Search

The rapid advances in wireless communications technology and mobile computing have enabled personal mobile devices that we use in everyday life to become information and service providers by complementing or replacing fixed-location hosts connected to the wireline network. Such mobile resources is highly important for other moving users, creating significant opportunities for many interesting and novel applications. The MobiShare architecture provides the infrastructure for ubiquitous mobile access and mechanisms for publishing, discovering and accessing heterogeneous mobile resources in a large area, taking into account the context of both sources and requestors. Any wireless communication technology could be used between a device and the system. Furthermore, the use of XML-related languages and protocols for describing and exchanging metadata gives the system a uniform and easily adaptable interface, allowing a variety of devices to use it. The overall approach is data-centric and service-oriented, implying that all devices are treated as producers or requestors of data wrapped as information services.

Temporal document databases are interesting in a number of contexts, in general document databases as well as more specialized applications like temporal XML/Web warehouses. In order to efficiently manage temporal document versions, a temporal document database system should be employed. In this paper, we describe the V2 temporal document database system, which supports storage, retrieval, and querying of temporal documents. We also give some performance results from a mini-benchmark run on the V2 prototype.

In a database system, read operations are much more common than write operations, and consequently, database systems have been read optimized. As the size of main memory increases, more of the database read requests will be satisfied front the buffer system, and the amount of disk write operations relative to disk read operations will increase. This calls for a focus on write optimized database systems. In this paper, we present solutions to this problem. We describe in detail the data structures and algorithms needed to realize a write optimized object-oriented database system in the context of Vagabond, an OODB currently being implemented at our department. In Vagabond, focus has been to provide support for applications which have earlier used file systems because of the limited data bandwidth in current database systems, typical examples are supercomputing applications and geographical information systems.

As main memory capacity increases, more of the database read requests will be satisfied from the buffer system. Consequently, the amount of disk write operations relative to disk read operations will increase. This calls for a focus on write optimized storage managers. We show how the Vagabond object storage manager uses no-overwrite sequential writing of long blocks to achieve high write performance. Vagabond also supports versioned/temporal objects, with the no-overwrite policy used, this does not imply any extra cost. Large objects, e.g., video and matrices, are divided into large chunks. This makes it easy to achieve high read and write bandwidth. This is important, since in many application areas, high data bandwidth is just as important as high transaction throughput. The buffer system in Vagabond is object based, rather than page based. This gives better utilization of main memory. Transparent compression of objects on disk is supported.

In an object database system using logical OIDs, an OID index (OIDX) is necessary to map from logical OID to the physical location of an object. In a temporal object database system (TODB), this OIDX also contains the timestamps of the object versions. OIDX maintenance can be very costly, and can easy become the bottleneck of such a system. The main reason for this, is that in a TODB the OIDX needs to be updated every time an object is updated. In order to reduce the access costs, a new index structure, particularly suitable to TODB requirements, is necessary. In this paper, we describe an OIDX for TODBs, the Vagabond Temporal OID Index (VTOIDX). The main goals of the VTOIDX are: 1) support for temporal data, while still having index performance close to a non-temporal (one-version) database system, 2) efficient object-relational operation, and 3) flexible tertiary storage migration of partitions of the index. In this paper, we describe the physical organization and the operations of the VTOIDX.

An alternative to in-place updating of data is to eliminate the database completely, and use a log-only approach. The log is written contiguously to the disk, in a no-overwrite way, in large blocks. The log-only approach is particularly interesting for transaction time object database systems (TODB), because keeping previous versions of objects is a feature that comes for free. One of the objections against log-only databases has been that the read cost will be prohibitively high because of loss of clustering. However in our comparison of the performance of log-only and in-place update TODBs, the analysis shows that with the workload we expect to be typical for future TODBs, the log-only approach is highly competitive with the traditional in-place update approach.