Publishing sensor data in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Publishing sensor data with MQTT

What is the output of this MQTT publish command when sending temperature data to topic home/sensor/temperature?

IOT Protocols

mosquitto_pub -h broker.example.com -t home/sensor/temperature -m "22.5"

A Message published successfully to topic home/sensor/temperature

B Syntax error in command

C Connection refused: broker.example.com

D Error: Topic home/sensor/temperature not found

💡 Hint

Check if the command syntax is correct and the broker is reachable.

The command publishes the message '22.5' to the topic 'home/sensor/temperature' on the MQTT broker. If successful, it confirms the message was published.

❓ Configuration

intermediate

Configuring QoS for sensor data publishing

Which MQTT QoS level ensures that the sensor data message is delivered exactly once to the broker?

A QoS 2 - Exactly once

B QoS 1 - At least once

C QoS 3 - Guaranteed delivery

D QoS 0 - At most once

💡 Hint

QoS levels 0, 1, and 2 are standard in MQTT.

QoS 2 guarantees that the message is delivered exactly once by using a four-step handshake between client and broker.

❓ Troubleshoot

advanced

Troubleshooting sensor data not received

A sensor publishes data to topic factory/machine1/status but the subscriber never receives it. Which is the most likely cause?

A Publisher uses QoS 0 and subscriber uses QoS 1

B Subscriber is subscribed to <code>factory/machine1/#</code>

C Subscriber is subscribed to <code>factory/machine1/status</code> but the publisher uses a different topic <code>factory/machine1/state</code>

D Publisher and subscriber use the same client ID

💡 Hint

Check if the topic names exactly match.

If the subscriber listens to a different topic than the publisher sends to, it will never receive the messages.

🔀 Workflow

advanced

Workflow for secure sensor data publishing

Which sequence correctly describes the steps to securely publish sensor data over MQTT?

A 3,1,2,4

B 2,1,3,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Security steps must happen before publishing data.

First establish a secure connection, then authenticate, publish data, and finally disconnect.

✅ Best Practice

expert

Best practice for sensor data payload format

Which payload format is best for publishing sensor data to ensure interoperability and easy parsing?

A Plain text string with comma-separated values

B JSON object with named fields

C Binary encoded proprietary format

D XML document with nested elements

💡 Hint

Consider readability and support across platforms.

JSON is widely supported, human-readable, and easy to parse in many programming languages, making it ideal for sensor data.

Practice

1. What is the main purpose of publishing sensor data in IoT protocols?

easy

A. To send sensor data to a topic so others can receive it

B. To store sensor data locally on the device

C. To encrypt sensor data before sending

D. To delete old sensor data from the cloud

Solution

1. Step 1: Understand publishing in IoT

   Publishing means sending data from a sensor to a topic so other devices can get it.

2. Step 2: Identify the main goal

   The goal is to share sensor data, not store or delete it locally or encrypt it by default.

3. Final Answer:

   To send sensor data to a topic so others can receive it -> Option A

4. Quick Check:

   Publishing = sending data to topic [OK]

Hint: Publishing means sending data to a topic for others [OK]

Common Mistakes:

• Confusing publishing with storing data locally
• Thinking publishing encrypts data automatically
• Assuming publishing deletes old data

2. Which MQTT publish command syntax is correct for sending temperature data to topic home/livingroom/temp?

easy

A. mqtt.publish('home/livingroom/temp', '22.5')

B. publish('home/livingroom/temp', temperature=22.5)

C. mqtt.publish('home/livingroom/temp')

D. publish('home/livingroom/temp', 22.5)

Solution

1. Step 1: Check MQTT publish method syntax

   The correct syntax uses the client object (e.g., mqtt) and calls publish with topic and message as string.

2. Step 2: Validate options

   mqtt.publish('home/livingroom/temp', '22.5') uses mqtt.publish with topic and string message, which is correct. publish('home/livingroom/temp', 22.5) misses client object, C misses message, B uses invalid keyword argument.

3. Final Answer:

   mqtt.publish('home/livingroom/temp', '22.5') -> Option A

4. Quick Check:

   mqtt.publish(topic, message) = correct syntax [OK]

Hint: Use client.publish(topic, message) with message as string [OK]

Common Mistakes:

• Omitting the client object before publish
• Not providing the message argument
• Using keyword arguments not supported by publish

3. Given this Python MQTT publish code snippet, what will be printed?

import paho.mqtt.client as mqtt
client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
result = client.publish('sensor/room1/humidity', '45')
print(result.rc)

medium

A. 1

B. 0

C. null

D. Error

Solution

1. Step 1: Understand publish return value

   client.publish returns a MQTTMessageInfo object with rc attribute showing result code: 0 means success.

2. Step 2: Check printed value

   Printing result.rc will print 0 if publish succeeded without error.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Publish success code = 0 [OK]

Hint: Publish returns rc=0 on success, print result.rc to see it [OK]

Common Mistakes:

• Assuming publish returns null
• Expecting printed message payload
• Confusing rc with error code

4. You wrote this MQTT publish code but no data appears on the subscriber side:

client.publish('sensor/temp', 25)

What is the likely problem?

medium

A. QoS level is missing and defaults to 2

B. The topic name is invalid

C. The message payload must be a string or bytes, not an integer

D. The client must call subscribe before publish

Solution

1. Step 1: Check payload data type

   MQTT publish requires message payload as string or bytes, not integer directly.

2. Step 2: Identify issue with given code

   Passing 25 as integer causes publish to fail silently or send empty data, so subscriber gets nothing.

3. Final Answer:

   The message payload must be a string or bytes, not an integer -> Option C

4. Quick Check:

   Payload must be string/bytes, not int [OK]

Hint: Always convert payload to string or bytes before publishing [OK]

Common Mistakes:

• Sending integer payload without conversion
• Thinking subscribe is needed before publish
• Assuming topic name is invalid without checking

5. You want to publish temperature data that stays available for new subscribers and is delivered reliably. Which MQTT publish options should you set?

hard

A. Set QoS=0 and retain=false

B. Set QoS=0 and retain=true

C. Set QoS=2 and retain=false

D. Set QoS=1 and retain=true

Solution

1. Step 1: Understand QoS levels

   QoS=1 ensures message delivery at least once, making it reliable.

2. Step 2: Understand retain flag

   retain=true keeps the last message on the broker so new subscribers get it immediately.

3. Step 3: Combine for reliable and persistent data

   Setting QoS=1 and retain=true meets both requirements.

4. Final Answer:

   Set QoS=1 and retain=true -> Option D

5. Quick Check:

   QoS=1 + retain=true = reliable & persistent [OK]

Hint: Use QoS=1 and retain=true for reliable, persistent data [OK]

Common Mistakes:

• Using QoS=0 which is unreliable
• Not setting retain so new subscribers miss data
• Confusing QoS=2 as always best choice