Scheduling transactions in mobile distributed real-time database systems

A DMVOCC-MVDA （distributed multiversion optimistic concurrency control with multiversion dynamic adjustment） protocol was presented to process mobile distributed real-time transaction in mobile broadcast environments. At the mobile hosts, all transactions perform local pre-validation. The local pre-validation process is carried out against the committed transactions at the server in the last broadcast cycle. Transactions that survive in local pre-validation must be submitted to the server for local final validation. The new protocol eliminates conflicts between mobile read-only and mobile update transactions, and resolves data conflicts flexibly by using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. Mobile read-only transactions can be committed with no-blocking, and respond time of mobile read-only transactions is greatly shortened. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. In global validation mobile distributed transactions have to do check to ensure distributed serializability in all participants. The simulation results show that the new concurrency control protocol proposed offers better performance than other protocols in terms of miss rate, restart rate, commit rate. Under high work load （think time is ls） the miss rate of DMVOCC-MVDA is only 14.6%, is significantly lower than that of other protocols. The restart rate of DMVOCC-MVDA is only 32.3%, showing that DMVOCC-MVDA can effectively reduce the restart rate of mobile transactions. And the commit rate of DMVOCC-MVDA is up to 61.2%, which is obviously higher than that of other protocols.

Quasi serializable concurrency control in mobile broadcast environments

The problem of maintaining data consistency in mobile broadcast environments is researched. Quasi serializability is formally defined and analyzed at first. It was shown that quasi serializability is less stringent than serializability when database consistency is maintained for transactions. Then, corresponding concurrency control protocol that supports both update transactions and read-only transactions is outlined for mobile broadcast environments. Finally, the simulation results confirmed that the proposed protocol could improve the response time significantly.

Weak Serializable Concurrency Control in Real-time Broadcast Environments

Given the asymmetric communication capabilities in mobile real-time environments, traditional serializability-based approaches are too restrictive, unnecessary, and impractical. The new notion of correctness called weak serializability is given. After the necessary and sufficient conditions for weak serializability are shown, corresponding concurrency control protocol based on this criterion is outlined for real-time broadcast environments. The simulation results confirmed that the proposed techniques help mobile transactions to meet their deadlines and improve query response time.

An Efficient Data Dissemination Scheme for Spatial Query Processing

Due to the personal portable devices and advances in wireless communication technologies, Location Dependent Information Services （LDISs） have received a lot of attention from both the industrial and academic communities. In LDISs, it is important to reduce the query response time, since a late query response may contain out-of-date information. In this paper, we study the issue of LDISs using a Voronoi Diagram. We introduce a new NN search method, called the Exponential Sequence Scheme （ESS）, to support NN query processing in periodic broadcast environment. This paper aims to provide research directions towards minimizing both the access latency and energy consumption for the NN-query processing.

Efficient Data Access for Location-Dependent Spatial Queries

When the mobile environment consists of light-weight devices, the loss of network connectivity and scarce resources, e.g., low battery power and limited memory, become primary issues of concern in order to efficiently support portable wireless devices. In this paper, we propose an index-based peer-to-peer data access method that uses a new Hierarchical Location-Based Sequential （HLBS） index. We then propose a novel distributed Nearest First Broadcast （NFB） algorithm. Both HLBS and NFB are specifically designed for mobile peer-to-peer service in wireless broadcast environments. The system has a lower response time, because the client only contacts a qualified service provider by accessing the HLBS and quickly retrieves the data to answer the query by using NFB. HLBS and NFB design the index for spatial objects according to the positions of individual clients and transfer the index in the order arranged so that the spatial query can be processed even after the user tunes the partial index. Hence, this design can support rapid and energy-efficient service. A performance evaluation is conducted to compare the proposed algorithms with algorithms based on R-tree and Hilbert-curve air indexes. The results show that the proposed data dissemination algorithm with the HLBS index is scalable and energy efficient in both range queries and nearest neighbor queries.

On-demand data broadcast scheduling based on AHP and GRA methods in wireless broadcast/UMTS integrated systems

The increasing demand for interactive mobile multimedia service is causing the integration of 3rd generation （3G） cellular systems and wireless broadcast systems. The key challenge is to support data dissemination with low response time, request drop rate, and the unfairness of request drop. This article proposes a novel scheduling algorithm called DAG （on-demand scheduling utilizing analytic hierarchy process （AHP） and grey relational analysis （GRA））, which takes multiple factors--waiting time, number of active requests, deadline--into consideration, and models the data scheduling process as a multiple factors＇ decision-making and best option-selecting process. The proposed approach comprises two parts. The first part applies AHP to decide the relative weights of multiple decision factors according to user requests, while the second adopts GRA to rank the data item alternatives through the similarity between each option and the ideal option. Simulation results are presented to demonstrate that DAG performs well in the multiple criterions mentioned above.