【作者】 侯卫红；

【导师】 杨进才；

【作者基本信息】 华中师范大学 ， 计算机应用， 2008， 硕士

【摘要】 随着无线通信技术和移动设备的飞速发展,移动环境日益成熟,移动计算成为一个新兴的热点研究领域。研究移动数据库技术,即支持移动环境的分布式数据库技术,已成为目前分布式数据库研究的一个新的方向。在移动数据库的关键技术中,缓存技术具有非常重要的地位,移动数据库的一个主要的目标就是要保证移动用户在断接情况下对数据的有效访问。因此,在客户端缓存经常存取的数据是一项提高移动环境性能的有效技术。融合移动数据库的缓存与广播技术,建立了一种缓存与广播相结合的系统结构。该系统结构分为固定网络和无线网络两部分。固定网络,即连接固定设备的网络,连接着若干个服务器及本地移动支持站MSS。本地MSS负责连接广播服务器与移动客户机通信;无线网络,即移动设备组成的网络,无线网络中的移动客户机通过本地MSS与广播服务器通信,来维护缓存数据与服务器端数据的一致性。在分析传统的基于失效报告的缓存失效机制的基础上,提出了一种移动数据库的自适应缓存管理机制(ACMS),通过在时间戳上引入计数器来记录向服务器提出请求的移动客户机数目。在服务器端,可以根据存取数据的移动客户机的数目,动态调整失效报告的内容及广播间隔,发送给移动客户机;在客户端,移动客户机根据接收到的内容,通过接收到的时间戳及时间间隔值来判断缓存中的内容是否有效,修改缓存的状态,从而实现缓存的动态管理。对提出的机制,通过仿真实验进行了评估,实验结果表明这种方法能更有效地适应移动环境中网络带宽不足、上下行链路的不对称性及网络的频繁断接性等特点,有效地提高带宽利用率,从而提高移动环境下缓存的效率,提高移动数据库数据的可用性、一致性。更多还原

【Abstract】 With the development of the wireless communication technology and mobile devices, the mobile environment increasingly mature, and mobile computing has become a hot research topic today. The research on mobile database technology that also called distributed databases technology which supports the mobile computing environment has become a new direction of the distributed database. Cache technology is one key technology of mobile database. One main object of mobile database is to ensure the validity of accessed data under disconnection. So caching of frequently accessed data at the mobile clients is an effective technology to improve mobile environmental performance.A new model is presented on the basis of cache and data broadcast, which includes two parts: the fixed network and the wireless network. The fixed network, connecting the fixed devices, includes several broadcast server and Home Mobile Support Station. The MSS is responsible for connecting the broadcast server and mobile clients. The wireless network, composed of mobile devices, enables mobile clients communicating with the broadcast server by MSS to maintain data consistency between cache and the server.A new adaptive cache management scheme (ACMS) for mobile database is proposed by analysis of the traditional cache invalidation mechanisms based on invalidation reports. In this mechanism, by utilizing a counter on the basis of the timestamps to record the number of mobile clients requesting to the server; at the server side, dynamically adjust the contents of invalidation reports and broadcast interval in accordance with the number of mobile clients accessing the data, and send to mobile clients; at the client side, the mobile clients judge the validity of the contents in cache by the timestamps and broadcast interval, and amend the state of cache so as to implement the dynamic management of the cache.The proposed algorithm is evaluated by experimental simulation, results show that this method can adapt to the characteristics of the mobile environment much more effectively, such as the deficiency bandwidth, the asymmetry of up-down-stream link, and the frequent disconnect of the network, and effective improve the bandwidth utilization, thereby enhancing the efficiency of the cache technology, data availability and consistency of mobile database.更多还原