The DSDs that are used to structure the data exchange may not coincide with the internal structural metadata, for example in the local data warehouse. It is, therefore, necessary to create a mapping between the local structures and the DSDs. In this case, subject-matter and metadata experts should be involved to create the mapping required.

The main goals of creating the mapping are:

• to translate the incoming SDMX queries to the internal structural metadata in order to output the correct internal data;
• output the internal data using the structure and coding of the SDMX project's DSDs.  

The image¹  below shows how a local data warehouse is queried by a SDMX query translated through the mapping (from DSDs to local structural metadata) and the resulting SDMX-ML file conforming to the data exchange's DSD structure is output.

There are existing tools that can be used to author the mappings, and use them to transform the data:

• The Mapping Assistant tool that is part of the SDMX Reference Infrastructure tool (SDMX-RI);
• create the mappings in SDMX Structure Sets and use the open-source CSPA Transform service to perform the automated remapping.

Output of this phase:

• The mappings between the internal structures and data exchange DSD(s);
• the transformation system

¹ Pictures taken from "SDMX Architecture Using the Pull Method for Data Sharing" - SDMX self-learning package No. 7 - Student book