What is "External State" in flink?

Question

While all the documentation on the Flink site does not mention an "external state", we came across this concept in a paper (http://www.vldb.org/pvldb/vol10/p1718-carbone.pdf section 4.1) . How do we enable this?

Answer 1

There's been an experiment or two with using an external database as a state backend for Flink, but nothing along these lines has made its way into the open source project.