ing maps around the world, including maps of the
continents, regions and the globe. The other data-
base maintains the directory, catalogue, etc., of
data sets in other databases. This description
includes information such as who owns the data
set, how it is organized, how much it will cost to
access and how it can be accessed, etc. The first
database is internal to the MetaView/GIS system,
and users cannot therefore influence its organiza-
tion and/or modification. The metadata database is
however available for updates by certain category
of users.
One unique characteristics of MetaView/GIS, is
that, beside the graphical interface of map display,
region selection, point-and-click selection using a
mouse device and a menu selection, advanced
users can utilize the full power of SQL to specify
customized queries. There are two main users of
the MetaView/GIS system. Those who will main-
tain the data in the database and those who will
interact with the database to access information
about data sets and databases.
The MetaView/GIS software is organized on a cli-
ent/server model. The server runs at a site having
typically a huge volume of data to be sold or made
publicly available. The server has the MetaView/
GIS databases. A client (which is free to access
information) runs at the user’s site. The user
invokes the MetaView/GIS system by issuing the
command xmvgis. Queries by the users are bundled
and transferred to the server, which retrieve the
necessary data from the database and returned to
the client. The communication between the client
and the server is done in an asynchronous mode
over a communication network. Figure 1 shows
how the system is modeled. The figure shows a
network of MetaView/GIS clients and servers in a
Local Area and Wide Area Networks.
3.0 User Interaction
3.1 Server
The MetaView/GIS server interfaces between the
MetaView/GIS clients and the databases of the sys-
tem. Basically, the server is responsible for:
e transaction management; this includes the mon-
itoring and scheduling of transactions, the
authentication of client requests, the administra-
tion of resources and the commit/abort of trans-
actions and recovery.
* Store, request executions and data management.
* connecting to a client to transmit data and com-
municating with other MetaView/GIS servers
for assistance when necessary.
« authentication of MetaView/GIS privileged cli-
ents.
3.2 Client
The client includes the GUI portion of the Meta-
View/GIS software that runs at the user's site.
Where a client's workstation cannot support X11/
Graphics display, a user invokes MetaView/GIS by
the command mvgis. When MetaView/GIS is
started it first displays its top level window which
is shown in figure 2. At this point the client is
attached to the defaults MetaView/GIS server. At
the top level, a user has the option of connecting to
DeltaX, a complimentary system of MetaView/
GIS. DeltaX provides interoperability among Geo-
graphical Information Systems by converting data
stored in one format and transporting data to users.
After user authentication, a user can perform some
system administrative functions by clicking the
System button at the top level window. To exit the
MetaView/GIS system, a user must select the Quit
button in this window. We will discuss the Help
button shortly. To start the MetaView/GIS itself,
one must select the Meta-Data button at this top
window. This button pops up the Database Infor-
mation Window shown in figure 3.
366
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