Comment by Nextgrid

You can model external system interactions with tables representing "mailboxes" - so for example if a DB stored procedure needs to call a third-party API to create a resource, it writes a row in the "outbox" table for that API, then application-level code picks that up, makes the API call, parses the response (extracts the required fields) and stores it in an "inbox" table so now the database has access to the response (and a trigger can run the remainder of the business process upon insertion of that row).