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.
