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Paper: Chapter 49: The VO Data Access Layer (DAL)
Volume: 382, The National Virtual Observatory: Tools and Techniques for Astronomical Research
Page: 527
Authors: Tody, D.
Abstract: The VO Data Access Layer (DAL) is the interface used by client software to remotely access astronomical data in the VO. DAL will already be familiar to most readers via Cone Search (SCS) and the Simple Image Access Protocol (SIAP), used to access catalog and image data respectively. As this is being written in early 2007, the Cone Search and SIA protocols are several years old, and a second generation of DAL interfaces, based upon the technology developed within the IVOA working groups over the past several years, are under development. The first of these second generation interfaces, the Simple Spectral Access Protocol (SSAP) for accessing one dimensional spectral data, is just now becoming available. Other second generation interfaces for accessing catalog data, image data including spectral and time data cubes, spectral line lists, theory data, and so forth are under development.

The second generation DAL protocols incorporate more comprehensive metadata and data models than previously, as well as functional enhancements for posing more general queries, describing the data thus discovered, and accessing data via robust web and grid technologies. Standard generic dataset metadata has been defined which can be used to uniformly describe any type of astronomical data. The query interface has been generalized to support queries by a variety of dataset attributes, hence queries no longer have to be based upon spatial position on the sky. The coordinate systems and reference frames used have been formalized.

In this chapter we review the currently planned hierarchy of DAL services representing the major classes of astronomical data. Each major class of data, and the associated data model and access protocol, is briefly covered. We present a special section on SSAP, the first of the second generation DAL services, which introduces much of the approach and technology to be used for the second generation services. Finally we introduce the VOClient, DALClient, and DALServer implementations from NVO, which provide reference implementations of the DAL protocols for both the client and server side of things.

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