https://github.com/bensheldon/good_job/blob/10e9d9b714a668dc...
That's heavily inspired by Rail's Action Cable (websockets) Adapter for Postgres, which is a bit simpler and easier to understand:
https://github.com/rails/rails/blob/be287ac0d5000e667510faba...
Briefly, it spins up a background thread with a dedicated database connection and makes a blocking Postgres LISTEN query until results are returned, and then it forwards the result to other subscribing objects.