How do I have RabbitMQ redeliver unacknowledged messages? - rabbitmq

Question

I'm following the following tutorial to the letter:
https://www.rabbitmq.com/tutorials/tutorial-two-java.html.
I start the RabbitMQ server as such:
docker pull rabbitmq
docker run -d --hostname my-rabbit-host --name my-rabbit -p 5672:5672 rabbitmq:3
From the tutorial:
Using this code we can be sure that even if you kill a worker using
CTRL+C while it was processing a message, nothing will be lost. Soon
after the worker dies all unacknowledged messages will be redelivered.
I spawn two consumers, and when I CTRL+C one of them, the other running one does not receive the messages that were originally destined to the former consumer. How do I get the messages to be redelivered after CTRL+C'ing out of one of the consumers?
Edit: I'm now installing RabbitMQ via 'brew', but I'm still seeing the same issue.
brew update
brew install rabbitmq
/usr/local/sbin/rabbitmq-server &

Answer 1

There is no need to put sleep or anything like that in the consumer code. On the link you provided, search for paragraph starting with Manual message acknowledgments and look at the code there. The key thing is not to acknowledge the message. If you have autoACK flag set to true, then you can call anything you want, the message is acknowledged as soon as it is received. So simply don't set that flag, and also for testing you could comment out the line channel.basicAck(envelope.getDeliveryTag(), false); in order not to do manual ACK either. So when the consumer exits, the message will still be in the queue.

Answer 2

Strange, RabbitMQ works for me out of the box.
Step 1, I started RabbitMQ:
$ docker run -d --hostname my-rabbit-host --name my-rabbit -p 5672:5672 rabbitmq:3
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e0c3257b8b49 rabbitmq:3 "docker-entrypoint.s…" 18 minutes ago Up 14 minutes 4369/tcp, 5671/tcp, 25672/tcp, 0.0.0.0:5672->5672/tcp my-rabbit
Step 2, I published a message (By the way, I tried with Node.js. See Appendix below for source code):
$ node src/producer.js
Publisher: TODO 1st
Step 3, I started two consumers one after another (my consumer is designed, for the testing purpose, not to acknowledge, so RabbitMQ will never dequeue a message).
Consumer 1 will receive the message, while consumer 2 won't.
Consumer 1:
$ node src/consumer.js
Consumer: TODO 1st
Consumer 2:
$ node src/consumer.js
Step 4, when I stop consumer 1 by 'Ctrl + c', Consumer 2 will immediately receive the message from RabbitMQ:
Consumer 2:
$ node src/consumer.js
Consumer: TODO 1st
Conclusion: Basically, when setting up a consumer, we need to tell RabbitMQ not to dequeue the message until its acknowledgement has been received from the consumer. As a result, if consumer 1 is stopped for any reason before it has a chance to acknowledge the message, RabbitMQ will redeliver the message to consumer 2.
Appendix
src/producer.js
var q = 'tasks'
function bail (err) {
console.error(err)
process.exit(1)
}
// Publisher
function publisher (conn) {
conn.createChannel(onOpen)
function onOpen (err, ch) {
if (err != null) bail(err)
ch.assertQueue(q)
const msg = 'TODO 1st'
ch.sendToQueue(q, Buffer.from(msg), { persistent: true })
console.log('Publisher: ', msg)
}
}
require('amqplib/callback_api')
.connect('amqp://guest:guest#localhost', function (err, conn) {
if (err != null) bail(err)
publisher(conn)
})
src/consumer.js
var q = 'tasks'
function bail (err) {
console.error(err)
process.exit(1)
}
// Consumer
function consumer (conn) {
conn.createChannel(onOpen)
function onOpen (err, ch) {
if (err != null) bail(err)
ch.assertQueue(q)
ch.consume(q, function (msg) {
if (msg !== null) {
console.log('Consumer: ', msg.content.toString())
// Commented out the line below, so RabbitMQ never dequeues a message
// ch.ack(msg)
}
}, { noAck: false })
}
}
require('amqplib/callback_api')
.connect('amqp://guest:guest#localhost', function (err, conn) {
if (err != null) bail(err)
consumer(conn)
})