12.1 Introduction
The ESAC Science Data Centre (ESDC; Arviset 2015) designs and implements 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 has implemented and operated the ESA Gaia archive from Gaia DR1. 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 with architectural principles that allow for easy data access and analysis, specially programmatically and from data mining platforms. For Gaia DR2 and Gaia EDR3, in which the main product is a catalogue, the approach has been to extend the IVOA tabular access protocol (TAP), in connection with the IVOA DataLink protocol, to allow for seamless, data model agnostic, product query and access.
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 https://www.cosmos.esa.int/web/gaia-users/archive/query-examples.
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+ based architecture can be found in Salgado et al. (2017).