UPDATE:

• Nothing have to be change in models but all fields should be nullable at the moment
• run ./manage.py migrate to add a special urlid field who will be the @id in DB if @id is set on the request

• Accept external @id on POST / PUT / GET
• Auto set urlid for local resources
• Allow not nullable model fields
• run a PUT request on server B when server A receive a modified server B object
• notify server B when server A receive a modified link with server B object (backlink, activitypub ?)