New SSAP services

[Doug Tody]

Hi All -

A couple of test SSAP services are now available.  These are intended
not for serious scientific use (since we have not yet finalized the
protocol and I don't have any spectral data to publish myself), but
rather for development purposes, to work out the remaining details
required before we can take SSAP, and the related technology it
is based upon, to PR.

The query interface and data model implemented in these services
reflects the current draft SSAP interface, which has not yet been
updated to reflect review comments and initial tests.  As mentioned
earlier, we are currently conducting the initial review of the draft
1.0 SSAP specification, while proceeding with initial implementations,
before updating the specification documents.  These services, and those
under development elsewhere, represent the initial implementations
required before we can take the SSAP specification to PR.

The new services can be most conveniently seen via the page at

     http://webtest.aoc.nrao.edu/ivoa-dal/

which provides a simple forms based interface for exercising the queryData
operation for each service, as well as a link to some documentation on
the service implementation and data models.

The two services provided are the following:

     SSAP Null/Echo Test Service

 	This service implments the SSAP protocol and responds to all
 	service operations, but a query never finds any data (which
 	is a valid query response).  It is useful for simple tests to
 	determine if a client can talk to the service.	The request
 	parameters are fully processed and echoed back in the query
 	response VOTable.  Full SSAP metadata is also returned.
 	Any errors will result in an error response.  You can access
 	the service directly at

 	    http://webtest.aoc.nrao.edu/ivoa-dal/ssap
 	e.g.,
 	    http://webtest.aoc.nrao.edu/ivoa-dal/ssap?
 	    REQUEST=queryData&POS=180,1&SIZE=0.2

 	(join the two lines to make the full query).


     SSAP Proxy for JHU Spectrum Services

 	This does everything that the null service does, plus
 	it implements a proxy SSAP interface to the JHU spectrum
 	services, to provide access to real data for testing and
 	software development.

 	What is provided is a proxy service, implementing the SSAP
 	protocol, which on the back end accesses the JHU spectrum
 	services to execute data queries and retrieve data.
 	Architecturally this is very similar to querying a local
 	archive database, so from a software point of view the
 	implementation is very close to what one would do to implement
 	a regular service.

 	Both data queries (for all the million+ spectra in the
 	JHU archives) and actual to the actual spectrum datasets
 	is provided.  Only access to the SDSS DR5 data collection has
 	been tested.  Spectra can be retrieved in CSV, VOTable, and the
 	"native project data format" (a JHU-specific XML format in this
 	case).	Support for FITS is planned but is not yet implemented.
 	The CSV and VOTable (and FITS) formats are SSA-compliant.

 	Service URL:

 	    http://webtest.aoc.nrao.edu/ivoa-dal/JhuProxySsap

For each service, the queryData, getData, and getCapabilities
operations are implemented using our new OpenGIS-based GET protocol
which supports multiple operations per service.  A fourth operation
for monitoring service health is planned but is not yet implemented
(a fifth operation, for data staging, is also planned but is further
off).  Versioning is included.  The getCapability operation functions
and returns an XML document, but does not yet return valid metadata
since we are still specifying the content.  The getData operation
provided is a real service operation, not just a URL, although the
access reference remains a simple URL (more on that later).

These service implementations are based on a new DALServer Java
package, which provides a reference implementation of the DAL services
(SSAP initially), as well as a toolkit for service implementors.
I will post more about that tomorrow.  The main point is that as our
services become more complex, reference implementation are needed,
both to ensure that good service specifications are produced, and
also to make it practical for the smaller data providers to implement
robust, correct, compliant services.

Although the services can be exercised via the web page in a browser,
anyone who wishes to look carefully at the metadata returned, at a
spectrum dataset, and so forth, is encouraged to download the data
and look at it with more powerful tools.  What I use is WGET to get
the data, and TOPCAT to view the VOTables.  It is much more pleasant
to view the metadata in a tool such as TOPCAT than in a web browser.
Other similar tools are available as well.

 	- Doug