With the rapid development and continuous penetration of network technology, access to the network to obtain various information at any place and at any time will become the common requirement of human beings in the 21st century. At the same time, the progress of mobile communication technology and people's processing of mobile data With the continuous improvement of demand, the embedded mobile database technology closely integrated with various intelligent communication devices has been highly valued by academics, industry, military, and civilian departments. Mobile computing and mobile database technologies will enable this need.
Mobile databases are distributed databases in mobile computing environments. Since mobile database applications are mostly embedded in mobile communication devices such as handheld computers, PDAs, and in-vehicle devices, mobile databases are sometimes referred to as embedded mobile databases. In the research history of database systems, the traditional research of distributed computing and distributed databases is based on wired networks and fixed hosts. These use some default implicit assumptions, such as fixed network connections, peer-to-peer communication costs, and fixed host nodes. However, since the 1990s, with the rapid development of mobile communication technology and network technology, coupled with the proliferation of mobile computers and mobile communication devices, many computing nodes can establish connections with the network during the mobile process, making the above assumptions unsatisfactory. The mobile computing environment has the characteristics of mobility, low bandwidth, frequent disconnection, asymmetry of network communication, and limited power supply, so that the methods and techniques in the traditional distributed database are continuously applied to the mobile database. At present, the application and research of mobile database is becoming a research hotspot in academia, and a large number of scholars have invested in this new research field.
In a traditional distributed computing system, each computing node is connected through a fixed network and maintains the technical connectivity of the network, and the mobile computing system changes this assumption. The mobile computing system is a bifurcation computing system composed of fixed nodes and mobile nodes. The mobile computing network environment has distinct characteristics: mobility, disconnection, bandwidth diversity, scalability, weak reliability, asymmetry of network communication, and limitations of power capabilities. A distributed database in a mobile environment is a mobile database. It is an extension of the traditional distributed database system and can be seen as a distributed system where clients are dynamically connected to fixed server nodes. The structure of the mobile database system is shown in Figure 1.
Among them, mobile client MC (Mobile Client) includes portable computers, PDAs, etc.; MSS (Mobile Support StaTIon) supports mobile computing fixed nodes, with wireless communication interface; FH (Fixed Host) does not have wireless communication interface, installed database and database Management system.
Mobile database system architecture
In order to realize the mobile database, it is necessary to solve the impact of factors such as the discontinuity of the mobile computing environment, mobility, and the asymmetry of network communication on the mobile database system.
(1) Data replication and caching technology
This technology is the key technology to solve the data connection of mobile databases. It is also used to adapt to the existing DBMS to adapt to mobile computing. Traditional replication/caching techniques assume that the client and server are constantly connected and maintain consistency based on this premise. This is not applicable in mobile computing. At present, the most representative research on data replication/caching technology for mobile computing features is: J.Gray's two-level replication mechanism, CODA system, and cache invalidation report broadcast technology. In addition, Sybase's mobile database products SQL ANYWHERE and SQL REMOTE also use this technology to support mobile computing environments.
(2) Data broadcasting
In general, data broadcasting refers to the use of asymmetry in communication between a client and a server in a mobile computing environment to send data to a client in the form of a periodic broadcast. The biggest advantage is that the broadcast overhead does not depend on the change of the number of mobile users. With the data broadcast, the disconnection problem of the mobile database system can be solved to some extent. The research of data broadcasting can be divided into two aspects: server and client: the server mainly considers how to use local cache to further reduce the time for querying broadcast data. In China, Academician Zhou Xingming of Changsha International University of Science and Technology conducted in-depth research on data broadcasting and proposed a multi-disc scheduling algorithm for data broadcasting. The parameters that measure the quality of the data broadcast scheduling algorithm are access time and tuning time.
(3) Location-related query optimization
In the mobile database, there are queries and updates to location-related information. Queries are usually location-dependent. Even in the same question, the results of the query are different in different places, such as "Where is the nearest hospital?". Mobile query optimization technology refers to the traditional distributed database query optimization technology, using a variety of methods to eliminate the impact of bandwidth diversity, disconnection and other factors, so that the query engine can take appropriate optimization strategies according to the currently available network conditions. At the same time, for the limited power supply capacity of mobile computers, reasonable management of local database management, remote database access and other energy-intensive operations to achieve energy-saving purposes and extend the available time of critical data.
When using a split-based address update policy, the mobile user object maintained by the location server contains the following data members and methods:
Split Set - Record the partitioning of the MSS, such as {Cell1, Cell2}, {Cell3, Cell4, Cell5};
LOC - records the address recently reported by the mobile user (the ID of the wireless unit), such as Cell1;
ERR - the current division of the mobile user, for example, if LOC = Cell1, then ERR = {Cell1, Cell2};
Loc() - A method that returns the actual address of the user, the address query procedure described above.
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