RabbitMQ tutorial - "Hello World!" SUPPRESS-RHS

Introduction

“Hello World”

(using the Go RabbitMQ client)

In this part of the tutorial we’ll write two small programs in Go; a producer that sends a single message, and a consumer that receives messages and prints them out. We’ll gloss over some of the detail in the Go RabbitMQ API, concentrating on this very simple thing just to get started. It’s a “Hello World” of messaging.

In the diagram below, “P” is our producer and “C” is our consumer. The box in the middle is a queue - a message buffer that RabbitMQ keeps on behalf of the consumer.

(P) -> [|||] -> ©“ height="60” />
</div>

<blockquote>
<h4 id=The Go RabbitMQ client library

RabbitMQ speaks multiple protocols. This tutorial uses AMQP 0-9-1, which is an open, general-purpose protocol for messaging. There are a number of clients for RabbitMQ in many different languages. We’ll use the Go amqp client in this tutorial.

First, install amqp using go get:

go get github.com/streadway/amqp

Now we have amqp installed, we can write some code.

Sending

(P) -> [|||]“ height="100” />
</div>

<p>We’ll call our message publisher (sender) <code>send.go</code> and our message consumer (receiver)
<code>receive.go</code>.  The publisher will connect to RabbitMQ, send a single message,
then exit.</p>

<p>In
<a href=send.go, we need to import the library first:

package main

import (
  "log"

  "github.com/streadway/amqp"
)

We also need a helper function to check the return value for each amqp call:

func failOnError(err error, msg string) {
  if err != nil {
    log.Fatalf("%s: %s", msg, err)
  }
}

then connect to RabbitMQ server

conn, err := amqp.Dial("amqp://guest:guest@localhost:5672/")
failOnError(err, "Failed to connect to RabbitMQ")
defer conn.Close()

The connection abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Next we create a channel, which is where most of the API for getting things done resides:

ch, err := conn.Channel()
failOnError(err, "Failed to open a channel")
defer ch.Close()

To send, we must declare a queue for us to send to; then we can publish a message to the queue:

q, err := ch.QueueDeclare(
  "hello", // name
  false,   // durable
  false,   // delete when unused
  false,   // exclusive
  false,   // no-wait
  nil,     // arguments
)
failOnError(err, "Failed to declare a queue")

body := "Hello World!"
err = ch.Publish(
  "",     // exchange
  q.Name, // routing key
  false,  // mandatory
  false,  // immediate
  amqp.Publishing {
    ContentType: "text/plain",
    Body:        []byte(body),
  })
failOnError(err, "Failed to publish a message")

Declaring a queue is idempotent - it will only be created if it doesn’t exist already. The message content is a byte array, so you can encode whatever you like there.

Here’s the whole send.go script.

Sending doesn’t work!

If this is your first time using RabbitMQ and you don’t see the “Sent” message then you may be left scratching your head wondering what could be wrong. Maybe the broker was started without enough free disk space (by default it needs at least 200 MB free) and is therefore refusing to accept messages. Check the broker logfile to confirm and reduce the limit if necessary. The configuration file documentation will show you how to set disk_free_limit.

Receiving

That’s it for our publisher. Our consumer listens for messages from RabbitMQ, so unlike the publisher which publishes a single message, we’ll keep the consumer running to listen for messages and print them out.

[|||] -> ©“ height="100” />
</div>

<p>The code (in <a href=receive.go) has the same import and helper function as send:

package main

import (
  "log"

  "github.com/streadway/amqp"
)

func failOnError(err error, msg string) {
  if err != nil {
    log.Fatalf("%s: %s", msg, err)
  }
}

Setting up is the same as the publisher; we open a connection and a channel, and declare the queue from which we’re going to consume. Note this matches up with the queue that send publishes to.

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()

q, err := ch.QueueDeclare(
  "hello", // name
  false,   // durable
  false,   // delete when unused
  false,   // exclusive
  false,   // no-wait
  nil,     // arguments
)
failOnError(err, "Failed to declare a queue")

Note that we declare the queue here, as well. Because we might start the consumer before the publisher, we want to make sure the queue exists before we try to consume messages from it.

We’re about to tell the server to deliver us the messages from the queue. Since it will push us messages asynchronously, we will read the messages from a channel (returned by amqp::Consume) in a goroutine.

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")

forever := make(chan bool)

go func() {
  for d := range msgs {
    log.Printf("Received a message: %s", d.Body)
  }
}()

log.Printf(" [*] Waiting for messages. To exit press CTRL+C")
<-forever

Here’s the whole receive.go script.

Putting it all together

Now we can run both scripts. In a terminal, run the publisher:

go run send.go

then, run the consumer:

go run receive.go

The consumer will print the message it gets from the publisher via RabbitMQ. The consumer will keep running, waiting for messages (Use Ctrl-C to stop it), so try running the publisher from another terminal.

If you want to check on the queue, try using rabbitmqctl list_queues.

Time to move on to part 2 and build a simple work queue.