Query Support
The Registry supports Queries in SDMX-ML as an HTTP POST, and the new 2.1 REST query syntax which allows queries for structures using a parameterised URL. The Registry defines a number of web services, each one supports a query in any SDMX format but each service will return the response in one of the following formats; SDMX 1.0, 2.0, 2.1 or SDMX-EDI.
HTTP POST
Each web service the Registry defines will accept a SDMX-ML query as an HTTP POST message. The Registry provides a simple web form allowing users to upload a query document, or alternatively the same service can be called by an application. Each web service will accept a Registry Query Document, or a simple Query document in SDMX 1.0, 2.0, or 2.1. The only difference between the web services is the response format, which can either be SDMX 1.0, 2.0, 2.1 or EDI.
REST
SDMX Version 2.1 introduced a powerful and simple query mechanism called REST. The REST query mechanism allows a query to be defined by creating a parameterised URL – a simple example of this is:
[URL_PREFIX]/DataStructure/METATECH/ALL/1.0?references=children&xxx=yyy
The above URL is querying for all version 1.0 Data Structure Definitions maintained by the Agency with Id METATECH. The Metadata Technology Registry fully supports the REST syntax, and depending on the web service queried, the response will come back in SDMX 1.0, 2.0, 2.1 or EDI.
Reference Resolution
As the registry has to maintain full referential integrity, any cross referenced structure must exist in the Registry, however SDMX does allow for ‘Stub’ Structures to exist providing they define the location where the full structure can be found. The location may be either a file, a REST URL, or a SDMX web service which returns the structure. This allows maintenance of structures such outside of the registry, such as codelists which can be large and change on a daily basis. These types of references are called ‘External References’.
The Registry will attempt to resolve any externally referenced structures when queried. This includes structures that can be resolved from URLs, other SDMX Web services and even SDMX files. If the URL, Web Service, or file contains more structures then the one the registry is looking for, then the registry will be able to determine which structure matches the criteria. If the request is for the structures to be returned in SDMX 2.1 but the external reference is referencing an earlier version of SDMX, then no problem, the Registry will automatically upgrade the message – this is also true for downgrading.