RabbitMQ pika.exceptions.ConnectionClosed

Question

I tried to send message and receive message using RabbitMQ. I dont have computer science background, the terms I used could not be very accurate.

I try to copy the tutorial file: When submitting my html form, my python script (cgi) the message is submitting to the queue

connection = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
        channel = connection.channel()
        channel.queue_declare(queue='task_queue', durable=True)
        message = PN
        channel.basic_publish(exchange='',
                              routing_key='task_queue',
                              body=message,
                              properties=pika.BasicProperties(
                                 delivery_mode = 2, # make message persistent
                              ))
        connection.close()

my receiver is running :

connection = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
channel = connection.channel()
channel.queue_declare(queue='task_queue', durable=True)
print(' [*] Waiting for messages. To exit press CTRL+C')

def callback(ch, method, properties, body):
    print(" [x] Received Project %r" % body)
    #ch.basic_ack(delivery_tag = method.delivery_tag) 
    if not (os.path.isfile(js_path)):
        print (' [*] ERROR files missing ')
        #ch.basic_ack(delivery_tag = method.delivery_tag)
        return
    p= subprocess.Popen(run a subprocess here)
    p.wait()

    print (' [*] Temporary Files removed')
    print(" [*] Waiting for messages. To exit press CTRL+C")

channel.basic_qos(prefetch_count=1)
channel.basic_consume(callback,queue='task_queue',no_ack=True)
channel.start_consuming()

It manages most of the time but randomly crash with the following error:

Traceback (most recent call last): File "Receive5.py", line 139, in channel.start_consuming() File "C:\Python27\lib\site-packages\pika\adapters\blocking_connection.py", line 1681, in start_consuming self.connection.process_data_events(time_limit=None) File "C:\Python27\lib\site-packages\pika\adapters\blocking_connection.py", line 647, in process_data_events self._flush_output(common_terminator) File "C:\Python27\lib\site-packages\pika\adapters\blocking_connection.py", line 426, in _flush_output raise exceptions.ConnectionClosed() pika.exceptions.ConnectionClosed

Answer 1

This is because you are keeping the main thread waiting, and because of this pika cannot handle incoming messages; in this case it cannot respond to the heartbeat until the subprocess is done. This causes RabbitMQ to think that the client is dead and forces a disconnection.

If you want this to work with heartbeats (which is recommend) you need to periodically call connection.process_data_events. This can be done by adding a loop that checks if the thread is done, and every 30s or so call process_data_events until the thread is done.

Answer 2

Here is pika document about how to avoid the connection being dropped because of heartbeat.

https://pika.readthedocs.io/en/stable/examples/heartbeat_and_blocked_timeouts.html

In a version of pika older than 0.11.2, although we can add an argument inside the pika.ConnectionParameters: heartbeat_interval=600, but it cannot help if the server side has a short heartbeat value of 60s. It can work only when the version is at least 0.11.2