Subscribing to control commands in IOT Protocols - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to the control topic using MQTT.

IOT Protocols

client.subscribe('[1]')

Drag options to blanks, or click blank then click option'

Acontrol/commands

Bdata/updates

Csensor/readings

Dalerts/warnings

Attempts:

3 left

💡 Hint

Common Mistakes

Subscribing to a data or sensor topic instead of the control topic.

Using an incorrect topic string with typos.

The control commands are sent on the control/commands topic, so subscribing to this topic allows the device to receive control messages.

2fill in blank

medium

Complete the code to set the callback function for incoming control messages.

IOT Protocols

client.on_message = [1]

Drag options to blanks, or click blank then click option'

Ahandle_data

Bprocess_sensor

Chandle_control

Don_connect

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the wrong callback function that does not handle control commands.

Confusing the on_connect callback with on_message.

The callback function handle_control is designed to process incoming control commands.

3fill in blank

hard

Fix the error in the subscription quality of service (QoS) level to ensure reliable delivery.

IOT Protocols

client.subscribe('control/commands', qos=[1])

Drag options to blanks, or click blank then click option'

A1

B3

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using an invalid QoS value like 3 or -1.

Using QoS 0 which may lose messages.

QoS level 1 ensures that the message is delivered at least once, which is suitable for control commands.

4fill in blank

hard

Fill both blanks to correctly initialize the MQTT client and connect to the broker.

IOT Protocols

client = mqtt.Client([1])
client.connect([2], 1883, 60)

Drag options to blanks, or click blank then click option'

A'device123'

B'broker.example.com'

C'control/commands'

D'user1'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a topic name as client ID or broker address.

Using username instead of client ID.

The client ID should be a unique identifier like 'device123'. The broker address is the server to connect to, here 'broker.example.com'.

5fill in blank

hard

Fill all three blanks to create a subscription with a callback that prints the received control command payload.

IOT Protocols

def [1](client, userdata, message):
    print('Received command:', message.[2].decode())

client.subscribe('control/commands', qos=1)
client.on_message = [3]

Drag options to blanks, or click blank then click option'

Ahandle_control

Bpayload

Dtopic

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the function and callback assignment.

Accessing message.topic instead of message.payload for the command data.

The function handle_control processes messages. The payload contains the message data, which must be decoded. The callback is assigned to handle_control.

Practice

1. What is the main purpose of subscribing to control commands in IoT devices?

easy

A. To update the device firmware manually

B. To listen and respond automatically to commands sent from a controller

C. To send data continuously to the cloud

D. To disconnect the device from the network

Solution

1. Step 1: Understand the role of subscribing

   Subscribing means the device listens for messages or commands sent to it.

2. Step 2: Identify the purpose of control commands

   Control commands tell the device what actions to perform automatically.

3. Final Answer:

   To listen and respond automatically to commands sent from a controller -> Option B

4. Quick Check:

   Subscribing = Listening for commands [OK]

Hint: Subscribing means listening for commands to act automatically [OK]

Common Mistakes:

• Confusing subscribing with sending data
• Thinking subscribing updates firmware
• Assuming subscribing disconnects device

2. Which of the following is the correct syntax to subscribe to a topic named device/control using an MQTT client in Python?

easy

A. client.subscribe('device/control')

B. client.sub('device/control')

C. subscribe(client, 'device/control')

D. client.listen('device/control')

Solution

1. Step 1: Recall MQTT client subscription method

   The standard method to subscribe is client.subscribe(topic).

2. Step 2: Match the correct syntax

   client.subscribe('device/control') uses the correct method and topic string format.

3. Final Answer:

   client.subscribe('device/control') -> Option A

4. Quick Check:

   Subscribe method = client.subscribe(topic) [OK]

Hint: Use client.subscribe('topic') to subscribe in MQTT Python [OK]

Common Mistakes:

• Using incorrect method names like sub or listen
• Passing client as first argument to subscribe
• Using wrong topic string format

3. Given this Python MQTT subscription code snippet, what will be printed when a message with payload ON arrives on topic device/control?

def on_message(client, userdata, message):
    if message.payload.decode() == 'ON':
        print('Turning device ON')
    else:
        print('Unknown command')

client.subscribe('device/control')
client.on_message = on_message

medium

A. Turning device ON

B. Unknown command

C. Error: decode() not found

D. No output

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 output

   Since payload is 'ON', it matches the if condition and prints 'Turning device ON'.

3. Final Answer:

   Turning device ON -> Option A

4. Quick Check:

   Payload 'ON' triggers print 'Turning device ON' [OK]

Hint: Decode payload and compare string to decide output [OK]

Common Mistakes:

• Forgetting to decode payload bytes
• Assuming payload is already string
• Ignoring else branch output

4. Identify the error in this MQTT subscription code snippet that prevents receiving messages:

client = mqtt.Client()
client.on_message = on_message
client.subscribe('device/control')
client.loop_start()

medium

A. loop_start() should be loop_forever()

B. Incorrect topic name format

C. Missing client.connect() call before subscribing

D. on_message function not defined

Solution

1. Step 1: Check connection setup

   MQTT client must connect to broker before subscribing.

2. Step 2: Identify missing connect call

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

3. Final Answer:

   Missing client.connect() call before subscribing -> Option C

4. Quick Check:

   Connect before subscribe to receive messages [OK]

Hint: Always connect client before subscribing to topics [OK]

Common Mistakes:

• Skipping client.connect() call
• Assuming subscribe works without connection
• Confusing loop_start() with connection

5. You want your IoT device to subscribe to control commands on topic home/lights/command and turn lights ON or OFF accordingly. Which code snippet correctly subscribes and handles these commands in Python using MQTT?

hard

A. client = mqtt.Client()
client.connect('broker.hivemq.com')
client.subscribe('home/lights/command')
client.loop_forever()

B. client = mqtt.Client()
client.subscribe('home/lights/command')
client.on_message = lambda c,u,m: print('Lights ON' if m.payload == 'ON' else 'Lights OFF')
client.loop_start()

C. def on_message(client, userdata, message):
if message.payload == 'ON':
print('Lights ON')
client = mqtt.Client()
client.connect('broker.hivemq.com')
client.subscribe('home/lights/command')
client.on_message = on_message
client.loop_forever()

D. def on_message(client, userdata, message):
cmd = message.payload.decode()
if cmd == 'ON':
print('Lights ON')
elif cmd == 'OFF':
print('Lights OFF')
client = mqtt.Client()
client.connect('broker.hivemq.com')
client.subscribe('home/lights/command')
client.on_message = on_message
client.loop_forever()

Solution

1. Step 1: Verify subscription and connection

   def on_message(client, userdata, message): cmd = message.payload.decode() if cmd == 'ON': print('Lights ON') elif cmd == 'OFF': print('Lights OFF') client = mqtt.Client() client.connect('broker.hivemq.com') client.subscribe('home/lights/command') client.on_message = on_message client.loop_forever() connects to broker, subscribes to correct topic, and sets message handler.

2. Step 2: Check message handling logic

   It decodes payload and handles both 'ON' and 'OFF' commands properly.

3. Final Answer:

   Complete subscription with connection and proper message handling -> Option D

4. Quick Check:

   Connect, subscribe, decode payload, handle ON/OFF [OK]

Hint: Connect, subscribe, decode payload, handle commands in on_message [OK]

Common Mistakes:

• Not decoding payload bytes to string
• Missing client.connect() call
• Ignoring OFF command handling
• Not assigning on_message callback