For Internet of Things (IoT) projects, there are a lot of different ways that sensors, devices, and client interfaces can be connected together. For many projects, using a simple Message Queue Telemetry Transport (MQTT) broker is all that you need. However, if you're trying to merge and build IoT projects that use both MQTT and Advanced Message Queue Protocol (AMQP), or you require a REST API, then you should take a look at RabbitMQ.

RabbitMQ [1] is an open source middleware solution that natively uses AMQP communications, but it has a good selection of plugins to support features like MQTT, MQTT WebSockets, HTTP REST API, and server-to-server communications (Figure 1).

Figure 1: RabbitMQ overview.

In this article, I will set up a RabbitMQ server, and I will look at some of the differences between MQTT and AMQP messaging. Finally, an example of an Arduino MQTT message will be presented as both an MQTT and an AMQP item in a Node-RED dashboard.

Getting Started Locally

RabbitMQ can be installed on Windows, Linux, and macOS systems, and there are also some cloud-based offerings. For small systems, lower-end hardware like a Raspberry Pi can be used. Complete RabbitMQ installation instructions are available online [2]. To install and run RabbitMQ on an Ubuntu system, enter:

sudo apt-get update
sudo apt-get install rabbitmq-server
sudo service rabbitmq-server start

The next step is to add some plugins. For my project, I loaded the MQTT and web administration plugins:

sudo rabbitmq-plugins enable rabbitmq_mqtt
sudo rabbitmq-plugins enable rabbitmq-management

The rabbitmqctl command-line tool allows you to configure and review the RabbitMQ server. To add a user (admin1) with a password (admin1) that has config, write, and read rights for management and administrator access, enter:

sudo rabbitmqctl add_user admin1 admin1
sudo rabbitmqctl set_permissions -p / admin1 ".*" ".*" ".*"
sudo rabbitmqctl set_user_tags admin1 management administrator

After you've defined an administrative user, the RabbitMQ web management plugin can be accessed at http://ip_address:15672 (Figure 2).

Figure 2: RabbitMQ web administration.

The RabbitMQ web management tool offers an overview of the present system load, connections, exchanges, and queues.

The RabbitMQ web management tool is excellent for small manual changes. However if you are looking at a doing a large number of additions or changes, then rabbitmqadmin, you can use the command-line management tool. Install the tool by entering:

# Get the cli and make it available to use.
wget http://127.0.0.1:15672/cli/rabbitmqadmin
sudo chmod +x rabbitmqadmin

Comparing MQTT and AMQP

It's useful to comment about some of the differences between MQTT and AMQP.

MQTT is a lightweight publish- and subscribe-based messaging protocol that works well with lower-end hardware and limited bandwidth. For Arduino-type applications where you only need to pass some sensor data, MQTT is an excellent fit.

AMQP has more overhead than MQTT, because it is a more advanced protocol that includes message orientation, queuing, routing, reliability, and security. Presently, there are no mainstream AMQP Arduino libraries, but numerous programming options for Raspberry Pi, Linux, Windows, and macOS systems exist. An AMQP IoT example would be to send sensor failures and alarms to dedicated maintenance and alarm queues.

MQTT and AMQP Queues

One of the biggest differences in queues is that MQTT queues are designed to show you the last available message, where as AMQP will store multiple messages in a queue.

A published MQTT message contains a message body, a retain flag, and a quality of service (QoS) value.

An AMQP message can be published with added properties, such as time stamp, type of message, and expiration information. AMQP messages also support the addition of custom header values. Listing 1 is a Python publish example that defines the message type to be "Pi Sensor", and it includes custom headers for status and alarm state.

01 #!/usr/bin/env python
02 import pika
03
04 node = "192.168.0.121"
05 user = "pete"
06 pwd = "pete"
07
08 # Connect to a remote AMQP server with a username/password
09 credentials = pika.PlainCredentials(user, pwd)
10 connection = pika.BlockingConnection(pika.ConnectionParameters(node,
11         5672, '/', credentials))
12 channel = connection.channel()
13
14 # Create a queue if it doesn't already exist
15 channel.queue_declare(queue='Rasp_1',durable=True)
16
17 # Define the properties and publish a message
18 props = pika.BasicProperties(
19     headers= {'status': 'Good Quality',"alarm":"HI"},
20     type ="Pi Sensor")
21 channel.basic_publish(exchange='',
22     routing_key='Rasp_1',body='99.5', properties = props)
23
24 connection.close()

The results from the Listing 1 example can be examined using the Queue | Get Message option in the RabbitMQ web management interface (Figure 3).

Figure 3: AMQP queue with custom properties.