DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 555

made in a synonym table at the LONDON site for MARY to refer to the ORDER
relation as M-ORD. Mary's queries will use the simpler name of M-ORD. When we
mention queries here, we do not confine data access requests only to queries. The
discussion applies equally to all transactions.
Network Component
Effective implementation of a distributed database depends on a good data com-
munications network. A distributed database system is built over a communications
network. The various sites have to be linked up properly so that pertinent portions
of the database may be stored at the respective sites. The communications network
must enable efficient flow of data from one site to another.
Data communications have made great progress over the past few years. We are
not purely interested in the various communications protocols and their specifica-
tions. However, we want to examine a few aspects of data communications as they
pertain to distribution of data in a distributed database environment. Two major
aspects are of importance from this standpoint: the linking of sites and the routing
of data.
Linking of Sites On the basis of whether the sites to be linked are within a
short distance or further apart, data communications networks are classified as
follows:
Local area network (LAN). Designed for connecting computers within the same
premises.
Wide area network (WAN). Intended for linking computers or LANs at vast dis-
tances from one another.
Figure 18-7 presents a comparison of the important features of LAN and WAN
arrangements.
Routing of Data Routing implies choosing a path on the network for the move-
ment of data. For this purpose, we may design a network either as a point-to-point
network or a broadcast network. For sending a single message to more than one
site in a point-to-point network, the sending site must send separate messages to
each of the receiving sites. In a broadcast network, the ID of the destination site
is attached as a prefix to the message. Virtually each message is sent around so
that all sites may listen in. The site for which the message is intended picks up the
message.
Configuration Options Figure 18-8 shows the common options for configuring a
network for a distributed database. The nature of the organization, the distribution
of data, and the data access pattern dictate the choice of the configuration.
Let us quickly review the configurations.
Fully connected. Each site is connected to every other site. Highly reliable and flex-
ible, but expensive.