Called once for each message produced to indicate delivery result.
        Triggered by poll() or flush().

def _delivery_report(err, msg):
    ''' Called once for each message produced to indicate delivery result.
        Triggered by poll() or flush(). '''
    if err is not None:
        log.error('Message delivery failed: %s', err)
    else:
        log.debug('Message delivered to %s [%s]', msg.topic(), msg.partition())