Data Management Issues

From a data management standpoint, mobile computing may be considered a variation of distributed computing. Mobile databases can be distributed under two possible scenarios:

1. The entire database is distributed mainly among the wired components, 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.

2. The database is distributed among wired and wireless components. Data management responsibility is shared among base stations or fixed hosts and mobile units.

(All the Distributed data management issues can be applied to Mobile)

• Data distribution and replication: Data is unevenly distributed among the base stations and mobile units. The consistency constraints compound the problem of cache management. Caches attempt to provide the most frequently accessed and updated data to mobile units that process their own transactions and may be disconnected over long periods.
• Transaction models: Issues of fault tolerance and correctness of transactions are aggravated in the mobile environment. A mobile transaction is executed sequentially through several base stations and possibly on multiple data sets depending upon the movement of the mobile unit. Central coordination of transaction execution is lacking, particularly in scenario (2) above. Moreover, a mobile transaction is expected to be long-lived because of disconnection in mobile units. Hence, traditional ACID properties of transactions may need to be modified and new transaction models must be defined.
• Query processing: Awareness of where the data is located is important and affects the cost/benefit analysis of query processing. Query optimization is more complicated because of mobility and rapid resource changes of mobile units. The query response needs to be returned to mobile units that may be in transit or may cross cell boundaries yet must receive complete and correct query results. 
• Recovery and fault tolerance: The mobile database environment must deal with site, media, transaction, and communication failures. Site failure at an MU is frequently due to limited battery power. If an MU has a voluntary shutdown, it should not be treated as a failure. Transaction failures are more frequent during handoff when an MU crosses cells. MU failure causes a network partitioning and affects routing algorithms.
• Mobile database design: The global name resolution problem for handling queries is compounded because of mobility and frequent shutdown. Mobile database design must consider many issues of metadata management—for example, the constant updating of location information.
• Location based service: As clients move location-dependent cache information may become stale. Eviction techniques are important in this case. Furthermore, updating location dependent queries, then applying these (spatial) queries in order to refresh the cache also poses a problem.
• Division of labor: Certain characteristics of the mobile environment force a change in the division of labor in query processing. In some cases, the client must function independent of the server. However, what are the consequences of allowing full independent access to replicated data? The relationship between client capabilities and their consequences have yet to be developed.
• Security: Mobile data is less secure than that which is left at the fixed location. Proper techniques for managing and authorizing access to critical data become more important in this environment. Data are also more volatile, and techniques must be able to compensate for its loss.