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Detailed Explanation of Mobile Agent Embedded Mobile Database and Implementation of Query Optimization
With the rapid advancement and widespread integration of network technology, accessing the internet to obtain information anytime and anywhere has become a fundamental expectation for people in the 21st century. Alongside this, the development of mobile communication technologies and the growing demand for processing mobile data have led to increased attention on embedded mobile database systems. These systems, tightly integrated with intelligent devices such as smartphones, PDAs, and in-vehicle computers, are now receiving significant focus from academia, industry, military, and civilian sectors. Mobile computing and mobile database technologies are essential in meeting these evolving needs.
A mobile database is essentially a distributed database that operates within a mobile computing environment. Since these databases are often embedded in mobile communication devices like handheld computers and PDAs, they are also known as embedded mobile databases. Historically, research into distributed databases was based on wired networks and fixed hosts, relying on assumptions such as stable network connections and consistent communication costs. However, with the emergence of mobile communication technologies and the proliferation of mobile devices since the 1990s, many nodes can now connect to the network dynamically, making traditional assumptions less valid. The mobile computing environment presents unique challenges: mobility, limited bandwidth, frequent disconnections, asymmetric communication, and constrained power resources. As a result, traditional distributed database techniques are being adapted to suit these new conditions. Today, the study and application of mobile databases have become a key area of academic interest, drawing researchers from various fields.
**Embedded Mobile Database Architecture**
In a traditional distributed system, each node is connected via a fixed network and maintains stable connectivity. In contrast, a mobile computing system introduces a more dynamic structure, consisting of both fixed and mobile nodes. The mobile computing environment is characterized by mobility, intermittent connectivity, varying bandwidth, scalability, weak reliability, asymmetric communication, and limited power. A mobile database is a type of distributed database designed for such environments, functioning as a distributed system where clients can dynamically connect to fixed server nodes. The architecture of a mobile database system is illustrated in Figure 1.
The mobile client (MC) includes portable devices like laptops and PDAs, while the Mobile Support Station (MSS) serves as a fixed node with wireless communication capabilities. The Fixed Host (FH), on the other hand, does not have wireless interfaces and is equipped with a database and its management system.
**Key Technologies of Embedded Mobile Databases**
To support mobile databases, several critical technologies must be addressed, including handling the challenges posed by mobility, disconnection, and asymmetric communication.
**(1) Data Replication and Caching**
This technology plays a crucial role in ensuring data availability in mobile environments. Unlike traditional replication methods, which assume constant connectivity, mobile systems require mechanisms that allow data to be cached and replicated across different locations. Research in this area includes Gray’s two-tier replication model, the CODA system, and broadcast-based cache invalidation. Companies like Sybase have also implemented these techniques in their mobile database products, such as SQL Anywhere and SQL Remote.
**(2) Data Broadcasting**
Data broadcasting involves sending data to mobile clients through periodic broadcasts, taking advantage of the asymmetry in communication between the client and the server. This approach reduces overhead and helps mitigate disconnection issues. Researchers like Academician Zhou Xingming have contributed significantly to this field, proposing algorithms for efficient data scheduling. Key performance metrics include access time and tuning time.
**(3) Location-Based Query Optimization**
Mobile databases often handle queries related to location, such as "Where is the nearest hospital?" These queries depend on the user's current position, and results can vary based on location. Query optimization techniques help adapt traditional distributed query methods to account for factors like limited bandwidth and intermittent connectivity. Additionally, energy-efficient management of local and remote database operations is essential to extend battery life and ensure continuous service.
When using a split-based address update policy, the location server maintains information about the mobile user, including the partition set, recent location, and current division. A method called Loc() retrieves the actual location of the user, enabling efficient and accurate query responses.