[RabbitMQ][AMQP] Failing to get and read single message with amqp_basic_get and amqp_read_message - rabbitmq

Question

I want to setup a consumer with amqp to read from a specific queue. Some googling pointed out that this can be done with amqp_basic_get, and looking into the documentation, the actual message is retrieved with amqp_read_message. I also found this example which I tried to follow for implementing the basic_get. Nevertheless, I am failing to get and read a message from a specific queue.
My scenario is like this: I have two programs that communicate by publishing and consuming from the rabbitmq server. In each, a connection is declared, with two channels, one meant for consuming, and one for publishing. The flow of information is like this: program A gets the current time and publishes to rabbitmq. Upon receiving this message, program B gets its own time, packages its time and the received time in a message that it publishes to rabbitmq. Program A should consume this message. However, I cannot succeed in reading from the namedQueue.
Program A (in c++, and uses the amqp.c) is implemented as follows:
... after creating the connection
//Create channels
amqp_channel_open_ok_t *res = amqp_channel_open(conn, channelIDPub);
assert(res != NULL);
amqp_channel_open_ok_t *res2 = amqp_channel_open(conn, channelIDSub);
assert(res2 != NULL);
//Declare exchange
exchange = "exchangeName";
exchangetype = "direct";
amqp_exchange_declare(conn, channelIDPub, amqp_cstring_bytes(exchange.c_str()),
amqp_cstring_bytes(exchangetype.c_str()), 0, 0, 0, 0,
amqp_empty_table);
...
throw_on_amqp_error(amqp_get_rpc_reply(conn), printText.c_str());
//Bind the exchange to the queue
const char* qname = "namedQueue";
amqp_bytes_t queue = amqp_bytes_malloc_dup(amqp_cstring_bytes(qname));
amqp_queue_declare_ok_t *r = amqp_queue_declare(
conn, channelIDSub, queue, 0, 0, 0, 0, amqp_empty_table);
throw_on_amqp_error(amqp_get_rpc_reply(conn), "Declaring queue");
if (queue.bytes == NULL) {
fprintf(stderr, "Out of memory while copying queue name");
return;
}
amqp_queue_bind(conn, channelIDSub, queue, amqp_cstring_bytes(exchange.c_str()),
amqp_cstring_bytes(queueBindingKey.c_str()), amqp_empty_table);
throw_on_amqp_error(amqp_get_rpc_reply(conn), "Binding queue");
amqp_basic_consume(conn, channelIDSub, queue, amqp_empty_bytes, 0, 0, 1,
amqp_empty_table);
throw_on_amqp_error(amqp_get_rpc_reply(conn), "Consuming");
// ...
// In order to get a message from rabbitmq
amqp_rpc_reply_t res, res2;
amqp_message_t message;
amqp_boolean_t no_ack = false;
amqp_maybe_release_buffers(conn);
printf("were here, with queue name %s, on channel %d\n", queueName, channelIDSub);
amqp_time_t deadline;
struct timeval timeout = { 1 , 0 };//same timeout used in consume(json)
int time_rc = amqp_time_from_now(&deadline, &timeout);
assert(time_rc == AMQP_STATUS_OK);
do {
res = amqp_basic_get(conn, channelIDSub, amqp_cstring_bytes("namedQueue"), no_ack);
} while (res.reply_type == AMQP_RESPONSE_NORMAL &&
res.reply.id == AMQP_BASIC_GET_EMPTY_METHOD
&& amqp_time_has_past(deadline) == AMQP_STATUS_OK);
if (AMQP_RESPONSE_NORMAL != res.reply_type || AMQP_BASIC_GET_OK_METHOD != res.reply.id)
{
printf("amqp_basic_get error codes amqp_response_normal %d, amqp_basic_get_ok_method %d\n", res.reply_type, res.reply.id);
return false;
}
res2 = amqp_read_message(conn,channelID,&message,0);
printf("error %s\n", amqp_error_string2(res2.library_error));
printf("5:reply type %d\n", res2.reply_type);
if (AMQP_RESPONSE_NORMAL != res2.reply_type) {
printf("6:reply type %d\n", res2.reply_type);
return false;
}
payload = std::string(reinterpret_cast< char const * >(message.body.bytes), message.body.len);
printf("then were here\n %s", payload.c_str());
amqp_destroy_message(&message);
Program B (in python) is as follows
#!/usr/bin/env python3
import pika
import json
from datetime import datetime, timezone
import time
import threading
cosimTime = 0.0
newData = False
lock = threading.Lock()
thread_stop = False
connection = pika.BlockingConnection(pika.ConnectionParameters('localhost'))
connectionPublish = pika.BlockingConnection(pika.ConnectionParameters('localhost'))
channelConsume = connection.channel()
channelPublish = connectionPublish.channel()
print("Declaring exchange")
channelConsume.exchange_declare(exchange='exchangeName', exchange_type='direct')
channelPublish.exchange_declare(exchange='exchangeName', exchange_type='direct')
print("Creating queue")
result = channelConsume.queue_declare(queue='', exclusive=True)
queue_name = result.method.queue
result2 = channelPublish.queue_declare(queue='namedQueue', exclusive=False, auto_delete=False)
channelConsume.queue_bind(exchange='exchangeName', queue=queue_name,
routing_key='fromB')
channelPublish.queue_bind(exchange='exchangeName', queue="namedQueue",
routing_key='toB')
print(' [*] Waiting for logs. To exit press CTRL+C')
def callbackConsume(ch, method, properties, body):
global newData, cosimTime
print("\nReceived [x] %r" % body)
#cosimTime = datetime.datetime.strptime(body, "%Y-%m-%dT%H:%M:%S.%f%z")
with lock:
newData = True
cosimTime = body.decode()
cosimTime = json.loads(cosimTime)
#print(cosimTime)
def publishRtime():
global newData
while not thread_stop:
if newData:
#if True:
with lock:
newData = False
msg = {}
msg['rtime'] = datetime.now(timezone.utc).astimezone().isoformat(timespec='milliseconds')
msg['cosimtime'] = cosimTime["simAtTime"]
print("\nSending [y] %s" % str(msg))
channelPublish.basic_publish(exchange='exchangeName',
routing_key='toB',
body=json.dumps(msg))
#time.sleep(1)
channelConsume.basic_consume(
queue=queue_name, on_message_callback=callbackConsume, auto_ack=True)
try:
thread = threading.Thread(target = publishRtime)
thread.start()
channelConsume.start_consuming()
except KeyboardInterrupt:
print("Exiting...")
channelConsume.stop_consuming()
thread_stop = True
connection.close()
What program A outputs is:
amqp_basic_get error codes amqp_response_normal 1, amqp_basic_get_ok_method 3932232
which is the code for AMQP_BASIC_GET_EMPTY_METHOD.
Program B gets the data, and publishes continuously.
If I slightly modify B to just publish all the time a specific string, then it seems that the amqp_basic_get returns successfully, however then it fails at amqp_read_message with the code AMQP_RESPONSE_LIBRARY_EXCEPTION.
Any idea how to get this to work, what I am missing the setup?

Answer 1

The issue was in the queue_declare where the auto_delete parameter was not matching on both sides.