Searching Moving Objects in a Spatio-temporal Distributed Database Servers System

Abstract

Querying about the time-varying locations of moving objects is particularly cumbersome in environments composed of a very large number of distributed spatio temporal database servers. In particular, searching for a specific object can require to visit each server. In this paper we propose a strategy to avoid such an exhaustive search that is based on the use of a centralized index, called meta-index, which is the entry point for spatio-temporal search queries. This index allows a software agent to determine a search plan for visiting the most likely servers to contain the target object. An important issue for large and dynamic distributed servers systems is to keep the meta-index as up-todate as possible with the real system. This paper defines and compares two different strategies for maintaining properly updated the meta-index: crawling, where the centralized system that keeps the index controls itself the updating process, and harvesting, where each distributed database server autonomously transfers data directly into the central index system. Both strategies were implemented and compared by using discrete-event simulators with demanding synthetic spatio-temporal data. The results show that crawling has better performance.

Complex Queries for Moving Object Databases in DHT-Based Systems

“…This value depends on the relative importance and observed activity in the server with respect to all other servers. Coordinators periodically determine the values of r c by computing the dynamic ranking of database servers along time, as we proposed for the centralized crawler in [8]. Basically, this method ranks servers considering how active objects are in their domain and what their relative importance is in terms of average number of objects.…”

“…A first evaluation was a comparison between results with the centralized crawler evaluated in [8] and the results obtained with the p2p crawler proposed in this paper. The differences in speed of crawling depend on the number of peers we deploy to be robots in our simulator.…”

“…We have reported work on object-location queries and aggregation queries, first for a centralized meta-index in [8], and then for a distributed meta-index on a p2p network in [6]. In this paper we extend the p2p meta-index proposed in [6] to include spatio-temporal range queries and top-k object queries.…”

A P2P Meta-index for Spatio-temporal Moving Object Databases

“…To maintain the meta-index, we define a crawling strategy that collects data from database servers asynchronously [3]. In every data collection from a server, a crawler transfers to the meta-index aggregated data and the ids of objects which have been in the server but not transfered in previous visits.…”

“…A preliminary study for this meta-index [3] analyzes the viability of creating and updating a centralized structure in a highly dynamic environment of moving objects. This paper continues with our previous work and describes the distributed meta-index structure using Chord [6].…”