python: how to get notifications for mysql database changes?

Question

In Python, is there a way to get notified that a specific table in a MySQL database has changed?

Answer 1

If by changed you mean if a row has been updated, deleted or inserted then there is a workaround.

You can create a trigger in MySQL

DELIMITER $$

CREATE TRIGGER ai_tablename_each AFTER INSERT ON tablename FOR EACH ROW
BEGIN
  DECLARE exec_result integer;

  SET exec_result = sys_exec(CONCAT('my_cmd '
                                    ,'insert on table tablename '
                                    ,',id=',new.id));
  IF exec_result = 0 THEN BEGIN
    INSERT INTO table_external_result (id, tablename, result) 
      VALUES (null, 'tablename', 0)
  END; END IF;
END$$

DELIMITER ;

This will call executable script my_cmd on the server. (see sys_exec fro more info) with some parameters.

my_cmd can be a Python program or anything you can execute from the commandline using the user account that MySQL uses.

You'd have to create a trigger for every change (INSERT/UPDATE/DELETE) that you'd want your program to be notified of, and for each table.
Also you'd need to find some way of linking your running Python program to the command-line util that you call via sys_exec().

Not recommended
This sort of behaviour is not recommend because it is likely to:

1. slow MySQL down;
2. make it hang/timeout if my_cmd does not return;
3. if you are using transaction, you will be notified before the transaction ends;
4. I'm not sure if you'll get notified of a delete if the transaction rolls back;
5. It's an ugly design

Links
sys_exec: http://www.mysqludf.org/lib_mysqludf_sys/index.php