Mobile Database Overview of Mobile Database

As an extension and extension of distributed database, mobile database has many advantages and unique characteristics of distributed database, which can meet the demand of people accessing information in the future and has broad application prospects. A typical mobile database prototype system structure is shown in the figure.

Mobile database can be regarded as an extension of traditional database system. Mobile database systems can be roughly divided into the following categories:

A. mobile multi-database system.

B. Mobile homogeneous database system.

C. Mobile heterogeneous database system.

D. mobile heterogeneous multi-database system. Mobile database is basically composed of three types of hosts: mobile host, mobile support station and fixed host.

A fixed host is a computer in the usual sense. They are connected through a high-speed fixed network and cannot manage mobile devices. The mobile support station has a wireless communication interface, which can communicate data with mobile devices. The communication between the mobile support site and the fixed host is carried out through the fixed network. The area covered by the mobile support site is called a cell, and the mobile host in the cell can communicate with the mobile support site covering this area through the wireless communication network to complete the retrieval of information and data. (1) replication and caching technology. In the mobile database environment, the response time is improved by adopting a weakly consistent server-level replication mechanism. Caching technology is to reduce the frequency of customers accessing the database server by caching some data on the client's data server.

(2) Data broadcasting technology. Taking advantage of the network asymmetry that the downlink bandwidth from the server to the mobile client is much larger than the uplink bandwidth from the mobile client to the server, the data frequently accessed by most mobile users are organized and provided to the mobile client in the form of periodic broadcast.

(3) Location management. The location management of mobile users mainly focuses on two aspects: one is how to determine the current location of mobile users, and the other is how to store, manage and update location information. The mobile computer can be permanently registered on its own home server, and when it moves to any other area, it will inform its home server of its current location.

(4) Query processing and optimization. In the mobile database environment, due to the mobility of users, frequent disconnection and the diversity of network environment in which users live, mobile query optimization must adopt dynamic strategies to adapt to the changing environment.

(5) Mobile transactions. (1) data consistency. A notable feature of mobile database is the weak connection between mobile terminal and server, that is, low bandwidth, long delay, instability and frequent disconnection. In order to support users to operate the database in a weak environment, optimistic replication method is widely used. It allows users to operate the data copy on the local cache, exchange data modification information with the database server or other terminals after network reconnection, and coordinate and restore data consistency through conflict detection mechanism.

(2) Efficient transaction processing. A new transaction management strategy and algorithm are designed and implemented by adopting perfect log strategy and real-time updating of user location attributes during transaction movement.

(3) Data security. By authenticating the mobile terminal, the fraudulent access of illegal terminals is prevented; Encrypt wireless communication to prevent data information from leaking; Encrypt and store the downloaded data copy to prevent data leakage after the physical loss of the mobile terminal and ensure data security. (1) miniaturization of kernel structure.

(2) Support standard SQL.

(3) Strengthen the transaction management function.

(4) Perfect data synchronization mechanism.

(5) Support serial communication, TCP/IP communication, infrared, Bluetooth and other connection protocols.

(6) Complete embedded database management function.

(7) Support popular embedded operating systems such as Windows CE and Palm OS. (1) data distribution and replication. Data is unevenly distributed in base stations and mobile units. A remarkable feature of mobile database is that the connection between mobile terminal and server is weak, that is, broadband, long delay, instability and frequent disconnection. Due to the constraint of database consistency, it is more difficult to manage cache. Now, the commonly used method allows the mobile unit to handle its own affairs and operate on the data copy on the local cache. When disconnected for a long time, the cache attempts to provide the most frequently accessed data or updated data to the mobile unit. . After reconnecting to the network, exchange data modification information with the database server or other terminals, and restore data consistency through conflict detection and coordination.

(2) trading mode. In the mobile environment, fault tolerance and error correction of transactions become very difficult. Mobile transactions are executed in sequence on several base stations. Due to the mobility of mobile devices, the execution process may involve multiple data sets, lacking centralized coordination of transaction execution. Therefore, it is necessary to modify the traditional ACID nature of things and define new transaction management strategies and algorithms.

(3) Query processing. Knowing the storage location of data will affect the cost performance analysis of query processing. Due to the mobility of mobile units and rapid resource changes, query optimization becomes more complicated. When the query results are returned to mobile units, these mobile units may be moving or crossing cell boundaries, but the query results received by users must be correct and complete.

(4) reply and fault tolerance. The mobile database environment must solve the problems caused by address errors, media failures, transaction and communication failures.

(5) Design of mobile database. Due to mobility and frequent shutdown, the problem of executing queries is that global name resolution becomes complicated. In the design of mobile database, many metadata management problems must be considered.

(6) Location-based services. As the client moves, the location-related cache information becomes invalid. In addition, frequently updating location-dependent queries and then applying them to update the cache can also cause problems.

(7) Safety. The security of mobile data is not as good as that of fixed location data.