python mqtt script on raspberry pi to send and receive messages

Question

MQTT question:

Hi, I’m trying to set up a MQTT network between multiple Raspberry Pis (starting with two). I have one raspberry pi (RPi-A), MQTT client, with a thermistor sensor attached and one raspberry (RPi-B), MQTT broker/client, acting as a hub for my network. Through python scripting I’d like the temperature to be sent every 30mins from RPi-A via MQTT to topic sensor/data and received by RPi-B. When RPi-B receives a message from RPi-A via topic sensor/data, I want it to respond with an instruction via MQTT topic sensor/instructions to RPi-A. Below is my script, so far RPi-A can send messages and RPi-B receive them but I cannot work out how RPi-B can respond.

Basically, what I’m trying to understand is, is it possible for a MQTT device to act as both broker and client at the same time? And, can a client both send and receive messages and if so how to implement all the above via python? I’ve read a lot of blogs, official MQTT articles and the paho module documentation (which for me is very hard to fathom) but still cannot figure this out. Your help would be most useful/ appreciated.

Code RPi-A ( with thermistor sensor):

from sense_hat import SenseHat
import time
import paho.mqtt.client as mqtt
import paho.mqtt.publish as publish
sense = SenseHat()

Broker = "192.168.1.252"

sub_topic = "sensor/instructions"    # receive messages on this topic

pub_topic = "sensor/data"       # send messages to this topic


############### sensehat inputs ##################

def read_temp():
    t = sense.get_temperature()
    t = round(t)
    return t

def read_humidity():
    h = sense.get_humidity()
    h = round(h)
    return h

def read_pressure():
    p = sense.get_pressure()
    p = round(p)
    return p

def display_sensehat(message):
    sense.show_message(message)
    time.sleep(10)

############### MQTT section ##################

# when connecting to mqtt do this;

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(sub_topic)

# when receiving a mqtt message do this;

def on_message(client, userdata, msg):
    message = str(msg.payload)
    print(msg.topic+" "+message)
    display_sensehat(message)

def publish_mqtt(sensor_data):
    mqttc = mqtt.Client("python_pub")
    mqttc.connect(Broker, 1883)
    mqttc.publish(pub_topic, sensor_data)
    #mqttc.loop(2) //timeout = 2s

def on_publish(mosq, obj, mid):
    print("mid: " + str(mid))


client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.connect(Broker, 1883, 60)


while True:
    sensor_data = [read_temp(), read_humidity(), read_pressure()]
    publish.single("monto/solar/sensors", str(sensor_data), hostname = Broker)
    time.sleep(1*60)

Code RPi-B (network hub):

import time
import paho.mqtt.client as mqtt
import paho.mqtt.publish as publish

Broker = "192.168.1.252"

sub_topic = "sensor/data"    # receive messages on this topic

pub_topic = "sensor/instructions"               # send messages to this topic


# mqtt section

# when connecting to mqtt do this;

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(sub_topic)

# when receiving a mqtt message do this;

def on_message(client, userdata, msg):
    message = str(msg.payload)
    print(msg.topic+" "+message)
    publish_mqtt(‘got your message’)

# to send a message

def publish_mqtt(sensor_data):
    mqttc = mqtt.Client("monto_hub")
    mqttc.connect(Broker, 1883)
    mqttc.publish(pub_topic, "this is the master speaking")
    #mqttc.loop(2) //timeout = 2s

def on_publish(mosq, obj, mid):
    print("mid: " + str(mid))


client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.connect(Broker, 1883, 60)
client.loop_forever()

Answer 1

The simplest way is to start the network loop on a separate thread using the client.loop_start() function, then use the normal client.publish method

from sense_hat import SenseHat
import time
import paho.mqtt.client as mqtt
import paho.mqtt.publish as publish
sense = SenseHat()

Broker = "192.168.1.252"

sub_topic = "sensor/instructions"    # receive messages on this topic

pub_topic = "sensor/data"       # send messages to this topic


############### sensehat inputs ##################

def read_temp():
    t = sense.get_temperature()
    t = round(t)
    return t

def read_humidity():
    h = sense.get_humidity()
    h = round(h)
    return h

def read_pressure():
    p = sense.get_pressure()
    p = round(p)
    return p

def display_sensehat(message):
    sense.show_message(message)
    time.sleep(10)

############### MQTT section ##################

# when connecting to mqtt do this;

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(sub_topic)

# when receiving a mqtt message do this;

def on_message(client, userdata, msg):
    message = str(msg.payload)
    print(msg.topic+" "+message)
    display_sensehat(message)

def on_publish(mosq, obj, mid):
    print("mid: " + str(mid))


client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.connect(Broker, 1883, 60)
client.loop_start()

while True:
    sensor_data = [read_temp(), read_humidity(), read_pressure()]
    client.publish("monto/solar/sensors", str(sensor_data))
    time.sleep(1*60)