the federation or authorizing user’s access to data 
stored in the federation, and viewing of data stored 
in Delta-X databases. Raster and vector data (maps) 
can be viewed graphically. Pre-viewing of data by a 
user prior to importing the data is important since a a 
data conversion transaction can take a long time to 
complete. 
The structure of the Delta-X client is shown in Fig- 
ure 4. 
4.1 Delta-X Client Modules 
The Delta-X client comprises the following mod- 
ules: the Administration and Data Transfer and Con- 
version graphical user interfaces (GUIs), the 
Transaction Management module, and the Network 
Communication module. 
The Administration GUI enables the user to perform 
admistrative tasks. The administration GUI is only 
packaged on Delta-X clients used by server adminis- 
trators. Regular users do not use it. The Data Trans- 
fer and Conversion GUI enables the user to select 
parameters for a data conversion and transfer trans- 
action (e.g. the format of the source or destination 
data or the location of the data), to start the transac- 
tion, to monitor the transaction's progress on the 
Transaction Status Display and to control some 
aspects of the transaction. À client can simulta- 
neously 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 Transaction Management module keeps track of 
all active transactions. If the client is terminated and 
there are active transactions started by the client, the 
Transaction Management module will cancel all out- 
standing active transactions. 
The network communication module performs all 
communication tasks with servers. The communica- 
tion functions can be either requests to view spatial 
data stored in one of the servers, or requests to per- 
form data transfer/conversion transactions. 
4.2 Delta-X Client Operation 
The mode of operation for the Delta-X client is as 
follows. Suppose a user wants to import some GIS 
data, stored on one of the Delta-X federation serv- 
ers, into her GIS. Suppose the GIS at the user’ 
workstation is ARC/INFO. Using the MetaView 
spatial browser [2] the user first locates the proper 
Delta-X client where the desired data resides. The 
user then starts the Delta-X client. During its start- 
up, the client contacts its dedicated server and que- 
ries the current configuration of the Delta-X federa- 
tion. 
If the server which owns the desired data is present 
in the federation, the user can contact the server and, 
if she has access rights to the data, pre-view the 
desired data from the server at the client’s display 
terminal. The user can pre-view spatial characteris- 
tics of the data (raster and vector data) as well as 
attributes and textual information associated with 
the spatial data. 
If satisfied with the data, the user specifies the trans- 
action parameters, such as the name of the desired 
data, the destination of the converted data (i.e. the 
machine, directory/database and file names), the 
destination data format (for example ARC/INFO), 
and then initiates a transaction to convert and trans- 
fer the data. Note that the destination machine does 
not necessarily have to be the machine on which the 
Delta-X client is currently running (e.g. it can be 
another workstation or a file server), but the user 
must have access rights to the destination machine 
and/or directory. 
The client has a status window for this transaction, 
which shows all status, log, and error messages for 
the transaction. The Delta-X client will notify the 
user when the transaction is completed and the 
desired data has been transferred to the specified 
directory/database at the destination machine. The 
user can then start a GIS (in our example ARC/ 
INFO) to load the converted data into the GIS. 
Similarly, when a user wishes to export data to the 
Delta federation, she first consults the MetaView 
spatial browser to determine where the data should 
be located. If the user has access and update rights 
for the specified server, she specifies the transaction 
parameters, such as the source data format, and loca- 
tion (the machine, directory and file names), the des- 
tination format and starts the export transaction. The 
Delta-X client has a status window for this transac- 
tion, which shows the progress of the transaction. 
The Delta-X client will notify the user when the 
transaction is completed and the desired data has 
been stored on the specified server. 
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