MQTT

Starting the Service

This is Fedorish, your mileage might vary:

# systemctl start mosquitto.service
# netstat -antp|grep mosquitto
tcp        0      0 0.0.0.0:1883            0.0.0.0:*               LISTEN      53767/mosquitto
tcp6       0      0 :::1883                 :::*                    LISTEN      53767/mosquitto

Ah, runs on port 1883.

See if it works. In one terminal, subscribe to a random topic. (Topic are created as soon as they are mentioned.)

$ mosquitto_sub --host localhost --port 1883 --topic /random/topic
...sit and wait for message...

In another terminal, publish a message,

$ mosquitto_pub --host localhost --port 1883 --topic /random/topic --message blah

You should see “blah” as the output of mosquitto_sub in the other terminal.

Publishing a Message in Python

This is the easiest, so lets start with that.

publish.py

#!/usr/bin/env python

from paho.mqtt import client

c = client.Client()
c.connect('localhost', 1883)
c.publish('/random/topic', 'blech')

Run it, and in the terminal running mosquitto_sub you’ll see "blech" on stdout.

Subscribing a Topic in Python

A little more complicated - we have to

• run an event loop: we want to see more than one message coming in

• register a callback function that is called by MQTT to notify us about an incoming message

subscribe.py

#!/usr/bin/env python

from paho.mqtt import client

def message_received(client, userdata, message):
    print(message.payload)

c = client.Client()
c.connect('localhost', 1883)
c.on_message = message_received

c.subscribe('/random/topic')
c.loop_forever()

Run it, possibly side by side with mosquitto_sub. Publish a message, using either the publish.py program above, or mosqitto_pub.

$ ./subscribe.py
b'blech'

Message Payload?

Note the b in the output: what comes in is not a string. MQTT’s transport is encoding-free; what is sent is completely up to the communicating parties.

You should probably read up on

• Encoding: Strings and Encoding

• JSON, a popular transport format, used for MQTT message transfer, and on The Web which is full of it.

Links

• Nice video about MQTT concepts, and a little Python code (German)

• Same, but with Node-RED (Styrian)

• Communicating from Arduino/ESP to Raspberry (German)

  • Python side

  • Arduino/ESP side

• Encryption, Users, from minute 11:30 (German)

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