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。

　随着移动通信技术的迅速发展，人们提出了新的应用要求：在移动环境下处理实时事务。而移动通信的带宽有限性引起较大的数据访问延迟，有时甚至由于网络传输的断接而使得事务得不到所需访问的数据，这些严重影响了移动环境下实时事务截止期的满足。实时事务的时间要求（典型地为截止性）的满足通常比事务执行的逻辑结果的完全正确更为重要。该文提出了移动环境下实时事务的“相对”时间正确性标准，只要事务访问的数据的陈旧度（staleness）在控制的范围内，就不一定要求实时事务访问的数据在事务提交时是有效的。文中提出了移动实时事务的相对时间正确性标准、保证事务相对时间正确性标准的算法及移动实时事务执行模型。所有这些策略一方面降低了移动实时事务的并发控制代价，另一方面以牺牲事务的“绝对”时间正确换取事务截止期的满足，这与实时数据库“宁愿要部分正确但及时的结果而不愿要绝对正确但过时的结果”的系统目标是相一致的。实验表明：相对时间正确性标准的提出及相应的事务执行模型能提高系统中满足截止期事务的比率，从而提高了系统性能。

实时数据库系统事务调度一般只考虑事务的截止期，这对保证满足事务访问数据的时间正确性是不够的。文中针对时序数据的特点给出了同时考虑事务的截止期入“数据截止期”的事务优先级分派、调度及并发控制方法。

实时数据库系统中事务可能有定时限制（典型地为截止期），事务超过截止期可能给系统带来灾难性后果。事务不光要满足数据库的完整性一致性，而且要满足在时间上的正确性以及事务之间在结构上的正确性。传统的事务不处理方法仅着眼于事务存取数据库的正确性，对于时间正确性与结构正确性无能为力。详细讨论了实时事务的正确性，包含结果正确性、时间正确性、行为正确性及结构正确性。已有的研究成果中大多用不同的算法及策略来保证不同的正确性要求：给出了保证实时事务正确性的一个统一的图论算法。

实时数据库系统中事务可能有定时限制（典型地为截止期），事务超过截止期可能给系统带来灾难性后果。这就要求系统能对事务的执行进行预分析与预安排。传统事务处理由于事务执行过程中的多种不确定因素（数据I/ O、页面缺失、事务之间的各种相关性等），事务执行具有动态不可预报性。提出了一种事务预分析处理的三段法：数据说明阶段的静态预分析提取事务静态信息；初始动态预分析阶段构造事务的“影子事务”，并在适当的时机调度影子事务执行，以保证事务操作之前所需数据已在内存；事务执行时动态预分析阶段构造事务的相关图，为事务执行的结构、行为及时间正确性提供统一的实现机制。事务执行具有更好的可预报性，从而事务能更好地满足截止期。

Data broadcast is a widely accepted data dissemimation method for mobile computing systems. When data broadcast is used to deliver frequently updated data to mobile read-only transactions, we call it updates dissemination. Existed updates dissemination protocols are unsuitable for mobile real-time read-only transaction processing since they neglect the time constraints on both data and transaction. In this paper a new updates dissemination protocol called hybrid forward forward multi-version data broadcast (HFMVB) is proposed. HFMVB not only guarantees the consistency of read-only transactions, it also provides higher data curency and lower miss rate by shortening the consistency interval, instantly broadcasting updates and making use of broadcast on-demand.