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MQTT with Arduino and Raspberry Pi

MQTT is a lightweight pub-sub framework that provides RabbitMQ-like functionality on internet-connected microcontrollers like ESP8266. Although it's different from the AMQP protocol, RabbitMQ does have a MQTT plugin that will allow MQTT clients to publish/subscribe to RabbitMQ. This allows full AMQP clients (like Raspberry Pi) to communicate with microcontrollers (like ESP8266/Arduino) via RabbitMQ.

Setting up the RabbitMQ Server

To install RabbitMQ on Rasperry Pi, first

# apt-get install rabbitmq-server
# rabbitmq-plugins enable rabbitmq_mqtt
# service rabbitmq-server restart

then set up a new user (called espuser) we can use from our Arduino:

# rabbitmqctl add_user espuser esppassword
# rabbitmqctl set_permissions -p "/" espuser "^mqtt-subscription-ESP8266.*" ".*" ".*"

where mqtt-subscription-XXXX is the queue that is declared by the MQTT plugin for RabbitMQ and XXXX is the clientID passed in as the first parameter to the client.connect() routine in the Arduino PubSubClient library.

# rabbitmqctl add_user rpiuser rpipassword
# rabbitmqctl set_permissions -p "/" rpiuser "^mqtt-subscription-ESP8266.*" ".*" ".*"

to connect, you would have to do (Arduino) client.connect("ESP8266...", "espuser", "esppassword")

Also add a user for the Raspberry Pi to use on the RabbitMQ user (called rpiuser):

# rabbitmqctl add_user rpiuser rpipassword
# rabbitmqctl set_permissions -p "/" rpiuser "^mqtt-subscription-ESP8266.*" ".*" ".*"

Connecting to RabbitMQ from Arduino

To connect to RabbitMQ from Arduino, you would issue something like

client.connect("ESP8266...", "espuser", "esppassword")

Strangely, the client.subscribe('XXX') doesn't seem to do anything with respect to XXX. Haven't figured this out yet.

From the pub side, you really only need

#!/usr/bin/env python
import pika
rmq_creds = pika.PlainCredentials('rpiuser', 'rpipassword')
rmq_conn_params = pika.ConnectionParameters('localhost', 5672, '/', rmq_creds)
conn = pika.BlockingConnection(rmq_conn_params)
channel = conn.channel()
channel.basic_publish(
    exchange='',
    routing_key='mqtt-subscription-ESP8266 Clientqos0',
    body='hello world')
conn.close()

That is, no need to declare any queues or exchanges, and use the default exchange. Note the routing key; in the Arduino sketch, we declare the MQTT client id as ESP8266 Client and use the qos=0. The RabbitMQ MQTT plugin then crafts a queue named mqtt-subscription-ESP8266 Clientqos0 based on this information when the Arduino declares its subscription:

Serial.print("Connecting to MQTT broker...");
if (client.connect("ESP8266 Client", mqtt_user, mqtt_password)) {
    Serial.println("connected");
    client.subscribe(mqtt_topic, 1);
} else {
    Serial.print("failed (");
    Serial.print(client.state());
    Serial.println(")");
}

Unresolved Mysteries

  • I can delete the wifi password and it still works. Does the router DHCP reservation just shoo in my client after the initial authentication and lease?

References