Triggers: Support simple MFA calls and messages in addition to stored procedures

[dumbbell]

This idea comes from some feedback on the Erlang forum:

Have you considered maybe supporting callbacks and/or notifications as well as Funs? e.g.

{callback, Mod :: atom(), Fun :: atom()}
{notify, Method :: call | cast | info, Pid :: pid()}

Indeed, it would be nice to have a simple way to specify a function call or a message+PID.

[dumbbell]

Some random thoughts from today:

• khepri_machine:register_trigger/5 could accept the following arguments instead of a stored procedure path:

  khepri_machine:register_trigger(..., {apply, {Mod, Fun, Args}}, ...).
  khepri_machine:register_trigger(..., {send, Pid | RegisteredName, Args}, ...).

• khepri_event_handler would have to monitor the passed PID and remove the trigger when the process exit. If the argument is a registerend process name, no need to monitor. Monitors would have to be recreated on Khepri startup.

• The #triggered{} record should be modified to:

  • have a reference or a sequence to distinguish retried executions; can we use the Ra term and index directly?
  • move the path of the stored procedure to the props field and use another field for the other argument types.

• In the case of apply, the function would be called with the given arguments, plus the one that would be passed to a stored procedure.

• In the case of send, the message would be of a record, e.g. #khepri_trigger{}, containing the given arguments and the map that would be passed to a stored procedure.