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.