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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Search and Retrieval of the AXAF Data Archive on the
Web using Java
Sumitra Chary and Panagoula Zografou
Smithsonian Astrophysical Observatory, AXAF Science Center, Boston,
MA 02138
Abstract. An important component of the AXAF Data Archive is the
interface through the WWW. Internet applications are moving beyond de­
livering static content to more complex dynamic content and transaction
processing. While HTML and CGI based interfaces have been possible
for some time, it was not until the advent of JDBC (Java 1 Database Con­
nectivity) that a uniform and powerful interface to the di#erent types of
servers in the archive, with secure access to proprietary data, could be­
come a reality.
This paper presents a multi­tier architecture which integrates the data
servers, Web server and a HTTP/JDBC gateway and enables data on any
of the data servers in the AXAF archive to be available to clients using
a Web browser. This Web­based solution to data browsing is currently
available for the contents of the AXAF observing catalog and other cata­
logs in the archive. A Java API for other database applications to access
the data is presented. Performance issues and security limitations and
workarounds are discussed.
1. Introduction
The AXAF archive architecture involves a number of servers, RDBMS servers
for managing relational data and archive servers that manage datafiles. Access
to datafiles is provided by existing client applications which send data ingest,
search and retrieval requests in a 4gl language to an archive server (Zografou
et al. 1997). An architecture was developed to build a powerful and uniform
interface to the various servers through the World Wide Web. The advent of
JDBC a specification for an API that allows Java applications to access various
DBMS using SQL became the obvious choice to build this interface.
2. System Architecture
Figure 1 shows both the client­server architecture developed using the Database
vendor API (Zografou et al. 1997) where both the client and the data servers
communicate using the same protocol and the 3­tier architecture which allows
1 Trademark of Sun Microsystems
408

Search and Retrieval of AXAF Data Archive 409
(CGI­data transformation)
Sybase Driver + API
JDBC (Javasoft)
(archive server)
ASCDS­db server
library
Database vendor API
(Sybase Open­Server
library)
TDS
TDS
TDS
TDS
SQL Server
Apache Web Server
Cascade Gateway
Open Server
Firewall
Server ­ side
Middleware
JConnect
Client Application
ASCDS­db client
library
Database vendor API
Network Protocol
Client side
Client ­ Server
3 Tier
HTTP
HTTP
Web Browser
(implements
Java Virtual
machine)
TDS ­ Native protocol for SQL Server and SYBASE
HTTP Hypertext Tansfer Protocol
Open Server Applications
(Sybase Open­Client
Client Library)
TDS
Tunnelled
Figure 1. System Architecture [Client­Server and 3­Tier]
a Java client applet to communicate with the data servers behind a firewall.
The middle tier consists of Sybase jConnect 2 for JDBC package. This includes
the Cascade HTTP Gateway 3 which is the gateway to the RDBMS and archive
servers.
According to Java security restrictions, applets can only connect back to
the host they were downloaded from. If the Web server and database server were
on the same machine then the Cascade gateway is not required. But in this case
the data server hosts are behind a firewall hence the Cascade gateway acts as
a proxy and provides the path to the data servers. The HTML documents and
Java code are served from the Apache Web Server 4 tree. The Apache Server is
also used to execute a cgi­script which performs transformations of input and
output data when required. Hence the Apache and the Cascade run on the same
host on di#erent ports so that the applet downloaded from the Apache server
tree can connect back to the same host at the port of the Cascade gateway.
3. Search Operation
Searching involves querying databases on the SQL Server and viewing the re­
sults on the client screen or saving them in a file. Both command­line client
applications and Java applets have been written to send browse requests to the
archive and the database servers. The primary di#erence between the above
2 http://www.sybase.com/
3 http://www.cascade.org.uk/
4 http://www.apache.org/

410 Chary and Zografou
User Request
Create file with result
contents
Send result
displayed to save
Cascade Gateway
FTP Server
result information
FTP File containing
Return result rows
Return result rows
User Request
User Request
Archive Server
Return result rows
SQL Server
Web Server
(cgi­script)
Applet
to ftp file
Return link
Save Result
Workaround
Figure 2. Search Element
two types of client requests is that the latter's request goes through the Cascade
HTTP/gateway and then to the data server (Figure 1). The way the browse
request is handled on the server­side is the same for either client. The results are
received in the same way on the client­side and displayed. One major di#erence
is in saving the results obtained.
Due to Java applet security restrictions, files cannot be created on the client
filesystem. Hence the result contents are sent by the applet to a cgi­script on the
Web Server host which creates a file on its filesystem from the binary stream.
This file is then FTP'ed to the FTP server in a previously created location. The
cgi­script informs the client of this path which is displayed to the user.
In the future with JDK1.1 and a compatible Web browser it may be possible to
re­create files on the client side depending on authentication and access control
schemes. This would eliminate the extra step of passing data to a cgi­script.
4. Retrieve Operation
This involves retrieving files stored in an archive from the archive server via
metadata browsing in the SQL server. As shown in Figure 3 the ideal retrieve
scenario would function the same way for either client type. The client would
receive the filenames and file contents as a binary stream, re­create the files
on the client filesystem. Since this is not possible with applets due to security
restrictions, two methods are presented to resolve this issue.
1. The file contents are forwarded to the middle­tier and the files are re­
created on the Web server's filesystem. These files are then FTP'ed to the
FTP Server and the client is notified of the link to access the files. This is
similar to the ``save results'' method outlined above.
2. The second method is much more e#cient both from the performance point
of view and because it avoids of excessive network activity. The applet in
this case only serves as the GUI and passes the user input to the Web
server. The Web server now behaves as the client to the archive server.
The CGI­script directly executes the ``arc4gl'' client application which is
the command­line client application. Hence it has the ability to send and
receive data from the archive server. The files retrieved are re­created on
the Web server filesystem. Then they are FTP'ed over to the FTP server.
The link to the files is returned and displayed at the client applet end. If
the files are of a format supported for viewing in the Web Browser e.g.,

Search and Retrieval of AXAF Data Archive 411
Archive Server
Web Server
Applet
SQL
Server
FTP Server
Workaround
Ideal
Binary Stream of File Contents
Binary Stream of File Contents
4GL
Binary Stream
FTP files
Re­create files
client application
(cgi­script )
Data Archive
Archive Server
Web Server
(cgi­script )
Applet
FTP files
4GL
4GL
Command­line (arc4gl)
Workaround
Executes arc4gl
Link to ftp file
Re­create files 4GL
SQL
Server
FTP Server
Binary Stream
of File contents
Link to ftp file
Figure 3. Retrieve Element
HTML, GIF, PS then the files, if placed under the Web Server tree, can
be directly viewed by the user.
To port this setup to various environments such as test and production, a cgi­
script is used to dynamically create the HTML file presented below with the
appropriate information.

code = "ocat.class"
codebase = "base URL of applet to be displayed"
archive = ocat.zip
width=700 height = 650>








References
Zografou P., Chary, S., DuPrie, K., Harbo, P., & Pak K. 1998, ``The ASC Data
Archive for AXAF Ground Calibration'', this volume