Module
General Query Protocol (GOP)
Request distribution module
Database Search Module
Map Browsing Module
Archive Request Module
Disk Access Module
Data Input Module
Data Processing Server
Functionality
The GQP connects the components of the GDSS on the OSI (Open Systems Interconnection) layer
4 (transport). GQP is a simple ASCII protocol to optimally address the issues of the GDSS. GQP
handles four categories of services, these are (1) spatial queries, (2) service requests, (3)
administrative queries and (4) data update commands. Furthermore the support of currently
evolving international data exchange standards such as IDN and Z39.50 is being investigated.
Distributes incoming requests and is responsible for security tasks (identification, authorization).
GDSS uses a distributed user management strategy, hence it is up to the Local Server administra-
tor to authorize users. The CS just takes an entire site into consideration.
This module consists of a massively parallel spatial RDBMS handling multiple queries at the same
time. It addresses coverage, points and regions of interest requests. The database holds coverage
information and meta info but not any image data. They remain stored in the Disk Access Module.
Provides the user with fast delivery of requested map tiles that should be currently displayed on
the screen. If more than one user requests tiles they are parallel loaded from the Disk Access
Module.
The Archive Request Module is responsible for the communication with the archive sites. Ar-
chives respond with the full resolution data and the billable costs. If the price has a non zero value,
the account server will be started and accounting with the Local Server takes place. Privacy will be
accomplished with a common key system (Stevenson, 1995). When the accounting process is
finished, the full resolution data are passed to the cropping server, which crops the data exactly to
the area defined by the user. The result, usually an image stack, is sent to the user.
The browsing maps of the entire planetary surface are stored here at different levels of detail with
the successive 2:1 reductions of these maps forming a resolution pyramid. Furthermore 1:4
reduced quicklooks will be kept within this module. An additional color reduction to 8 bits is
applied to all image data. In combination with low quality JPEG compression (>1:25) a data re-
duction of 100:1 or better is possible. The Disk Access Module is designed for giving many users
parallel stream access to the image data.
The Data Input Module is responsible for bringing in data as well as updating existing data.
This module provides users with remote access to image processing facilities which just can only
run with expensive hard- or software. This feature makes it possible to achieve an efficient re-
source sharing, accounting for the usually non-free services will take place.
Table 2: The main modules of the Central Server and a brief description of their functionality
"f R equest
cR Distribution
Module
Input
from
Local Servers
GQP D atabasc
Scarch
Module
D isk GQP | D ata
Access CoD" qe Input
Modulc
Module
| M ap
rl Browsing
Module
| Data
rA Processing
| Server
O utput
to
Local Servers
A rchive
Request
Module
ea
GQP |
1/0 with
vendor
Archives
Figure 2: The main modules of the Central Server, with the internal dataflow and the links to external components.
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