Why hands-on MQTT implementation matters in IOT Protocols - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to connect an MQTT client to a broker.

IOT Protocols

client.connect('[1]')

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

Amqtt.example.com

Blocalhost

C192.168.1.1

Dbroker.hivemq.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using an IP address that is not reachable

Leaving the broker address empty

Using localhost when the broker is remote

The code connects the MQTT client to the public broker at broker.hivemq.com.

2fill in blank

medium

Complete the code to subscribe to the topic 'home/temperature'.

IOT Protocols

client.subscribe('[1]')

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

Ahome/temperature

Bhome/pressure

Coffice/temperature

Dhome/humidity

Attempts:

3 left

💡 Hint

Common Mistakes

Subscribing to the wrong topic

Using a topic with a typo

Using a topic from a different location

Subscribing to 'home/temperature' allows the client to receive messages published to that topic.

3fill in blank

hard

Fix the error in publishing a message to 'home/light'.

IOT Protocols

client.publish('[1]', 'ON')

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

Ahome_light

Bhome/light

Chome/light/status

Dhome.light

Attempts:

3 left

💡 Hint

Common Mistakes

Using underscores instead of slashes

Using dots instead of slashes

Adding extra topic levels unintentionally

The correct topic format uses slashes, so 'home/light' is valid. Other options use different separators or wrong topic names.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps topics to their message lengths.

IOT Protocols

{topic: len([1]) for topic, [2] in messages.items()}

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

Amessage

Bmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for both blanks

Using a variable not defined in the loop

Confusing topic and message variables

The comprehension iterates over messages.items() with variables topic and msg. The length is calculated on msg.

5fill in blank

hard

Fill all three blanks to filter messages with payload length greater than 5 and create a new dictionary with uppercase topics.

IOT Protocols

{topic.upper(): [1] for topic, [2] in messages.items() if len([3]) > 5}

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

Apayload

Bmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing variable names between blanks

Using topic instead of message for length check

Not converting topic to uppercase

The comprehension uses 'msg' as the dictionary value, 'msg' as the loop variable for message, and filters by length of 'msg'.

Practice

1. Why is hands-on implementation important when learning MQTT?

easy

A. It replaces the need to read any documentation.

B. It allows you to memorize MQTT commands without practice.

C. It helps you understand how devices actually send and receive messages.

D. It makes MQTT work without any configuration.

Solution

1. Step 1: Understand MQTT basics

   MQTT is a messaging protocol where devices send and receive messages. Hands-on practice shows this in action.

2. Step 2: Compare theory vs practice

   Reading alone doesn't show how messages flow or how devices interact. Hands-on helps you see and try it yourself.

3. Final Answer:

   It helps you understand how devices actually send and receive messages. -> Option C

4. Quick Check:

   Hands-on = Understand message flow [OK]

Hint: Hands-on shows real message flow, not just theory [OK]

Common Mistakes:

• Thinking memorization replaces practice
• Assuming no need to configure MQTT
• Believing documentation is unnecessary

2. Which of the following is the correct MQTT command to subscribe to a topic named home/temperature using the mosquitto_sub client?

easy

A. mosquitto_pub -t home/temperature

B. mosquitto_sub -t home/temperature

C. mosquitto_sub -p home/temperature

D. mosquitto_pub -p home/temperature

Solution

1. Step 1: Identify subscribe command

   The command to subscribe to a topic is mosquitto_sub.

2. Step 2: Use correct option for topic

   The option -t specifies the topic name. So mosquitto_sub -t home/temperature subscribes correctly.

3. Final Answer:

   mosquitto_sub -t home/temperature -> Option B

4. Quick Check:

   Subscribe uses mosquitto_sub -t [OK]

Hint: Subscribe uses mosquitto_sub with -t topic [OK]

Common Mistakes:

• Using mosquitto_pub instead of mosquitto_sub
• Using -p instead of -t for topic
• Confusing publish and subscribe commands

3. What will be the output when you run this command after subscribing to home/light and then publishing ON to the same topic?

mosquitto_sub -t home/light

In another terminal:

mosquitto_pub -t home/light -m ON

medium

A. ON

B. home/light

C. Error: Topic not found

D. No output

Solution

1. Step 1: Understand subscription

   The mosquitto_sub command listens to messages on home/light.

2. Step 2: Understand publishing

   The mosquitto_pub command sends the message ON to home/light. The subscriber receives this message.

3. Final Answer:

   ON -> Option A

4. Quick Check:

   Publish message received by subscriber = ON [OK]

Hint: Publish message appears on subscriber terminal [OK]

Common Mistakes:

• Expecting topic name as output
• Thinking error occurs if topic not pre-created
• Assuming no output without extra flags

4. You tried to subscribe to sensor/data using mosquitto_sub -p sensor/data but got an error. What is the likely cause?

medium

A. Using -p instead of -t for topic option

B. The MQTT broker is down

C. The topic name is invalid

D. You need to publish before subscribing

Solution

1. Step 1: Check command options

   The -t option specifies the topic name in mosquitto_sub. Using -p is incorrect here.

2. Step 2: Understand error cause

   Using -p causes the command to fail because it expects a port number, not a topic.

3. Final Answer:

   Using -p instead of -t for topic option -> Option A

4. Quick Check:

   Topic option must be -t, not -p [OK]

Hint: Use -t for topic in subscribe command [OK]

Common Mistakes:

• Confusing -p (port) with -t (topic)
• Assuming broker is always down on error
• Thinking topic must exist before subscribing

5. You want to test an MQTT setup where multiple devices send temperature data to a central server. Which hands-on approach best helps you verify the system works correctly?

hard

A. Only read the MQTT specification document carefully.

B. Use a single client to publish and ignore subscriptions.

C. Publish messages without subscribing to any topic.

D. Subscribe to the temperature topic and publish sample temperature messages from multiple clients.

Solution

1. Step 1: Identify testing goal

   You want to verify multiple devices send data and the server receives it correctly.

2. Step 2: Choose hands-on method

   Subscribing to the temperature topic and publishing sample messages from multiple clients simulates real device communication and tests message flow.

3. Final Answer:

   Subscribe to the temperature topic and publish sample temperature messages from multiple clients. -> Option D

4. Quick Check:

   Test with multiple publishers and subscriber = success [OK]

Hint: Test with multiple publishers and one subscriber [OK]

Common Mistakes:

• Skipping subscription step
• Relying only on reading docs
• Testing with single client only