RabbitMQ / AMQP: single queue, multiple consumers for same message?

Question

I am just starting to use RabbitMQ and AMQP in general.

• I have a queue of messages
• I have multiple consumers, which I would like to do different things with the same message.

Most of the RabbitMQ documentation seems to be focused on round-robin, ie where a single message is consumed by a single consumer, with the load being spread between each consumer. This is indeed the behavior I witness.

An example: the producer has a single queue, and send messages every 2 sec:

var amqp = require('amqp');
var connection = amqp.createConnection({ host: "localhost", port: 5672 });
var count = 1;

connection.on('ready', function () {
  var sendMessage = function(connection, queue_name, payload) {
    var encoded_payload = JSON.stringify(payload);  
    connection.publish(queue_name, encoded_payload);
  }

  setInterval( function() {    
    var test_message = 'TEST '+count
    sendMessage(connection, "my_queue_name", test_message)  
    count += 1;
  }, 2000) 


})

And here's a consumer:

var amqp = require('amqp');
var connection = amqp.createConnection({ host: "localhost", port: 5672 });
connection.on('ready', function () {
  connection.queue("my_queue_name", function(queue){
    queue.bind('#'); 
    queue.subscribe(function (message) {
      var encoded_payload = unescape(message.data)
      var payload = JSON.parse(encoded_payload)
      console.log('Recieved a message:')
      console.log(payload)
    })
  })
})

If I start the consumer twice, I can see that each consumer is consuming alternate messages in round-robin behavior. Eg, I'll see messages 1, 3, 5 in one terminal, 2, 4, 6 in the other.

My question is:

• Can I have each consumer receive the same messages? Ie, both consumers get message 1, 2, 3, 4, 5, 6? What is this called in AMQP/RabbitMQ speak? How is it normally configured?

• Is this commonly done? Should I just have the exchange route the message into two separate queues, with a single consumer, instead?

Answer 1

A topic exchange is probably not what you want.

Use a direct exchange. For every consumer that you want the message copied to, create a new queue. What you do, is you match the data transfer using the routing key, not the queue name.

   err = ctx.rmqChannel.Publish(
    rabbitMQExchange, // direct exchange
    rk,               // routing key
    false,            // mandatory
    false,            // immediate
    amqp.Publishing{
        ContentType: "application/json",
        Body:        v,
    },
   )

    q, err := ch.QueueDeclare(
        uuid.New().String(), // random unique name
        true,                // durable
        true,                // delete when unused
        false,               // exclusive
        false,               // no-wait
        nil,                 // arguments
    )

    if err := ch.QueueBind(
        q.Name,           // queue name
        rk,               // routing key matches above rk
        rabbitMQExchange, // direct exchange
        false,            // no-wait
        nil,              // arguments
    ); err != nil {
        return
    }

    msgs, err := ch.Consume(
        q.Name, // queue
        "",     // id, a uuid is probably good too
        true,  // auto-ack
        false, // exclusive
        false, // no-local
        false, // no-wait
        nil,   // args
    )

Answer 2

Newer versions of RabbitMq allow 'streams'. One of the use case that solves streams is 'Large fan-outs' and I think that it is the correct solution from here:

When wanting to deliver the same message to multiple subscribers users currently have to bind a dedicated queue for each consumer. If the number of consumers is large this becomes potentially inefficient, especially when wanting persistence and/or replication. Streams will allow any number of consumers to consume the same messages from the same queue in a non-destructive manner, negating the need to bind multiple queues. Stream consumers will also be able to read from replicas allowing read load to be spread across the cluster.

Ref.: RabbitMQ streams

Answer 3

You just need to assign different groups to the consumers.

Answer 4

Can I have each consumer receive the same messages? Ie, both consumers get message 1, 2, 3, 4, 5, 6? What is this called in AMQP/RabbitMQ speak? How is it normally configured?

No, not if the consumers are on the same queue. From RabbitMQ's AMQP Concepts guide:

it is important to understand that, in AMQP 0-9-1, messages are load balanced between consumers.

This seems to imply that round-robin behavior within a queue is a given, and not configurable. Ie, separate queues are required in order to have the same message ID be handled by multiple consumers.

Is this commonly done? Should I just have the exchange route the message into two separate queues, with a single consumer, instead?

No it's not, single queue/multiple consumers with each consumer handling the same message ID isn't possible. Having the exchange route the message onto into two separate queues is indeed better.

As I don't require too complex routing, a fanout exchange will handle this nicely. I didn't focus too much on Exchanges earlier as node-amqp has the concept of a 'default exchange' allowing you to publish messages to a connection directly, however most AMQP messages are published to a specific exchange.

Here's my fanout exchange, both sending and receiving:

var amqp = require('amqp');
var connection = amqp.createConnection({ host: "localhost", port: 5672 });
var count = 1;

connection.on('ready', function () {
  connection.exchange("my_exchange", options={type:'fanout'}, function(exchange) {   
 
    var sendMessage = function(exchange, payload) {
      console.log('about to publish')
      var encoded_payload = JSON.stringify(payload);
      exchange.publish('', encoded_payload, {})
    }

    // Recieve messages
    connection.queue("my_queue_name", function(queue){
      console.log('Created queue')
      queue.bind(exchange, ''); 
      queue.subscribe(function (message) {
        console.log('subscribed to queue')
        var encoded_payload = unescape(message.data)
        var payload = JSON.parse(encoded_payload)
        console.log('Recieved a message:')
        console.log(payload)
      })
    })
  
    setInterval( function() {    
      var test_message = 'TEST '+count
      sendMessage(exchange, test_message)  
      count += 1;
    }, 2000) 
 })
})

Answer 5

Fan out was clearly what you wanted. fanout

read rabbitMQ tutorial: https://www.rabbitmq.com/tutorials/tutorial-three-javascript.html

here's my example:

Publisher.js:

amqp.connect('amqp://<user>:<pass>@<host>:<port>', async (error0, connection) => {
    if (error0) {
      throw error0;
    }
    console.log('RabbitMQ connected')
    try {
      // Create exchange for queues
      channel = await connection.createChannel()
      await channel.assertExchange(process.env.EXCHANGE_NAME, 'fanout', { durable: false });
      await channel.publish(process.env.EXCHANGE_NAME, '', Buffer.from('msg'))
    } catch(error) {
      console.error(error)
    }
})

Subscriber.js:

amqp.connect('amqp://<user>:<pass>@<host>:<port>', async (error0, connection) => {
    if (error0) {
      throw error0;
    }
    console.log('RabbitMQ connected')
    try {
      // Create/Bind a consumer queue for an exchange broker
      channel = await connection.createChannel()
      await channel.assertExchange(process.env.EXCHANGE_NAME, 'fanout', { durable: false });
      const queue = await channel.assertQueue('', {exclusive: true})
      channel.bindQueue(queue.queue, process.env.EXCHANGE_NAME, '')

      console.log(" [*] Waiting for messages in %s. To exit press CTRL+C");
      channel.consume('', consumeMessage, {noAck: true});
    } catch(error) {
      console.error(error)
    }
});

here is an example i found in the internet. maybe can also help. https://www.codota.com/code/javascript/functions/amqplib/Channel/assertExchange

Answer 6

The last couple of answers are almost correct - I have tons of apps that generate messages that need to end up with different consumers so the process is very simple.

If you want multiple consumers to the same message, do the following procedure.

Create multiple queues, one for each app that is to receive the message, in each queue properties, "bind" a routing tag with the amq.direct exchange. Change you publishing app to send to amq.direct and use the routing-tag (not a queue). AMQP will then copy the message into each queue with the same binding. Works like a charm :)

Example: Lets say I have a JSON string I generate, I publish it to the "amq.direct" exchange using the routing tag "new-sales-order", I have a queue for my order_printer app that prints order, I have a queue for my billing system that will send a copy of the order and invoice the client and I have a web archive system where I archive orders for historic/compliance reasons and I have a client web interface where orders are tracked as other info comes in about an order.

So my queues are: order_printer, order_billing, order_archive and order_tracking All have the binding tag "new-sales-order" bound to them, all 4 will get the JSON data.

This is an ideal way to send data without the publishing app knowing or caring about the receiving apps.

Answer 7

There is one interesting option in this scenario I haven`t found in answers here.

You can Nack messages with "requeue" feature in one consumer to process them in another. Generally speaking it is not a right way, but maybe it will be good enough for someone.

https://www.rabbitmq.com/nack.html

And beware of loops (when all concumers nack+requeue message)!

Answer 8

I think you should check sending your messages using the fan-out exchanger. That way you willl receiving the same message for differents consumers, under the table RabbitMQ is creating differents queues for each one of this new consumers/subscribers.

This is the link for see the tutorial example in javascript https://www.rabbitmq.com/tutorials/tutorial-one-javascript.html

Answer 9

If you happen to be using the amqplib library as I am, they have a handy example of an implementation of the Publish/Subscribe RabbitMQ tutorial which you might find handy.

Answer 10

As I assess your case is:

• I have a queue of messages (your source for receiving messages, lets name it q111)

• I have multiple consumers, which I would like to do different things with the same message.

Your problem here is while 3 messages are received by this queue, message 1 is consumed by a consumer A, other consumers B and C consumes message 2 and 3. Where as you are in need of a setup where rabbitmq passes on the same copies of all these three messages(1,2,3) to all three connected consumers (A,B,C) simultaneously.

While many configurations can be made to achieve this, a simple way is to use the following two step concept:

• Use a dynamic rabbitmq-shovel to pickup messages from the desired queue(q111) and publish to a fanout exchange (exchange exclusively created and dedicated for this purpose).
• Now re-configure your consumers A,B & C (who were listening to queue(q111)) to listen from this Fanout exchange directly using a exclusive & anonymous queue for each consumer.

Note: While using this concept don't consume directly from the source queue(q111), as messages already consumed wont be shovelled to your Fanout exchange.

If you think this does not satisfies your exact requirement... feel free to post your suggestions :-)

Answer 11

RabbitMQ / AMQP: single queue, multiple consumers for same message and page refresh.

rabbit.on('ready', function () {    });
    sockjs_chat.on('connection', function (conn) {

        conn.on('data', function (message) {
            try {
                var obj = JSON.parse(message.replace(/\r/g, '').replace(/\n/g, ''));

                if (obj.header == "register") {

                    // Connect to RabbitMQ
                    try {
                        conn.exchange = rabbit.exchange(exchange, { type: 'topic',
                            autoDelete: false,
                            durable: false,
                            exclusive: false,
                            confirm: true
                        });

                        conn.q = rabbit.queue('my-queue-'+obj.agentID, {
                            durable: false,
                            autoDelete: false,
                            exclusive: false
                        }, function () {
                            conn.channel = 'my-queue-'+obj.agentID;
                            conn.q.bind(conn.exchange, conn.channel);

                            conn.q.subscribe(function (message) {
                                console.log("[MSG] ---> " + JSON.stringify(message));
                                conn.write(JSON.stringify(message) + "\n");
                            }).addCallback(function(ok) {
                                ctag[conn.channel] = ok.consumerTag; });
                        });
                    } catch (err) {
                        console.log("Could not create connection to RabbitMQ. \nStack trace -->" + err.stack);
                    }

                } else if (obj.header == "typing") {

                    var reply = {
                        type: 'chatMsg',
                        msg: utils.escp(obj.msga),
                        visitorNick: obj.channel,
                        customField1: '',
                        time: utils.getDateTime(),
                        channel: obj.channel
                    };

                    conn.exchange.publish('my-queue-'+obj.agentID, reply);
                }

            } catch (err) {
                console.log("ERROR ----> " + err.stack);
            }
        });

        // When the visitor closes or reloads a page we need to unbind from RabbitMQ?
        conn.on('close', function () {
            try {

                // Close the socket
                conn.close();

                // Close RabbitMQ           
               conn.q.unsubscribe(ctag[conn.channel]);

            } catch (er) {
                console.log(":::::::: EXCEPTION SOCKJS (ON-CLOSE) ::::::::>>>>>>> " + er.stack);
            }
        });
    });

Answer 12

The send pattern is a one-to-one relationship. If you want to "send" to more than one receiver you should be using the pub/sub pattern. See http://www.rabbitmq.com/tutorials/tutorial-three-python.html for more details.

Answer 13

To get the behavior you want, simply have each consumer consume from its own queue. You'll have to use a non-direct exchange type (topic, header, fanout) in order to get the message to all of the queues at once.

Answer 14

Just read the rabbitmq tutorial. You publish message to exchange, not to queue; it is then routed to appropriate queues. In your case, you should bind separate queue for each consumer. That way, they can consume messages completely independently.

Answer 15

Yes each consumer can receive the same messages. have a look at http://www.rabbitmq.com/tutorials/tutorial-three-python.html http://www.rabbitmq.com/tutorials/tutorial-four-python.html http://www.rabbitmq.com/tutorials/tutorial-five-python.html

for different ways to route messages. I know they are for python and java but its good to understand the principles, decide what you are doing and then find how to do it in JS. Its sounds like you want to do a simple fanout (tutorial 3), which sends the messages to all queues connected to the exchange.

The difference with what you are doing and what you want to do is basically that you are going to set up and exchange or type fanout. Fanout excahnges send all messages to all connected queues. Each queue will have a consumer that will have access to all the messages separately.

Yes this is commonly done, it is one of the features of AMPQ.