mapped to the global database conceptual model. 
Our present implementaton suports databases 
that must be relational or simple file systems. 
However, we will assume a global data model 
that is based on extended-entity relationship data 
model. Unlike a general multi-database system 
where the problem of schema integration is a 
much complex issue to handle, this problem does 
not arise in our constrained multi-database con- 
figuration. The global data model is such that 
it can be mapped onto either an underlying rela- 
tional DBMS or an object-oriented DBMS. 
A typical configuration consists a network of 
independent autonomous databases management 
systems that form the nodes of the Delta-X 
servers. These form the local database manage- 
ment systems (LDBMS) of the Delta-X federated 
database management system. The clients on 
which the GIS softwares run on are grouped into 
client nodes of a LAN. A LAN will data server 
that runs the Delta-X server component software 
(DæServer). A DxServer runs on a capable Unix 
system that communicates with other DzServer 
in a wide-area-network. 
The Delta-X services are split into two com- 
ponents: the server services (DzServer) and the 
client services (DxClient). The Delta-X global 
data model integrates spatial (vector and raster), 
non-spatial data, free text and documents. We 
anticipate incorporating a knowledge base but 
this is not addressed in the current implemen- 
tation. The defined global schemes of the re- 
spective data classes are virtual and materialize 
only as the stored data of the corresponding local 
schemes. The global schemes are therefore re- 
ferred to as virtual. However, the global schema 
definitions are supported in a Delta-X directory 
information service ( DzDIS), that is fully repli- 
cated at every DzServer node. 
The concept of a.federation is established at 
the database level. A database administrator 
(DBA), manages one of the DzServer nodes. The 
DBA is responsible for registering databases of 
an LDBMS into the federation. Registering a 
database implies exporting the schema definition 
of a local database to the global database of 
the DxDIS. A Delta-X master node (DxMaster), 
maintains a master copy of DxDIS. This node is 
maintained currently, by the server of the GIS 
Division, Natural Resources, Canada. The Dx- 
Master node administers the functions of admit- 
ting and retiring Delta-X servers (and not the 
databases). Once a server is admitted, into the 
federation, the DBA of the DxzServer freely regis- 
ters and de-registers databases maintained by the 
local DBMS. 
3.2 A DxServer Node 
Due to the different data types required in a GIS, 
a DzServer may be configured to maintain and 
deliver more than one data type. For example, 
a server may be configured for only vector data 
that is topological structured. Another may be 
configured to manage either only remote sensed 
data, a national toponomy database, geological 
database or hydrographic data. A DzServer node 
runs cooperatively with a client node during data 
translation to the respective GIS format. 
Transaction services of the DzServer will be 
generally long-lived. If the anticipated number of 
access is sufficiently large, the DzServer must be 
computational powerful to handle the workload. 
A DzServer node may be a super-computer, a 
highend graphics engine or a cluster of worksta- 
tion that perform parallel and distributed com- 
putation using virtual parallel machine (PVM) 
tools [7]. In some case, due to the specialized 
nature of data processing activity, availability of 
special skilled personnel, the operations of the 
server may be dedicated to special operations. An 
inexpensive manner to support a DzServer is to 
use a cluster of workstations that pool their pro- 
cessing power together using the parallel virtual 
machine paradigm. 
3.3 Remote Data Access and Queries 
A DxClient runs on a Unix workstation that is 
configured also as an X11R5 server with PEX ex- 
tensions and a Motif GUIinterface. The DxClient 
is scheduled to be ported to a PCWindow's en- 
vironment. Normally, a DxClient workstation is 
also configured to a GIS software and as such, 
will have the required graphics support needed. A 
DxClient node has connectivity to some DzServer 
node. Clients perform remote data access either 
directly using a graphical user interface of the Dx- 
Client component from the client node or by ex- 
plicit query specification using a common global 
Delta-X Data Access Language (DxDAL). The 
GUI interface mode of data access is the sim- 
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