- The ticket adding external ids to the data model - T133381: Add support for foreign entities to EntityId
- The 2 PRs on github adding the parsing ability for external IDS https://github.com/wmde/WikibaseDataModelServices/pull/146 https://github.com/wmde/WikibaseDataModelServices/pull/149
So, the current docs & blockers that need to be overcome can be seen in https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/docs/federation.wiki
Currently a shared DB is needed / DB access is needed.
Also mixing local and foreign entities currently will not work
Note that as of March 2017 it is only possible for Wikibase to use entities from a single repository, either local or foreign. For instance, it is not possible to use both local and foreign items in statements.