TRENDS IN DATABASE TECHNOLOGY - Database Design and Development: An Essential Guide for IT Professionals - page 670

Types of data content In a mobile computing environment, you can have private
data owned, managed, and restricted to a single user. Public data is maintained at
one unit, such as stock prices, airline fares, and so on. Public data is available for
access by any user. Another type of data content may be read and updated by any
user. Product inventory falls in this shared category.

Application types Vertical applications are restricted within a cell. For example,
local information about emergency services and accommodation information is
valuable only within small areas. On the other hand, horizontal applications are
meant for users in all cells.

Data distribution On method is to distribute the data only among the fixed hosts
and support stations, using full or partial replication techniques. This is the popular
method. The other method is to distribute data among all units—fixed and mobile.
Data management is more difficult in this approach.

Data synchronization Frequently, users download relevant data from the common
database and keep the downloaded data for off-line work. This adds another dimen-
sion to data management. The downloaded data must be kept synchronized with
the main database.

Query processing Query processing costs depend on the location of the mobile
unit. Another consideration of query processing relates to the requirement that
complete and correct query results must be provided even while the mobile unit is
moving from cell to cell.

Transaction processing A transaction emanating from a mobile unit may be exe-
cuted through several support stations and perhaps on different copies of the data-
base fragments. Traditional ACID properties of transactions do not apply to the
mobile computing environment.

Recovery Recovery from failures becomes quite involved. Failures at mobile units
commonly result from battery failure. Support station failures cause routing read-
justments. In this environment, recovery has to address many types of failure—
mobile unit,

fixed host, support station, network links, wireless links, and

transactions themselves.

Geographic Databases

Geographic databases store and manage spatial data. Examples of spatial data
in geographic databases include maps with political boundaries and administra-
tive boundaries, digital satellite images showing natural contours, transportation
networks, highways, rivers, and so on. These databases also hold another type
of data that is nonspatial but related to the spatial data. This second type of
data includes census counts, population distribution by economic factors, and sales
data.
Geographic databases support a wide variety of applications. They range exten-
sively in scope and use. Your latest automobile is probably fitted with a GPS (Global