Subscribing to control topics in Raspberry Pi - Step-by-Step Execution

Concept Flow - Subscribing to control topics

Start Program

↓

Initialize MQTT Client

↓

Connect to Broker

↓

Subscribe to Control Topic

↓

Wait for Messages

↓

On Message Received

↓

Process Control Command

↓

Loop Back to Wait

The program starts, connects to the MQTT broker, subscribes to control topics, waits for messages, and processes commands when received.

Execution Sample

Raspberry Pi

import paho.mqtt.client as mqtt

def on_message(client, userdata, msg):
    print(f"Received: {msg.topic} -> {msg.payload.decode()}")

client = mqtt.Client()
client.on_message = on_message
client.connect("broker.hivemq.com", 1883)
client.subscribe("home/control")
client.loop_forever()

This code connects to a public MQTT broker, subscribes to the 'home/control' topic, and prints any received messages.

Execution Table

Step	Action	Evaluation	Result
1	Import MQTT library	Success	paho.mqtt.client module ready
2	Define on_message callback	Function created	Ready to handle messages
3	Create MQTT client	Client object created	Client ready
4	Assign on_message callback	Callback set	Client will handle messages
5	Connect to broker	Connection attempt	Connected to broker.hivemq.com:1883
6	Subscribe to 'home/control'	Subscription request sent	Subscribed successfully
7	Start loop_forever	Listening for messages	Waiting for messages
8	Message received on 'home/control'	Trigger on_message	Print message content
9	Process message	Print output	Output: Received: home/control -> <message>
10	Loop continues	Wait for next message	Ready for next message

💡 Program runs indefinitely until manually stopped

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	After Step 8
client	None	MQTT Client object	Connected client	Subscribed client	Client with message callback triggered

Key Moments - 3 Insights

Why do we assign the on_message function to client.on_message?

Assigning on_message to client.on_message tells the client what to do when a message arrives, as shown in step 4 and triggered in step 8.

What happens if the client does not subscribe to the topic?

If the client does not subscribe (step 6), it will not receive messages on that topic, so on_message will never be called.

Why does the program use loop_forever()?

loop_forever() (step 7) keeps the program running and listening for messages continuously, as explained in the exit note.

Visual Quiz - 3 Questions

Look at the execution table, what is the client variable after step 5?

A Disconnected client object

B Connected client object

C None

D Subscribed client object

💡 Hint

Check variable_tracker column 'After Step 5' and execution_table step 5

At which step does the program start listening for messages?

A Step 4

B Step 6

C Step 7

D Step 8

💡 Hint

Look at execution_table step 7 description

If the subscription to 'home/control' is removed, what changes in the execution?

A on_message will never be triggered

B Client will not connect to broker

C Program will crash at step 6

D Messages will still be received

💡 Hint

Refer to key_moments question 2 and execution_table step 6

Concept Snapshot

Subscribing to control topics with MQTT:
- Import MQTT client library
- Define on_message callback to handle messages
- Connect client to broker
- Subscribe to control topic
- Use loop_forever() to listen continuously
- on_message triggers on receiving messages

Full Transcript

This example shows how a Raspberry Pi program subscribes to control topics using MQTT. First, it imports the MQTT library and defines a function to handle incoming messages. Then, it creates an MQTT client, assigns the message handler, and connects to a broker. After subscribing to the 'home/control' topic, the program enters a loop waiting for messages. When a message arrives, the on_message function runs and prints the message. The program keeps running until stopped.