This tutorial assumes RabbitMQ is installed and running on localhost on the standard port (5672). In case you use a different host, port or credentials, connections settings would require adjusting.
If you're having trouble going through this tutorial you can contact us through the mailing list or RabbitMQ community Slack.
In the previous tutorial we created a work queue. The assumption behind a work queue is that each task is delivered to exactly one worker. In this part we'll do something completely different -- we'll deliver a message to multiple consumers. This pattern is known as "publish/subscribe".
To illustrate the pattern, we're going to build a simple logging system. It will consist of two programs -- the first will emit log messages and the second will receive and print them.
In our logging system every running copy of the receiver program will get the messages. That way we'll be able to run one receiver and direct the logs to disk; and at the same time we'll be able to run another receiver and see the logs on the screen.
Essentially, published log messages are going to be broadcast to all the receivers.
In previous parts of the tutorial we sent and received messages to and from a queue. Now it's time to introduce the full messaging model in Rabbit.
Let's quickly go over what we covered in the previous tutorials:
The core idea in the messaging model in RabbitMQ is that the producer never sends any messages directly to a queue. Actually, quite often the producer doesn't even know if a message will be delivered to any queue at all.
Instead, the producer can only send messages to an exchange. An exchange is a very simple thing. On one side it receives messages from producers and the other side it pushes them to queues. The exchange must know exactly what to do with a message it receives. Should it be appended to a particular queue? Should it be appended to many queues? Or should it get discarded. The rules for that are defined by the exchange type.
There are a few exchange types available: direct, topic, headers and fanout. We'll focus on the last one -- the fanout. Let's create an exchange of this type, and call it logs:
err = ch.ExchangeDeclare( "logs", // name "fanout", // type true, // durable false, // auto-deleted false, // internal false, // no-wait nil, // arguments )
The fanout exchange is very simple. As you can probably guess from the name, it just broadcasts all the messages it receives to all the queues it knows. And that's exactly what we need for our logger.
Listing exchanges
To list the exchanges on the server you can run the ever useful rabbitmqctl:
sudo rabbitmqctl list_exchangesIn this list there will be some amq.* exchanges and the default (unnamed) exchange. These are created by default, but it is unlikely you'll need to use them at the moment.
The default exchange
In previous parts of the tutorial we knew nothing about exchanges, but still were able to send messages to queues. That was possible because we were using a default exchange, which is identified by the empty string ("").
Recall how we published a message before:
err = ch.PublishWithContext(ctx, "", // exchange q.Name, // routing key false, // mandatory false, // immediate amqp.Publishing{ ContentType: "text/plain", Body: []byte(body), })Here we use the default or nameless exchange: messages are routed to the queue with the name specified by routing_key parameter, if it exists.
Now, we can publish to our named exchange instead:
err = ch.ExchangeDeclare( "logs", // name "fanout", // type true, // durable false, // auto-deleted false, // internal false, // no-wait nil, // arguments ) failOnError(err, "Failed to declare an exchange") ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second) defer cancel() body := bodyFrom(os.Args) err = ch.PublishWithContext(ctx, "logs", // exchange "", // routing key false, // mandatory false, // immediate amqp.Publishing{ ContentType: "text/plain", Body: []byte(body), })
As you may remember previously we were using queues that had specific names (remember hello and task_queue?). Being able to name a queue was crucial for us -- we needed to point the workers to the same queue. Giving a queue a name is important when you want to share the queue between producers and consumers.
But that's not the case for our logger. We want to hear about all log messages, not just a subset of them. We're also interested only in currently flowing messages not in the old ones. To solve that we need two things.
Firstly, whenever we connect to Rabbit we need a fresh, empty queue. To do this we could create a queue with a random name, or, even better - let the server choose a random queue name for us.
Secondly, once we disconnect the consumer the queue should be automatically deleted.
In the amqp client, when we supply queue name as an empty string, we create a non-durable queue with a generated name:
q, err := ch.QueueDeclare( "", // name false, // durable false, // delete when unused true, // exclusive false, // no-wait nil, // arguments )
When the method returns, the queue instance contains a random queue name generated by RabbitMQ. For example it may look like amq.gen-JzTY20BRgKO-HjmUJj0wLg.
When the connection that declared it closes, the queue will be deleted because it is declared as exclusive.
You can learn more about the exclusive flag and other queue properties in the guide on queues.
We've already created a fanout exchange and a queue. Now we need to tell the exchange to send messages to our queue. That relationship between exchange and a queue is called a binding.
err = ch.QueueBind( q.Name, // queue name "", // routing key "logs", // exchange false, nil, )
From now on the logs exchange will append messages to our queue.
Listing bindings
You can list existing bindings using, you guessed it,
rabbitmqctl list_bindings
The producer program, which emits log messages, doesn't look much different from the previous tutorial. The most important change is that we now want to publish messages to our logs exchange instead of the nameless one. We need to supply a routingKey when sending, but its value is ignored for fanout exchanges. Here goes the code for emit_log.go script:
package main import ( "context" "log" "os" "strings" "time" amqp "github.com/rabbitmq/amqp091-go" ) func failOnError(err error, msg string) { if err != nil { log.Panicf("%s: %s", msg, err) } } func main() { conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/") failOnError(err, "Failed to connect to RabbitMQ") defer conn.Close() ch, err := conn.Channel() failOnError(err, "Failed to open a channel") defer ch.Close() err = ch.ExchangeDeclare( "logs", // name "fanout", // type true, // durable false, // auto-deleted false, // internal false, // no-wait nil, // arguments ) failOnError(err, "Failed to declare an exchange") ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second) defer cancel() body := bodyFrom(os.Args) err = ch.PublishWithContext(ctx, "logs", // exchange "", // routing key false, // mandatory false, // immediate amqp.Publishing{ ContentType: "text/plain", Body: []byte(body), }) failOnError(err, "Failed to publish a message") log.Printf(" [x] Sent %s", body) } func bodyFrom(args []string) string { var s string if (len(args) < 2) || os.Args[1] == "" { s = "hello" } else { s = strings.Join(args[1:], " ") } return s }
As you see, after establishing the connection we declared the exchange. This step is necessary as publishing to a non-existing exchange is forbidden.
The messages will be lost if no queue is bound to the exchange yet, but that's okay for us; if no consumer is listening yet we can safely discard the message.
The code for receive_logs.go:
package main import ( "log" amqp "github.com/rabbitmq/amqp091-go" ) func failOnError(err error, msg string) { if err != nil { log.Panicf("%s: %s", msg, err) } } func main() { conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/") failOnError(err, "Failed to connect to RabbitMQ") defer conn.Close() ch, err := conn.Channel() failOnError(err, "Failed to open a channel") defer ch.Close() err = ch.ExchangeDeclare( "logs", // name "fanout", // type true, // durable false, // auto-deleted false, // internal false, // no-wait nil, // arguments ) failOnError(err, "Failed to declare an exchange") q, err := ch.QueueDeclare( "", // name false, // durable false, // delete when unused true, // exclusive false, // no-wait nil, // arguments ) failOnError(err, "Failed to declare a queue") err = ch.QueueBind( q.Name, // queue name "", // routing key "logs", // exchange false, nil, ) failOnError(err, "Failed to bind a queue") msgs, err := ch.Consume( q.Name, // queue "", // consumer true, // auto-ack false, // exclusive false, // no-local false, // no-wait nil, // args ) failOnError(err, "Failed to register a consumer") var forever chan struct{} go func() { for d := range msgs { log.Printf(" [x] %s", d.Body) } }() log.Printf(" [*] Waiting for logs. To exit press CTRL+C") <-forever }
If you want to save logs to a file, just open a console and type:
go run receive_logs.go > logs_from_rabbit.log
If you wish to see the logs on your screen, spawn a new terminal and run:
go run receive_logs.go
And of course, to emit logs type:
go run emit_log.go
Using rabbitmqctl list_bindings you can verify that the code actually creates bindings and queues as we want. With two receive_logs.go programs running you should see something like:
sudo rabbitmqctl list_bindings # => Listing bindings ... # => logs exchange amq.gen-JzTY20BRgKO-HjmUJj0wLg queue [] # => logs exchange amq.gen-vso0PVvyiRIL2WoV3i48Yg queue [] # => ...done.
The interpretation of the result is straightforward: data from exchange logs goes to two queues with server-assigned names. And that's exactly what we intended.
To find out how to listen for a subset of messages, let's move on to tutorial 4
Please keep in mind that this and other tutorials are, well, tutorials. They demonstrate one new concept at a time and may intentionally oversimplify some things and leave out others. For example topics such as connection management, error handling, connection recovery, concurrency and metric collection are largely omitted for the sake of brevity. Such simplified code should not be considered production ready.
Please take a look at the rest of the documentation before going live with your app. We particularly recommend the following guides: Publisher Confirms and Consumer Acknowledgements, Production Checklist and Monitoring.
If you have questions about the contents of this tutorial or any other topic related to RabbitMQ, don't hesitate to ask them on the RabbitMQ mailing list.
If you'd like to contribute an improvement to the site, its source is available on GitHub. Simply fork the repository and submit a pull request. Thank you!