Allow querying for past data (previous "vintages")

[dosse]

SDMX rest should provide a way to retrieve the data (and only that) as it would have been retrieved at a specific point in time in the past, like a snapshot.

In SDMX-RI NSI web service, the following rest feature was already successfully implemented:

• A URL parameter asOf=[date-time-and-zone] can be used to specific the time in point in the past, with a value type similar to the updatedAfter parameter (allowing to specify a date-time with a time zone).
• The response is one dataset, which has the validFrom and validTo properties set to the asOf input value.
• The response action property is set to information, despite its deprecation earlier this year.

[sosna]

This feature might be interesting for other types or resources as well (e.g. structures or schemas). So, you could retrieve all information about a dataflow (dataflow, DSD, codelists, etc.), as it was at a certain point in time.