Ne 
format can be any common GIS format and the target 
machine can be either a UNIX workstation or a PC. 
The Delta-X server handles requests both from clients and 
from other servers in the federation. A transaction may start 
on one server, but it may migrate to another server, for 
example, to carry out a conversion from one data format into 
another, or to finish a data transfer. When a GIS wants to 
import or export data to Delta-X, it relinquishes control to 
the Delta-X client. The Delta-X client initiates the data 
transfer/conversion transaction. In data import operations, 
after a transaction has been completed, the GIS will use the 
data imported in the transaction. 
The mode of communication between a Delta-X client and a 
Delta-X server or between Delta-X servers is asynchronous. 
Delta-X supports conversational transactions, which proceed 
as follows. A client connects to a server, requests a 
transaction, and then disconnects from the server. When the 
server finishes the requested transaction, or when it has to 
report the transaction's status, the server connects to the 
client and delivers requested data or reports the status. 
The main reason for introducing the asynchronous 
communication paradigm into Delta-X is the long duration 
of the conversion/transfer transactions. A typical transaction 
can take some time to complete and it would not be 
reasonable to maintain a connection between a client and a 
server and thus to tie up network resources for the whole 
duration of the transaction. 
The asynchronous communication between the server and 
the client has several other advantages. First, the server can 
simultaneously process multiple transactions from multiple 
clients without running out of communication channels. 
Second, asynchronous communication between Delta-X 
entities facilitates nesting and chaining of Delta-X 
transactions. The implementation of client-server and 
server-server asynchronous communication is based on the 
SUN remote procedure calls over TCP/IP. 
2.5 Client Services 
Delta-X client is a software that runs on the user’s machine 
and enables Delta-X users to start and control Delta-X data 
conversion and movement transactions. Users can either 
export their data to Delta-X or import data from Delta-X. 
The Delta-X client also supports administrative functions, 
such as joining or leaving the Delta-X federation or access 
rights control, and viewing of data stored in Delta-X 
databases. Raster and vector data can be viewed. Display of 
data by a user prior to importing the data is important since 
a data conversion transaction can take a long time to 
complete. 
As shown in Figure 3, the Delta-X client comprises the 
following modules: the Administration and Data Transfer 
and Conversion graphical user interfaces (GUIs), the 
5 
Transaction Management module, and the Network 
Communication module. 
The Administration GUI enables the user to perform 
administrative tasks. The administration GUI is only 
packaged on Delta-X clients used by Delta-X site 
administrators. A Delta-X client can simultaneously control 
multiple transactions on multiple servers. The Data 
Transfer and Conversion GUI also supports viewing of 
textual, raster and vector data to which the user has access 
rights and which is stored on any server in the Delta-X 
federation. 
The network communication module performs all 
communication tasks with Delta-X servers. The 
communication functions can be either requests to view 
spatial data stored in one of Delta-X data repositories, or 
requests to perform data transfer/conversion transactions. 
3. METAVIEW/GIS SPATIAL BROWSER 
MetaView/GIS Spatial Browser was developed as a front 
end to the Delta-X to assist the GIS system users in 
identifying the source of data required for their applications. 
MetaView/GIS facilitates access to metadata of various 
database, e.g.: information on specific datasets, ownership, 
geographic coverage, format, availability, access mode, cost, 
etc. 
MetaView/GIS is configured as a client-server model to run 
over Internet. The client is a user UNIX workstation, e.g. a 
GIS, and the server component runs on MetaView/GIS site. 
A number of other client-server directory services exist for 
locating and retrieving information across the Internet. Such 
directory services include Wide Area Information Server 
(WAIS), World-Wide Web (WWW), Archie and Gopher. 
WAIS, Archie and Gopher provide the user with an 
overview of likely places to find the desired information, and 
then help the user locate the specified information items. In 
WWW, a GUI software known as mosaic allows a user to 
navigate through databases with a mouse click in a hypertext 
mode. 
MetaView/GIS differs from these systems in a number of 
ways. First, MetaView/GIS, operationally, serves as a 
partial front end of a commercial service Delta-X. Second, 
it provides an X11 graphical interface at the user’s client 
which relies on commercial databases management system 
and not a hypertext system. Third, users can specify spatial 
queries. Fourth, MetaView/GIS provides more information 
related to a data set than those existing in these directory 
services. However in designing MetaView/GIS, the services 
provided by these systems were taken into consideration and 
in fact are utilized by the MetaView/GIS system. For 
example, if a data set is available free at a site, one may 
choose to retrieve it by employing the services of the other 
directory services. 
3.1 MetaView/ GIS Operational Concepts 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996