gistfile1.txt

http://acme.org/document/1?transformer=untrusted-service

This calls -> 
Document Service (our trusted application that produces a json document)

Document Service then calls Transformer Service via coreos service layer, passing in the json document.

Transformer Service looks up 'untrusted service' code, executes it, passing in the json and letting the untrusted transform logic run.

Transformer Service then responds with the transformed json to the Document Service.

Document Service returns the json.