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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
A Generalized Mosaic­to­SQL Interface with Extensions
to Distributed Archives
F. Pasian and R. Smareglia
Osservatorio Astronomico di Trieste, Via G B Tiepolo 11, Trieste, I
34131 Italy
Abstract. A graphical user interface based on Mosaic, and allowing
form­based access to SQL­driven relational databases, is described. The
system is general, due to some simple naming conventions for database
tables. This allows a description of the database structure (contents of
tables, joins, and views) to be stored in the database itself, and the Mosaic
pages to be prepared on­the­fly, with no need for external definition files.
Furthermore, the interface allows X­Y plotting of any two numeri­
cal quantities retrieved from the database, and access to data archives
distributed over Internet, following the WWW concept. Quick­look data
can be displayed, and observational data can be retrieved in their orig­
inal format (FITS) inside compressed tar files. All of these operations
are performed on the archive hosts by proper server processes, and no
additional software, besides Mosaic, is required at the user's site.
The functionality of the system has been tested on a small archive
of objective prism data (images and extracted spectra) and will be im­
plemented for a distributed archive of ISM data.
1. Introduction
The World­Wide­Web (WWW) and NCSA Mosaic can be considered a de facto
standard for information retrieval, and their wide acceptance is leading to a
different and more homogeneous design of interfaces to archives and databases
in astronomy. The advantages of using Mosaic as an interface are numerous:
distribution and development of the software are taken care of at NCSA, the
Mosaic client handles all the intricacies of the man­machine interface, the devel­
opment rules are easy to learn, and data centers do not need to bother about
distributing and maintaining user interface or commercial software at a number
of external sites.
The information providers can therefore concentrate on building servers
able to guarantee efficient access to their data, without the need of building ad­
hoc interfaces to their systems. It is essential that data access tools are flexibly
built and can easily communicate with this standard user interface. These are
the basic concepts we followed while implementing the system described in this
work.
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2. System Architecture
The user interface allows access to the underlying structure of the archive: a
catalogue managed by a commercial SQL­based database management system,
and a data bank holding the original data and compressed quick­look data.
The interface itself, which is based on NCSA Mosaic, allows a number of
operations on the archive: selection of specific database tables or views to be ac­
cessed; browsing of the catalogue (database tables or views) by specifying query
constraints inside forms to be filled by the user; possible editing of the resulting
SQL query; display of the results in a HTML page; graphical visualization of the
numerical results derived from a query; display of quick­look 2­D images and
finding charts; display of quick­look 1­D graphs (e.g., extracted spectra); and
retrieval of observational data from the data bank.
The structure of the system is shown in Figure 1. Users access the archive
through a Mosaic interface; at the host node, the http messages coming from
the interface are interpreted by a Protocol Handler, and different specialized
servers can be started:
ffl The SQL Server builds a SQL query from information received from
the interface, and passes back to the interface the result of the query or
quick­look data (if any).
-- The DBMS Server extracts from the database information on the
table or view to be accessed and builds the Mosaic interface page; in
other cases passes the SQL query on to the DBMS and gets results.
ffl If the data are stored locally, the Data Server performs a username/pass­
word check through the DBMS Server, accesses on­line databank files and
sends them to the user, or sends retrieval requests for off­line data to the
archive operator.
ffl The Graphic Server plots the numerical results of a database query (or
of archive data, in the case that quick­look data do not exist), and passes
the graphic files back to the interface.
In keeping with the concept of the WWW, this system allows access to
archives of data distributed over the Internet. From a logical point of view, it
makes no difference if the archive is local or remote: the pointers to the data
are stored in the database and, once accessed, are fed back to the interface
for standard WWW access. If necessary, the Protocol Handler sends a message
asking the remote archive to perform some specific action which may be required
(e.g., storing files to be transferred in a specific world­readable directory, running
a dedicated process to perform format conversion on archive data, or packing
data in a compressed tar file).
3. Database Structure and Naming Conventions
The SQL Server uses standard SQL calls to access the database in retrieve
mode, therefore guaranteeing independence from the SQL dialect used by the
specific DBMS. It is, however, another feature that allows the system to be fully

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http
Protocol
Handler
Graphic
Server
Data
Server
SQL
Server
Server
DBMS
FITS GIF
local archive
remote archive
remote
hyperlink
DBMS
$paths
$mothertab
$systemtab
Figure 1. Structure of the system, based on a number of different
servers, and allowing access to remote archives and hypertext media via
the HTML protocol. The possibility of sending communications to re­
mote archives through the Protocol Handler is shown. In the database,
the three special tables allowing system portability are evidenced.
general: all of the interface information is dynamically stored in the database
and is retrieved when needed to build on­the­fly Mosaic interface pages.
This mechanism works as follows: when it is necessary (i.e., when a user has
issued a request to access data stored in the database), the DBMS server first
retrieves from the database information on the table (or view joining a number
of different tables) that needs to be accessed. Such information is used to build
on­the­fly the form the user will to use to browse the contents of the database.
The form, in HTML format, is then fed to the Mosaic interface.
The mechanism can be considered similar to the one described by Ras­
mussen & Pirenne (1994) for Sybase, but in our case form definition files (FDFs)
are never built, nor stored in the database. This design feature allows modifi­
cation of the database structure without changing the archive software, keeping
description files up­to­date, or maintaining additional software to be run on the
user's computer.
To allow independence of the structure from the specific DBMS used, some
conventions are enforced. First, no DBMS system table is directly used; a table
called $SYSTEMTAB is used instead. This table contains the names of the various
tables stored in the database, together with their fields, descriptions, and for­
mats; information on joins and views are also stored. $SYSTEMTAB may be built
from a DBMS system table by means of a specific utility.
Two additional tables must be available in the system. $MOTHERTAB contains
the list of all tables and views which are explicitly available to the user when
accessing the archive. $PATHS contains information on the location of both
quick­look and original FITS data files.

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Some additional naming conventions are enforced on fields. If a field in
a table is called $tablename, it contains names of other tables stored in the
database; this convention allows the creation of a hierarchy of database tables,
if needed. The $filename field contains names of files stored in the archive,
either on disk, on a juke box, or off­line. The $hostname field contains the
name of a (remote) archive server. The $path—id, $gif—path, and $fits—path
fields contain the location of quick­look and FITS data files in terms of directory
paths; $path—id is given as a numerical value corresponding to a directory path,
and is used to save space in the database.
4. Applications
The scheme developed allows the interface, and the overall system, to be com­
pletely independent from operating system, DBMS, archive structure, and data­
base structure (except for the conventions described in the previous section).
These concepts have been tested on a prototype archive containing a few
digitized objective prism images, related tables containing the position of de­
tected spectra, and the extracted spectra themselves. Both original and quick
look data are contained in the archive. Details on the implementation of the
archive are given in Pasian & Smareglia (1994b). The related URL is SQL­
ARCHIVE interface 1 .
A distributed archive containing interstellar medium spectra is currently
being designed (Porceddu et al. 1994), and an initial version should be available
before the end of 1994. This archive will use the interface described here; its
metadata will be contained in a database in Trieste, while the data will be
stored in Cagliari, Pisa (Italy) and Torun (Poland). Finally, this architecture
is planned to be used (Pasian 1994) as the interface to the technical archive of
TNG, the Italian National 3.5 m telescope to be located at La Palma, in the
Canary Islands.
Acknowledgments. M. Albrecht, A. Balestra, P. Marcucci, B. Pirenne,
G. Russo, and C. Vuerli are gratefully acknowledged for many interesting dis­
cussions on the topics of archives and information retrieval.
References
Pasian, F. 1994, Archives at the TNG Telescope -- Architecture Design Docu­
ment, TNG Project Technical Report (draft 0.9)
Pasian, F., & Smareglia, R. 1994a, in New tools for network information re­
trieval, eds I. Porceddu, & S. Corda (Cagliari, Oss. Astr. Cagliari)
Pasian, F., & Smareglia, R. 1994b, Int. Jour. of Mod. Phys. C ­ Physics and
Computers, in press
Porceddu, I., Corda, S., Pasian, F., & Smareglia, R., 1994, Proc. Conf. Boulder,
Colorado, in press
Rasmussen, B. F. 1995, this volume, p. ??
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http://atlantis.oat.ts.astro.it