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.

Practice

1. What is the main purpose of subscribing to control topics on a Raspberry Pi using MQTT?

easy

A. To send commands from the Raspberry Pi to other devices

B. To update the Raspberry Pi operating system automatically

C. To store data locally on the Raspberry Pi

D. To listen and react to control commands sent to the Raspberry Pi

Solution

1. Step 1: Understand MQTT subscription

   Subscribing means the Raspberry Pi listens to messages on a topic.

2. Step 2: Identify the purpose of control topics

   Control topics send commands that the Raspberry Pi should react to.

3. Final Answer:

   To listen and react to control commands sent to the Raspberry Pi -> Option D

4. Quick Check:

   Subscribing = Listening to commands [OK]

Hint: Subscribing means listening, not sending commands [OK]

Common Mistakes:

• Confusing subscribing with publishing
• Thinking subscription stores data
• Assuming it updates the OS

2. Which of the following is the correct Python syntax to subscribe to an MQTT topic named "home/lights/control" using the Paho MQTT client?

easy

A. client.connect("home/lights/control")

B. client.sub("home/lights/control")

C. client.subscribe("home/lights/control")

D. client.listen("home/lights/control")

Solution

1. Step 1: Recall Paho MQTT subscribe method

   The method to subscribe to a topic is subscribe().

2. Step 2: Match the correct syntax

   Using client.subscribe("home/lights/control") is correct syntax.

3. Final Answer:

   client.subscribe("home/lights/control") -> Option C

4. Quick Check:

   Subscribe method is subscribe() [OK]

Hint: Subscribe uses .subscribe(), not .sub() or .listen() [OK]

Common Mistakes:

• Using .sub() instead of .subscribe()
• Confusing subscribe with connect
• Using non-existent methods like .listen()

3. Given the following code snippet, what will be printed when a message with payload "ON" is received on the topic "home/lights/control"?

def on_message(client, userdata, message):
    if message.payload.decode() == "ON":
        print("Turning lights ON")
    else:
        print("Turning lights OFF")

client.subscribe("home/lights/control")
client.on_message = on_message

medium

A. Turning lights ON

B. Turning lights OFF

C. No output

D. Error: decode() not found

Solution

1. Step 1: Decode the message payload

   The payload is decoded to string and compared to "ON".

2. Step 2: Check the condition and print output

   Since payload is "ON", it prints "Turning lights ON".

3. Final Answer:

   Turning lights ON -> Option A

4. Quick Check:

   Payload "ON" triggers print "Turning lights ON" [OK]

Hint: Decode payload before comparing strings [OK]

Common Mistakes:

• Not decoding payload before comparison
• Assuming no output without explicit print
• Thinking decode() causes error

4. Identify the error in this MQTT subscription code snippet for Raspberry Pi:

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

client.on_message = on_message
client.subscribe("home/lights/control")
client.loop_start()

medium

A. Incorrect callback function name

B. Missing client.connect() before subscribing

C. Using loop_start() instead of loop_forever()

D. No error, code is correct

Solution

1. Step 1: Check connection setup

   Before subscribing, the client must connect to the MQTT broker.

2. Step 2: Identify missing connect call

   The code lacks client.connect() call, so subscription won't work.

3. Final Answer:

   Missing client.connect() before subscribing -> Option B

4. Quick Check:

   Must connect before subscribe [OK]

Hint: Always connect client before subscribing [OK]

Common Mistakes:

• Skipping client.connect() call
• Confusing callback function names
• Thinking loop_start() is wrong here

5. You want your Raspberry Pi to subscribe to multiple control topics: "home/lights/control" and "home/thermostat/control". Which code snippet correctly subscribes to both and handles messages differently based on the topic?

hard

A. client.subscribe(["home/lights/control", "home/thermostat/control"])
def on_message(client, userdata, message):
topic = message.topic
if topic == "home/lights/control":
print("Light control received")
elif topic == "home/thermostat/control":
print("Thermostat control received")
client.on_message = on_message

B. client.subscribe("home/lights/control")
client.subscribe("home/thermostat/control")
def on_message(client, userdata, message):
if message.payload == "light":
print("Light control")
else:
print("Thermostat control")
client.on_message = on_message

C. client.subscribe("home/lights/control", "home/thermostat/control")
def on_message(client, userdata, message):
print("Control received")
client.on_message = on_message

D. client.subscribe("home/lights/control")
def on_message(client, userdata, message):
print("Control received")
client.on_message = on_message
client.subscribe("home/thermostat/control")

Solution

1. Step 1: Subscribe to multiple topics correctly

   client.subscribe(["home/lights/control", "home/thermostat/control"]) def on_message(client, userdata, message): topic = message.topic if topic == "home/lights/control": print("Light control received") elif topic == "home/thermostat/control": print("Thermostat control received") client.on_message = on_message uses a list to subscribe to both topics at once, which is valid.

2. Step 2: Handle messages based on topic

   The on_message function checks message.topic to differentiate actions.

3. Final Answer:

   correctly subscribes to both topics and handles them separately -> Option A

4. Quick Check:

   Subscribe list + topic check = correct [OK]

Hint: Subscribe with list and check message.topic [OK]

Common Mistakes:

• Passing multiple topics as separate arguments
• Checking payload instead of topic for routing
• Subscribing after setting callback incorrectly