Data management technology that can support easy data access from and to mobile devices is among the main concerns in mobile information systems. Mobile computing may be considered a variation of distributed computing. The two scenarios in which mobile databases is distributed are: Among the wired components, the entire database is distributed, possibly with full or partial replication. A base station or fixed host manages its own database with a DBMS like functionality, with additional functionality for locating mobile units and additional query and transaction management features to meet the requirements of mobile environments.
Among the wired and wireless components, the database is distributed. Among the base stations or fixed hosts and mobile units, the data management responsibility is shared.
Here are some of the issues which arises in data management of the mobile databases:
1. Mobile database design –
Because of the frequent shutdown and for handling the queries, the global name resolution problem is compounded.
2. Security –
The data which is left at the fixed location is more secure as compared to mobile data. That is mobile data is less secure. Data are also becoming more volatile and techniques must be able to compensate for its loss. The most important thing needed in this environment is the authorizing access to critical data and proper techniques.
3. Data distribution and replication –
Uneven distribution of data among the mobile units and the base stations take place here. Higher data availability and low cost of remote access is there in data distribution and replication. The problem of Cache management is compounded by the consistency constraints. The most updated data and frequently accessed data is provided by the Caches to the mobile units. It process their own transactions. There is most efficient access of data and higher security is available.
4. Replication issues –
There is increase of costs for updates and signalling due to increase in number of replicas. Mobile hosts can move anywhere and anytime.
5. Division of labour –
There is a certain change in the division of labour in query processing because of certain characteristics of the mobile environment. There are some of the cases in which the client must function independently of the server.
6. Transaction models –
In mobile environment, the issues of correctness of transactions and fault tolerance are aggravated. All transactions must satisfy the ACID properties, these are atomic, consistency, isolation, and durability.
Depending upon the movement of the mobile unit, possibly on multiple data sets and through several base station, a mobile transaction is executed sequentially. When the mobile computers are disconnected, ACID properties gets hard to enforce. Because of the disconnection in mobile units, there is expectation that a mobile transaction will be lived long.
7. Recovery and fault tolerance –
Fault tolerance is the ability of a system to perform its function correctly even in the presence of internal faults. Faults can be classified in two types: transient and permanent. Without any apparent intervention, a transient fault will be eventually disappeared but a permanent fault will remain unless it is removed by some external agency.
The mobile database environment must deal with site, transaction, media, and communication failures. Due to limited battery power there is a site failure at MU. If a voluntary shutdown occurs in MU, then it should not be treated as a failure. Whenever Mu crosses the cells, most frequently there will be a transaction failures during handoff. Due to failure of MU, there is a big cause of network partitioning and affection of the routing algorithms. The characterization of mobile computing is done by:
- Limiting resource availability
- Frequent disconnection
- High mobility
- Low bandwidth
8. Location based service –
One of the most challenging tasks which must be undertaken is determining the location of mobile users, which must be undertaken in order to enable a location based service. A cache information becomes sale when clients move location dependent. Eviction techniques are important in this case.Issues that arises in location and services are:
- User Privacy
- Diverse mobile mapping standards
- Market capability
Updation of the location dependent queries and then applying spatial queries to refresh the cache causes a problem.
9. Query processing –
Because of the mobility and rapid resource changes of mobile units, Query optimization becomes the most complicated. That is query processing is affected when mobility is considered. There is a need to returned a query response to mobile units that may be in transit.The cost that affects the most in centralized environments is the input/output.
Communication cost is the most important in distributed environments. It is possible to formulate location dependent queries. There is difficulty in estimating the communication costs in distributed environments because the mobile host may be situated in different locations. There is a requirement of dynamic optimization strategies in the mobile distributed context.
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