Looking for solution.
I'm using MySQL triggers to perform after insert/update and before delete logging of changes to a MySQL table.
Not considering server-side scripting at this point.
In the context of the trigger, the MySQL function USER() evaluates to "appuser@localhost" as predicted, since DF is configured to access mysql running on localhost using "appuser" credentials.
What I think I'm looking for is a connection-related or session-related variable that gets SET by DF whenever a REST API call results in a SQL statement.
I'm running on a resent version of MySQL, and so I have the performance_schema database, if that helps.
I've already looked at performance_schema.user_variables_by_thread, but I don't see anything helpful within the context of a trigger instance.