DAL Working Group meeting in Cambridge

[Doug Tody]

VO Data Access Layer (DAL) Working Group
IVOA, Cambridge, UK  May 12-16 2003


Hi Folks -

We will have the kick-off meeting of the IVOA Data Access Layer (DAL)
working group on Monday May 12 at the IVOA Interoperability meeting in
Cambridge.  The initial discussion will be followed by smaller breakout
meetings throughout the week, with a wrap-up discussion in plenary session
on Friday morning.

The primary purpose of the DAL working group will be to define, over the
next year or so, the initial set of IVOA standards for VO science data
access.  In the Cambridge meeting we will need to clarify the scope of our
working group and develop a roadmap of our activities for the next year.
This will include identifying what data access standards are needed, how
these will interoperate with each other, and how these will integrate
with the infrastructure being developed by the other working groups
(registry, UCDs, data models, VOTable, etc.).  A special topic, possibly
for a subgroup, will be to discuss what should be included in the second,
"IVOA" version of the simple image access protocol due out this summer.

Our first item of business is to prepare an agenda for the meeting.
***Anyone who is interested in presenting any material at the meeting
should contact the WG organizers immediately.***  Please contact either
myself (dtody at nrao.edu) or Markus Dolensky (Markus.Dolensky at eso.org),
who is helping to organize the working group.  We can't promise anything
at this point about presentations (it will depend upon the final agenda
and the time available) but we do want to benefit from our collective
experiences in preparing these standards.  Suggestions for agenda items
or topics are also welcome.

	- Doug


To help identify the areas where work is needed some thoughts on the
scope of the VO data access layer follow.


Scope of VO Data Access Layer

The scope of the VO Data Access Layer includes the services and protocols
used to access remote science data via the VO, as well as the software
used to implement data access services.  Ultimately this will include
advanced capabilities such as data subsetting, data model mediation,
and server-side analysis, i.e., for grid computing.  We must also include
some client-side software to demonstrate an end-user analysis capability
and perform end-to-end testing and integration.  Ultimately most analysis
software will come from the user community, not from VO.  DAL builds upon
and integrates other VO technology for metadata, data models, data formats,
registries, and queries.

The science data dealt with by the DAL potentially includes all of the
following:

    catalogs
        e.g., object catalogs

	Catalog analysis is a fundamental VO capability which is beyond
	the scope of mere data access, but at the DAL level catalog
	and image/spectra access are often performed together.

    images
        e.g., 2D sky images
    data cubes
        e.g., 2D sky images with a spectral axis
    spectra, SEDs
        mainly 1D spectra
        2D/longslit spectra? (maybe not right away)
    time series
        e.g., time-resolved photometry
        spectrally-resolved light curves
        synoptic imagery (e.g., all-sky camera, synoptic surveys)

    event list (photon counting) data
    visibility data (interferometry)

The highest priority goes to object catalogs and 2D sky images, for which
prototype data access services are already available.  Spectral data cubes
are probably best treated as a general type of image.  Next in priority
are spectra, especially simple 1D spectra; spectra are a high priority for
the next phase of DAL development.  Time series data is less common but is
similar to 1D spectra and could possibly benefit from a similar approach.
It would be useful to integrate event list data and visibility data into
the VO, although our expectation is that most VO users will be interested
in images produced from such data rather than the original data (image
generation may need to be on-the-fly since there is in general no one
best way to produce images from event data or UV data).

The highest priority for IVOA data access standards is probably in the area
of standard data access protocols - this is what we should emphasize in
the first year.  Protocols are, or should be, implementation-independent
and hence are one of these easiest software elements to standardize.
As the VO software and infrastructure becomes more complicated it will
become increasingly important to provide some reusable VO framework-level
software to simplify the job of those putting up services or writing
client-side applications.  Finally as we move to grid computing it will
become necessary to dynamically deploy computational components on any
grid-enabled computational resource.  For this to be feasible we will need
some interoperability standards in the areas of computational frameworks
and components.