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Delta-X has a client component and a server component. On
a client, Delta-X provides a GUI that enables GIS users to
start and control transfer/conversion transactions and a
software which communicates with the server. On a server,
Delta-X provides the client-server and server-server
communication software, a module that transfers data, a
network management agent, an interface to a database, and a
set of data conversion routines. The server's network
management module facilitates setting of server’s
operational parameters, authentication keys, the entering and
leaving of the Delta-X federation, and it also raises alarms to
network management when the server malfunctions. Data
on the server can be stored in any commercial database
model - relational or object-oriented.
The exact internal format of data stored in Delta-X servers is
database-specific, but all database servers share the same
data model and logical schema, and they can store the same
types of data. The internal database schema used at a
particular server is transparent to all other servers in the
federation. A separate database interface module adapts
each internal database schema to a uniform data interface
used within Delta-X. Conversion routines are also database-
specific and GIS-specific. Note that the conversion routines
and the database schema are the only entities in Delta-X that
are specific to standard data formats or GIS systems.
The clear separation of data storage functions (the Delta-X
server) from data processing functions (GIS) in a network
not only facilitates data sharing and creation of logically
organized application-specific data repositories, but it also
enables to take advantage of the latest development in both
of the DBMS and GIS worlds.
The Delta-X server is a node in a network of Delta-X servers
which has a data repository containing a part of shared data
stored in the Delta-X federation. As shown in Figure 1, the
Delta-X comprises modules to perform the following
functions:
e Transaction management and surveillance
e Data storage, access, and management
e Data conversion
e Client-server and server-server communication
over a LAN or WAN
e Naming and authentication for Delta-X clients
The Transaction Monitor (TM) module performs transaction
management tasks, such as transaction scheduling,
transaction monitoring, authentication of transaction
requests, locking of data, resource administration, and
transaction commits, aborts and recoveries. TM receives
requests for new transactions from Delta-X clients or
requests to process already running transactions from fellow
servers in the Delta-X federation. The Delta-X server
3
validates the requestor’s permissions for accessing the
requested data and notifies the clients about the status of
transactions in progress. TM also logs accounting data in
order to support billing of users who access data stored in
the federation.
If required, TM will relocate the transaction to another
server to, for example, perform a conversion or finish a data
transfer. After the data conversion has been finished, TM
will initiate the transfer of converted data, temporarily
stored on the server, to its final destination. There is one
Convertor module for the conversion of each GIS data format
to and from the Delta-X internal data format. All
Convertors are database and GIS dependent.
The Data Mover module is responsible for sending or
receiving data between the clients and servers and/or
between the servers themselves. It is implemented on top of
the TCP/IP protocol stack.
2.4 Network Configuration
As shown in Figure 2, Delta-X is a loosely coupled network
of servers and clients. Clients and servers attached to the
same LAN form a cluster. Clusters are connected to each
other via a WAN, which forms the backbone of the Delta-X
system. Servers and clients can also be connected directly to
the WAN or, via a dial-up line, to one of the servers. The
set of all clusters, clients and servers connected directly to
the WAN, and clients remotely connected to cluster servers
forms the Delta-X federation. Communication between
clients and servers in the federation is TCP/IP-based (both
over the LAN and the backbone WAN). Any client can
request a transaction from any server in the federation. A
dedicated server in a cluster performs name and
authentication services for all clients in the cluster. A server
can also perform name and authentication services for
clients remotely attached to the server. Although having a
dedicated server for certain functions, a client can connect to
and request data from any server in the federation.
One dedicated server in the federation "the Delta-X Master
", monitors and coordinates the activities of the other
servers, and holds the configuration of the Delta-X
federation. When a server wants to join or leave the Delta-X
federation, it must contact the Delta-X Master Server, which
updates its configuration tables, and distributes them to all
other in the federation. The Delta-X Master Server is
duplicated for uninterrupted availability.
A server is essentially a database server which is also
capable of converting data from client into an internal Delta-
X format and vice versa, and storing the converted data in its
database. When another client wishes to import this data,
the Delta-X server translates the data into the target client’s
format and transfers the data to the target client. The target
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
