I have a document to that includes a foreign key to an external data source. When a client retrieves the document, I'd like ES to transparently query the external source and materialize the data into the retrieved document just as if it was stored in ES.
I can't find some off-the-shelf functionality that does this except, possibly, rivers which are deprecated. I thought t get started by extending the FetchPhase by adding a new FetchSubFetch. Am I on the right track? Is there a more appropriate extension point for this type of activity?