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Patz, A., Harbo, P., Moran, J., Van Stone, D., & Zografou, P. 2003, in ASP Conf. Ser., Vol. 295 Astronomical Data Analysis Software and Systems XII, eds. H. E. Payne, R. I. Jedrzejewski, & R. N.
Hook (San Francisco: ASP), 249
Middle Tier Services Accessing the Chandra X-Ray Center Data Archive
Alexandra Patz, Peter Harbo, John Moran, David Van Stone, Panagoula
Zografou
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge MA 02138
Abstract:
The Chandra Data Archive team at the
Chandra X-ray
Center
has developed middle tier services
that are used by both our search and retrieval applications to
uniformly access our data repository. Accessible through an
HTTP URL
interface, these services can be called by our J2EE web application
(WebChaser) and our Java Swing application (Chaser), as well as any
other
HTTP client. Programs can call the services to retrieve observation
data such as a single FITS file, a proposal abstract or a detailed
report of observation parameters. Having a central interface to the
archive, shared by client applications, facilitates code
reusability and easier maintenance.
These middle tier services have been written in Java and packaged into
a single J2EE application called the Search and Retrieval (SR)
Services. The package consists of a web application front-end and an
Enterprise Java Beans back-end. This paper describes the design and
use of the SR Services.
The Chandra Data Archive team has two search and retrieval
applications: Chaser, a Java Swing application and WebChaser, a J2EE
web application. Both perform similar functions: they search the
Chandra Data Archive for observations matching a set of search
criteria, display information about the found observations, and allow
the user to download data for these observations from the
archive. Until recently, they have used two different bodies of
code to perform these functions causing maintenance issues. In an
effort to make our code more reusable, these
shared functions have been broken out into a
separate middle tier that both applications can use to access and
retrieve data from the archive. This middle layer is comprised of a
collection of services, accessible through an HTTP URL interface,
called the Search and Retrieval (SR) Services.
In addition to being used by these two internally developed applications,
the SR Services can also be used by external programs needing to
access the Chandra Data Archive. There is a growing
need from the astronomical community for programmatic interfaces to
the astronomical data archives. Interest in Web Services is growing with
virtual observatory projects needing to query various data
archives for observation data. While we have not yet implemented
actual Web Services, these HTTP URL services do provide programmatic
access to the Chandra Data Archive.
The following SR Services are available:
Given a set of search criteria (position coordinates, target name,
observation start
date etc.) this service returns a summary report of the observations
found. The data is returned in one of two formats: RDB table format or HTML
format. In an effort to comply with emerging virtual observatory
standards, there will also be a future option for the data to be
returned in VO Table format.
Given the same set of search criteria used by the Observation Summary Service,
this service returns a detailed observation report including
instrument settings and multiple ACIS window settings if
applicable. The data is returned in the same formats as the Observation
Summary Service.
Given a single observation ID, this service returns the Verification and
Validation Report. The data is returned as a formatted text file.
Given a single observation ID, this service returns proposal information,
such as the
principal investigator, proposal title and abstract. The data is returned
as an HTML page.
Given an observation ID, filetype and data-processing level, this service
returns the content of a single file from the Data Archive. The file is
returned as a stream of binary data.
Given an observation ID and an image filetype, this service returns
a JPEG image.
Given a single observation ID, this service looks up the NASA Astrophysics
Data System (ADS) bibcodes in our Data Archive. It then uses these
bibcodes to forward to the relevant page on
the ADS website.
To connect to the SR Services the HTTP URL query
mechanism is used. Both Chaser and WebChaser connect to the SR Services using
the
HttpUrlConnection Java class, as could other Java
programs. Clients running scripts could access the SR Services using an
application such as
wget
to retrieve the content via HTTP.
HTTP error codes are returned to the client application
when the user passes in a bad parameter, when no data is found, when
authentication fails or when proprietary data not belonging to the
user is requested.
If an observation has not yet
been made public, the Image Service, the V&V Report Service and the Archive
File Service
require authentication to retrieve proprietary data. Client
applications must send a username and
password to these services using www-authentication to
retrieve proprietary data, otherwise only publicly available data may be
requested.
Both internally developed applications and
externally developed applications could use the SR Services.
The Java Swing application Chaser makes calls to the individual SR
Services and displays the results in a manner dependent on the data type
the service returns. For example, when a service, such as the
Observation Details Service, returns an HTML
page the results are displayed in the Swing JEditorPane widget. When a
service, such as the Observation Summary Service, returns data in
RDB table format the results are parsed and displayed in a JTable control.
After an initial observation search has been performed in WebChaser, the new
Observation Viewer page can be used to examine
information about these observations before products are selected for
retrieval. The Observation Viewer page includes a set of menu options:
Summary, Detail, V&V Report, Proposal Abstract, Images and
Publications. Each menu option makes a call to the corresponding
service in the SR Services middle tier to display the requested data.
There has been interest from other astronomical data
archives in providing links to Chandra data from their client search
applications. These links could be made to the SR Services through the
HTTP URL interface.
There has also been interest in using
these services from educational astronomy software, astronomical
analysis programs and virtual observatory applications.
The SR Services have been written in Java and packaged into a single
J2EE application. This consists of a web application front-end and an
Enterprise Java Beans back-end.
The web application follows a Model-View-Controller design pattern
implemented on top of the Apache Struts
framework. The Struts
framework provides a controller for the application which invokes the
relevant SR Service object. The model layer is made up
of form beans that encapsulate the set of parameters each service
accepts. The view layer consists of JSP pages that display the results.
The Enterprise Java Beans (EJBs) are called from the web application to do
the actual archive lookup. The EJBs connect to the
back-end archive servers using the Sybase JConnect JDBC driver.
Putting all the SR Services together in one application allows the
services to share code. The front-end components share authentication
and logging code, for example, while the back-end components share
code that checks the proprietary status of the data.
Acknowledgments
This project is supported by the Chandra Xray Center under NASA
contract NAS8-39073.
© Copyright 2003 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: The Automated Data Processing Pipeline for SIRTF IRS
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