An important issue in the design of a mobile computing system is how to manage the location information of mobile clients. In the existing commercial cellular mobile computing systems, a two-tier architecture is adopted. However, the two-tier architecture is not scalable. In the literatures, a hierarchical database structure is proposed in which the location information of mobile clients within a cell is managed by the location database responsible for the cell. The location databases of different cells are organized into a tree-like structure to facilitate the search of mobile clients. Although this architecture can distribute the updates and the searching workload amongst the location databases in the system, location update overheads can be very expensive when the mobility of clients is high. In this paper, we study the issues on how to generate location updates under the distance-based method for systems using hierarchical location databases. A cost-based method is proposed for calculating the optimal distance threshold with the objective to minimize the total location management cost. Furthermore, under the existing hierarchical location database scheme, the tree structure of the location databases is static. It cannot adapt to the changes in mobility patterns of mobile clients. This will affect the total location management cost in the system. In the second part of the paper, we present a reorganization strategy to restructure the hierarchical tree of location databases according to the mobility patterns of the clients with the objective to minimize the location management cost. Extensive simulation experiments have been performed to investigate the reorganization strategy when our location update generation method is applied.

In distributed computing systems, processes in different hosts take checkpoints to survive failures. For mobile computing systems, due to certain new characteristics such as mobility, low bandwidth, disconnection, low power consumption and limited memory, conventional distributed checkpointing schemes need to be reconsidered. In this paper, a novel min-process coordinated checkpointing algorithm that makes full use of the computation ability and power of mobile support stations is proposed. During normal computation message transmission, the checkpoint dependency information among mobile hosts is recorded in the corresponding mobile support stations. When a checkpointing procedure begins, the initiator concurrently informs relevant mobile hosts, which minimizes the identifying time. Moreover, compared with the existing coordinated checkpointing schemes, our algorithm blocks the minimum number of mobile support stations during the identifying procedure, which leads to the improvement of the system performance. In addition, the proposed algorithm is a min-process, domino-free checkpointing algorithm, which is especially desirable for mobile computing systems. Quantitative analysis and experimental simulation show that our algorithm outperforms other coordinated checkpointing schemes in terms of the identifying time and the number of blocked mobile support stations and then can provide a better system performance for mobile computing systems.

在实时事务执行时，事务故障或数据竞争会导致事务重启，为减少事务重启损失的工作量，可以采用检验点技术保证事务的时间正确性。在一类分布式实时数据库应用中，不同结点的事务通过消息交换形成合作关系，为保证合作事务间的全局一致性，当某一事务记检验点时，相关事务也要记检验点。传统协同检验点方法没有考虑应用的定时约束，不能很好地支持分布式合作实时事务处理。该文提出了一种基于图论的协同检验点方法，利用在每个计算结点上为每个合作事务集维护的局部有向图，使用一个基于图论的计算过程标识出应记检验点的事务，该方法既具有最小协同检验点特性，又使全局检验点的时延最小。实验表明该算法减少了全局检验点时延，有利于实时事务截止期的满足。