13.1 Introduction

The ESAC Science Data Centre (ESDC; Arviset 2015) is responsible for the design and implementation of ESA’s space science archives for all astronomy, planetary science, and heliophysics missions. This group is also responsible for the long-term preservation of the data. In particular, ESDC is responsible for the implementation and maintenance of the ESA Gaia archive. This archive has been designed to (i) allow easy access and exploitation of the Gaia data products, (ii) act as a central repository to enable the validation of the catalogue before its public release and, (iii) enable the distribution of the Gaia products to other data centres.

Due to the size and complexity of the Gaia products, the ESA Gaia Archive has been designed following the approach of ’bringing code to the data’. For Gaia DR1 and Gaia DR2, in which the main product is a catalogue, the approach has been to extend the IVOA tabular access protocol (TAP). TAP provides a remote database invocation using the astronomical data query language (ADQL), itself based on structured query language (SQL). ADQL is a subset of SQL that incorporates geometrical functions like CIRCLE, DISTANCE, OVERLAPS, and CONTAINS, which simplifies typical queries done by scientists when exploring big catalogues. ADQL examples can be found at http://archives.esac.esa.int/gaia/help/adql/examples/index.html.

Standard IVOA TAP does not provide clear rules on how to handle proprietary data and/or how to share data or user tables. Since these were requirements for the Gaia Archive, ESDC has developed the concept of TAP+, a backwards-compatible extension of TAP that fulfils the needs of the Gaia mission. TAP+ is an extension of the standard IVOA TAP that, in some ways, has the popular service CasJobs, from MAST/STScI, as a precursor. Similar to this service, two different execution queues (synchronous and asynchronous) are defined. Also, users have their own database schemas with persistent areas and there are quotas and maximum execution times for queries. Since TAP+ has been defined using REST (instead of SOAP) protocols and uses a standard query language (ADQL), it maintains compatibility with standard IVOA protocols. As a result, the Gaia Archive can be queried by most of the VO-compatible applications and users can interact with the Archive in a standard way. More information on the Gaia Archive and TAP+ can be found in Salgado et al. (2017).