different temporal types within the same system.

A dynamic hierarchical VANET architecture for Named Data Networking applications

In this paper a hierarchical architecture for content distribution in vehicular network is proposed. A cluster-based organization of vehicles is considered based on the Named Data Networking (NDN) model where a subset of reference vehicles named barycenters plays the NDN router role. Complementary functionalities of both short-range and cellular communications are used, the former for vehicle-to-vehicle (V2V), the latter for vehicle-to-infrastructure (V2I) communications, to off-load the fixed NDN infrastructure, and increase the chance of finding a content in the VANET, while limiting the number of broadcasted information. The proposed model is described and analytical evaluations are presented to show the potential of the approach.

Temporal Conversion Functions for Multitemporal Relational Databases

Most research in the field of algebras for temporal relational databases concerns the study of operations within each of the formats transaction-time, valid-time, bitemporal and snapshot. Nevertheless, data of different temporal format (multitemporal relational data) should be allowed to interact. Two possible scenarios for this problem are: (1) federated databases, where the interaction is required of databases of different temporal format; (2) databases supporting selective versioning, where, due to the different update frequencies, the attributes are maintained in tables of different temporal formats. A solution to this interoperability problem is neither straightforward nor univocal, since the semantics differences among such data are quite deep. As suggested in [DGS94] for the multitemporal natural join, a first step for the definition of a multitemporal relational algebra can be the conversion to a common temporal format for the execution of operations. The semantic criteria at the basis of the temporal conversion functions in [DGS94] have been long debated for their safeness in the deduction or reconstruction of temporal information. In this paper we revise such criteria, discuss their problems and present several alternative solutions.

A Modular History-Oriented Access Structure for Bitemporal Relational Databases

Two time dimensions are usually considered in temporal databases: transaction-time, which tells when an event is recorded in a database, and valid-time, which tells when an event occurs, occurred or is expected to occur in the real world. Transaction-time is defined by the system and can only grow, since it records the time of successive transactions. This peculiarity makes it difficult to adapt traditional access structures for non-temporal data, such as indices or grid files, since transaction-time unbalances such structures. The maintenance of access structures for temporal data can thus be very expensive, also due to the large amount of data contained in temporal relations. In this paper, we propose a variant of an index composed by as many sub-indices as the number of objects. The main feature of this structure is to localize the unbalancements and thus, whenever a sub-index needs reorganization, not to burden the whole structure.

Smart City Pilot Projects Using LoRa and IEEE802.15.4 Technologies

Information and Communication Technologies (ICTs), through wireless communications and the Internet of Things (IoT) paradigm, are the enabling keys for transforming traditional cities into smart cities, since they provide the core infrastructure behind public utilities and services. However, to be effective, IoT-based services could require different technologies and network topologies, even when addressing the same urban scenario. In this paper, we highlight this aspect and present two smart city testbeds developed in Italy. The first one concerns a smart infrastructure for public lighting and relies on a heterogeneous network using the IEEE 802.15.4 short-range communication technology, whereas the second one addresses smart-building applications and is based on the LoRa low-rate, long-range communication technology. The smart lighting scenario is discussed providing the technical details and the economic benefits of a large-scale (around 3000 light poles) flexible and modular implementation of a public lighting infrastructure, while the smart-building testbed is investigated, through measurement campaigns and simulations, assessing the coverage and the performance of the LoRa technology in a real urban scenario. Results show that a proper parameter setting is needed to cover large urban areas while maintaining the airtime sufficiently low to keep packet losses at satisfactory levels.

A First Approach to Temporal Predicate Locking for Concurrency Detection in Temporal Relational Databases Supporting Schema Versioning

Recently, a great deal of attention has been devoted to the problem of schema versioning. A possible solution [3, 7] maintains all the temporal intensional information in a unique “completed schemata” and all the underlying temporal data in a unique structure, here called “single-pool”. As in [3, 7], the data can be accessed through any schema version, thus the problem arises of the concurrency control of the access to the single-pool. A simple solution is here proposed that detects conflicts checking first the intensional and then, if needed, the extensional level.