<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def engine(self):
 """ Return an engine instance, creating it if it doesn't exist. Recreate the engine connection if it wasn't originally created by the current process. """

        pid = os.getpid()
        conn = SQLAlchemyTarget._engine_dict.get(self.connection_string)
        if not conn or conn.pid != pid:
            # create and reset connection
            engine = sqlalchemy.create_engine(
                self.connection_string,
                connect_args=self.connect_args,
                echo=self.echo
            )
            SQLAlchemyTarget._engine_dict[self.connection_string] = self.Connection(engine, pid)
        return SQLAlchemyTarget._engine_dict[self.connection_string].engine