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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.
Mirroring the ADS Bibliographic Databases
Alberto Accomazzi, Guenther Eichhorn, Michael J. Kurtz, Carolyn S.
Grant and Stephen S. Murray
Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge,
MA 02138, USA
Abstract.
During the past year the Astrophysics Data System has set up two
mirror sites for the benefit of users around the world with a slow network
connection to the main ADS server in Cambridge, MA. In order to clone
the ADS abstract and article services on the mirror sites, the structure
of the bibliographic databases, query forms and search scripts has been
made both site­ and platform­independent by creating a set of configu­
ration parameters that define the characteristics of each mirror site and
by modifying the database management software to use such parameters.
Regular updates to the databases are performed on the main ADS server
and then mirrored on the remote sites using a modular set of scripts capa­
ble of performing both incremental and full updates. The use of software
packages capable of authentication, as well as data compression and en­
cryption, permits secure and fast data transfers over the network, making
it possible to run the mirroring procedures in an unsupervised fashion.
1. Introduction
Due to the widespread use of its abstract and article services by astronomers
worldwide, the NASA Astrophysics Data System (ADS) has set up two mirror
sites in Europe and Asia. The European mirror site is hosted by the Centre De
Donn’ees Stellaires (CDS) in Strasbourg, while the Asian mirror is hosted by the
National Astronomical Observatory of Japan (NAO) in Tokyo.
The creation of the ADS mirrors allows users in di#erent parts of the world
to select the most convenient site when using ADS services, making best use of
bandwidth available to them. For many users outside the USA this has meant
an increase in throughput of orders of magnitude. For instance, Japanese users
have seen typical data transfer rates going from 10 bytes/sec to 10K bytes/sec.
In addition, the existence of replicas of the ADS services has taken some load o#
of the main ADS site at the Smithsonian Astrophysical Observatory, allowing
the server to respond better to incoming queries.
The cloning of databases on remote sites does however present new chal­
lenges to the data providers. First of all, in order to make it possible to replicate
a complex database system elsewhere, the database management system and the
underlying data sets have to be independent of the local file structure, operating
system, hardware architecture, etc. Additionally, networked services which rely
395

396 Accomazzi, Eichhorn, Kurtz, Grant and Murray
on links with both internal and external Web resources (possibly available on
di#erent mirror sites) need to have procedures capable of deciding how the links
should be created, possibly giving users the option to review and modify the sys­
tem's linking strategy. Finally, a reliable and e#cient mechanism should be in
place to allow unsupervised database updates, especially for those applications
involving the publication of time­critical data.
2. System Independence
The database management software and the search engine used for the ADS
bibliographic services have been written to be system­independent.
Hardware independence is made possible by writing portable software that
can be either compiled under a standard compiler and environment framework
(e.g., GNU gcc) or interpreted by a standard language (e.g., Perl5). All the
software used by the ADS mirrors is first compiled and tested for the di#erent
hardware platforms on the main ADS server, and then the appropriate binary
distributions are mirrored to the remote sites.
Operating System independence is achieved by using a standard set of Unix
tools which abiding to a well­defined standard (e.g., POSIX.2). Any additional
enhancements to the standard Unix system tools are achieved by cloning more
advanced software utilities (e.g., GNU shell­utils) and using them when neces­
sary.
File­system independence is made possible by organizing the data files for
a specific database under a single directory tree, and creating configuration files
with parameters pointing to the location of these top­level directories. Similarly,
host name independence is achieved by storing the host names of ADS servers
in configuration files.
3. Resolution of Hyperlinks
The strategy used to generate links to networked services external to the ADS
which are available on more than one site follows a two­tiered approach. First,
a ``default'' mirror can be specified in a configuration file by the ADS adminis­
trator. This configuration file is site­specific, so that appropriate defaults can
be chosen for each of the ADS mirror sites depending on their location. Then,
ADS users are allowed to override these defaults by using a ``Preference Settings''
page to have the final say as to which site should be used for each link category
(see Figure 1). The use of preferences is implemented using HTTP ``cookies''
(Kristol & Montulli, 1997). The URLs relative to external links associated with
a particular bibliographic references are looked up in a hash table and variable
substitution is done if necessary to resolve those URLs containing mirror site
variables, as shown in the examples below.
1997Icar..126..241S # $IDEAL$/cgi­bin/links/citation/0019­1035/126/241
# http://www.idealibrary.com/cgi­bin/links/citation/0019­1035/126/241
1997astro.ph..8232H # $PREPRINTS$/abs/astro­ph/9708232
# http://xxx.lanl.gov/abs/astro­ph/9708232

Mirroring the ADS Bibliographic Databases 397
# $Id: ads_sites.config,v 1.1 1997/09/08 19:52:16 ads Exp ads $
# Sets possible values for mirror sites used by the ADS
# bibliographic services, with default values for the SAO site.
# ADS article sites
SET_1=ARTICLE
DESC_1=ADS Article Mirrors
TAGS_1=ARTICLE
DEF_1=1
ARTICLE_DESC_1=SAO, Cambridge, MA, USA
ARTICLE_DESC_2=NAO, Tokyo, Japan
ARTICLE_1=http://adsbit.harvard.edu
ARTICLE_2=http://ads.nao.ac.jp
# SIMBAD DB mirrors
SET_2=SIMBAD
DESC_2=SIMBAD Mirrors
TAGS_2=SIMBAD
DEF_2=2
SIMBAD_DESC_1=CDS, Strasbourg, France
SIMBAD_DESC_2=SAO, Cambridge, MA, USA
SIMBAD_1=http://simbad.u­strasbg.fr/simbo.pl
SIMBAD_2=http://simbad.harvard.edu/simbo.pl
# AAS publications (University Chicago Press)
SET_3=AAS
DESC_3=AAS Mirrors
TAGS_3=AAS_APJ AAS_AJ
DEF_3=1
AAS_DESC_1=University of Chicago Press, USA
AAS_DESC_2=CDS, Strasbourg, France
AAS_APJ_1=http://www.journals.uchicago.edu/ApJ/cgi­bin/resolve
AAS_APJ_2=http://cdsaas.u­strasbg.fr:2001/ApJ/cgi­bin/resolve
AAS_AJ_1=http://www.journals.uchicago.edu/AJ/cgi­bin/resolve
AAS_AJ_2=http://cdsaas.u­strasbg.fr:2001/AJ/cgi­bin/resolve
# Preprint Servers
SET_5=PREPRINTS
DESC_5=PREPRINTS Mirrors
TAGS_5=PREPRINTS
DEF_5=1
PREPRINTS_DESC_1=LANL, Los Alamos, NM, USA
PREPRINTS_1=http://xxx.lanl.gov
PREPRINTS_DESC_2=SISSA, Trieste, Italy
PREPRINTS_2=http://xxx.sissa.it
PREPRINTS_DESC_3=Yukawa, Kyoto, Japan
PREPRINTS_3=http://xxx.yukawa.kyoto­u.ac.jp
# IDEAL library (Academic Press publications)
SET_6=IDEAL
DESC_6=Ideal Library Mirrors
TAGS_6=IDEAL
DEF_6=1
IDEAL_DESC_1=Ideal, North America
IDEAL_DESC_2=Ideal, Europe
IDEAL_1=http://www.idealibrary.com
IDEAL_2=http://www.europe.idealibrary.com
Figure 1. Left: the Preference Setting form allows users to select
which mirror sites should be used when following links on ADS pages.
Right: mirror sites configuration file for the ADS server at SAO.
1997ApJ...486L..75F # $AAS APJ$?1997ApJ...486L..75FCHK
# http://www.journals.uchicago.edu/ApJ/cgi­bin/resolve?1997ApJ...486L..75FCHK
While more sophisticated ways to create dynamic links are being used by
other institutions (Fernique et al. 1998), there is currently no reliable way to
automatically choose the ``best'' mirror site for a particular user. By saving
these settings in a user preference database indexed on the cookie ID, users only
need to define their preferences once and our interface will retrieve and use the
appropriate settings as necessary.
4. Mirroring Software
The software used to perform the actual mirroring of the databases consists
of a main program running on the ADS master site initiating the mirroring
procedure, and a number of scripts, run on the mirror sites, which perform
the transfer of files and software necessary to update the database. The main
program, which can be run either from the command line or as a CGI script, is
an Expect/Tcl script that performs a login on the mirror site to be updated, sets
up the environment by evaluating the mirror site and master site's configuration
files, and then initiates the updating process.
The updating procedures are specialized scripts which check and update
di#erent parts of the database and database management software (including
the procedures themselves). The actual updating of the database files is done
by using a public domain implementation of the rsync algorithm (Tridgell &

398 Accomazzi, Eichhorn, Kurtz, Grant and Murray
Mackerras, 1996), with local modifications. The advantages of using rsync to
update data files rather than performing complete transfers are:
Incremental updates: rsync updates individual files by scanning their con­
tents and copying across the network only those parts of the files that have
changed. Since only a small fraction of the data files actually changes dur­
ing our updates (usually less than 5% of them), this has proved to be a great
advantage.
Data integrity: should the updating procedure be interrupted by a network
error or human intervention, the update can be resumed at a later time and
rsync will pick up transferring data from where it had left o#. File integrity is
checked by comparing file attributes and via a 128­bit MD4 checksum.
Data compression: rsync supports internal compression of the data stream
by use of the zlib library (also used by GNU gzip).
Encryption: rsync can be used in conjunction with the Secure Shell package
(Ylonen 1997) to transfer the data for added security. Unfortunately, transfer of
encrypted data could not be performed at this point due to foreign government
restrictions and regulations on the use of encryption technology.
5. Conclusions
The approach we followed in the implementation of automated mirroring pro­
cedures for the ADS bibliographic services has proved to be very e#ective and
flexible. The use of the rsync algorithm makes it practical to update portions
of the database and have only such portions automatically transferred to the
mirror sites, without requiring us to keep track of what individual files have
been modified. Because of the reduced amount of data that needs to be trans­
ferred over the network, we typically achieve speed gains from 1 to 2 orders of
magnitude, which makes the updating process feasible despite poor network con­
nections. We plan to improve the reliability of the individual transfers (which
occasionally are interrupted by temporary network dropouts) by using sensible
time­outs and adding appropriate error handlers in the main transfer procedure.
As a result of the proliferation of mirror sites, we have provided a user­
friendly interface which allows our users to conveniently select the best possible
mirror site given their local network topology. This model, currently based on
HTTP cookies, can be easily adapted by other data providers for the benefit
of the user. An issue which still needs to be resolved concerns providing a fall­
back mechanism allowing users to retrieve a particular document from a backup
mirror site should the default site not be available. It is possible that new
developments in the area of URN definition and management will help us to
find a solution to this problem.
Acknowledgments. This work is funded by the NASA Astrophysics Pro­
gram under grant NCCW­0024.
References
Fernique, P., Ochsenbein, & F., Wenger, M. 1998, this volume

Mirroring the ADS Bibliographic Databases 399
Kristol, D., & Montulli, L. 1997, HTTP State Management Mechanism, RFC2109,
Internet O#cial Protocol Standards, Network Working Group.
Tridgell, A., & Mackerras, P. 1996, The rsync algorithm, Joint Computer Science
Technical Report Series TR­CS­96­05, Australian National University.
Ylonen, T. 1997, SSH (Secure Shell) Remote Login Program, Helsinki University
of Technology, Finland.