Short question:

I’m publishing a message to a dead letter queue with specific headers in x-dead. When the message is consumed by the dead letter consumers, the headers are gone. Why is this?

Long question:

I have an exchange with a dead letter queue:

def create_dlx(channel):
    channel.exchange_declare(exchange=f"{EXCHANGE_NAME}.dlx", exchange_type='fanout')
    channel.queue_declare(DLX_QUEUE)
    channel.queue_bind(exchange=f"{EXCHANGE_NAME}.dlx", queue=DLX_QUEUE)

def create_exchange(channel):
    args = {
        "x-dead-letter-exchange": f"{EXCHANGE_NAME}.dlx"
    }
    channel.exchange_declare(exchange=EXCHANGE_NAME, exchange_type='topic', arguments=args)

    # some queues with specific routing keys are declared here

This is the dlx consumer:

def get_dlx_channel():
    print(f"Consumer: Starting connection to DLX {EXCHANGE_NAME}.dlx on queue {DLX_QUEUE}")

    connection = pika.BlockingConnection(rabbitmq_params)
    channel = connection.channel()
    
    channel.basic_qos(prefetch_count=1)
    channel.basic_consume(queue=DLX_QUEUE, on_message_callback=dlx_callback)
    return channel, connection

def dlx_callback(ch, method, properties, body):    
    original_routing_key = properties.headers['x-death'][0]['routing-keys'][0]
    request_key = original_routing_key
    failure_reason = 'Dead Letter'

    print(f'{str(datetime.now())} - {failure_reason} {request_key}')
    ch.basic_ack(delivery_tag=method.delivery_tag)

Currently the setup works. If a consumer rejects or nacks a message, it is correctly routed to the dlx queue.

Now I’m trying to make the rejections more informative. A lot of my consumers have the logic:

try:
    do_thing()
    ch.basic_ack(delivery_tag=method.delivery_tag)
    send_response()
except:
    ch.basic_reject(delivery_tag=method.delivery_tag, requeue=False)

I want to update this so the rejected message is sent to the dlx queue with an error reason. In the except block, I do this:

def get_reject_properties(method, reason):
    properties = pika.BasicProperties(
        headers={
            'x-death': [{
                'reason': 'rejected',
                'queue': method.routing_key,
                'time': int(time.time()),
                'exchange': method.exchange,
                'routing-keys': [method.routing_key]
            }],
            'x-rejection-reason': reason 
        }
    )
    return properties

reject_properties = get_reject_properties(method, reason)
ch.basic_publish(
    exchange=f"{EXCHANGE_NAME}.dlx",
    routing_key=method.routing_key,
    body=body,
    properties=reject_properties
        )

This creates an error on the consumer side in the line properties.headers['x-death'][0]['routing-keys'][0]. When the message is received, properties.headers['x-death'] is an empty list even though the x-death headers are explicitly specified in the message properties when publishing.

Why is this? How can I send a message to the dlx queue with the original message header in x-death and an x-rejection-reason header?