Publish-subscribe architecture in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does the broker function in a publish-subscribe system?

In a publish-subscribe architecture, what is the main role of the broker?

A It stores all messages permanently for future retrieval by subscribers.

B It directly connects publishers and subscribers without storing messages.

C It encrypts messages to ensure secure communication between devices.

D It receives messages from publishers and forwards them to subscribers based on topics.

💡 Hint

Think about how messages flow from senders to receivers without direct connection.

The broker acts as a middleman that receives messages from publishers and forwards them to subscribers who have expressed interest in specific topics.

💻 Command Output

intermediate

MQTT subscribe command output

What is the expected output when a client successfully subscribes to the topic home/temperature using the MQTT command below?

IOT Protocols

mosquitto_sub -t home/temperature

A Subscribed (QoS 0)

B Error: Topic not found

C No output until a message is published

D Connection refused: not authorized

💡 Hint

Consider what happens immediately after subscribing before any messages arrive.

The subscribe command waits silently for messages; it does not print confirmation unless a message arrives.

🔀 Workflow

advanced

Order of steps to set up a publish-subscribe system

Arrange the steps below in the correct order to set up a basic publish-subscribe communication using MQTT.

A 1,3,2,4

B 3,1,2,4

C 1,2,3,4

D 2,1,3,4

💡 Hint

The broker must be running before clients connect; subscribers listen before messages are sent.

First, start the broker. Then subscribe to the topic so the client is ready to receive. Next, publish the message. Finally, the subscriber receives it.

❓ Troubleshoot

advanced

Why does a subscriber not receive messages?

A subscriber client connected to the MQTT broker subscribes to sensor/data but never receives messages. Which is the most likely cause?

A The publisher is sending messages to a different topic than <code>sensor/data</code>.

B The subscriber client has a firewall blocking outgoing connections.

C The MQTT broker is offline but the subscriber shows connected.

D The subscriber client is using QoS level 2 which is unsupported.

💡 Hint

Check if the topic names match exactly between publisher and subscriber.

If the publisher sends messages to a different topic, the subscriber won't receive them because subscriptions are topic-specific.

✅ Best Practice

expert

Choosing QoS level for critical IoT messages

In a publish-subscribe system for critical IoT sensor data, which MQTT QoS level ensures messages are delivered exactly once without duplicates?

A QoS 0 - At most once delivery

B QoS 2 - Exactly once delivery

C QoS 1 - At least once delivery

D QoS 3 - Guaranteed delivery with retries

💡 Hint

Consider the QoS levels defined by MQTT and their delivery guarantees.

QoS 2 ensures messages are delivered exactly once, preventing duplicates, which is critical for reliable IoT data.

Practice

1. What is the main role of a broker in a publish-subscribe architecture?

easy

A. To receive messages from publishers and forward them to subscribers

B. To directly connect publishers and subscribers

C. To store all messages permanently

D. To encrypt messages between devices

Solution

1. Step 1: Understand the broker's function

   The broker acts as a middleman that receives messages from publishers.

2. Step 2: Identify message forwarding role

   The broker forwards these messages to subscribers interested in specific topics.

3. Final Answer:

   To receive messages from publishers and forward them to subscribers -> Option A

4. Quick Check:

   Broker forwards messages [OK]

Hint: Broker forwards messages, it doesn't connect devices directly [OK]

Common Mistakes:

• Thinking broker connects devices directly
• Assuming broker stores messages permanently
• Confusing broker with encryption service

2. Which of the following is the correct way to describe a topic in publish-subscribe?

easy

A. A label used to organize messages for subscribers

B. A direct connection between two devices

C. A security protocol for message encryption

D. A device that publishes messages

Solution

1. Step 1: Define what a topic is

   A topic is a label or category that organizes messages so subscribers receive only relevant ones.

2. Step 2: Eliminate incorrect options

   Topics are not connections, security protocols, or devices.

3. Final Answer:

   A label used to organize messages for subscribers -> Option A

4. Quick Check:

   Topic organizes messages [OK]

Hint: Topics organize messages, not devices or connections [OK]

Common Mistakes:

• Confusing topics with device connections
• Thinking topics are security features
• Mixing up topics with publishers

3. Given this scenario: A device publishes messages to topic home/temperature. Which device will receive the messages?

medium

A. All devices on the network

B. Any device subscribed to home/humidity

C. Any device subscribed to home/temperature

D. Only the publishing device

Solution

1. Step 1: Identify the topic published to

   The device publishes to home/temperature.

2. Step 2: Determine which subscribers get the message

   Only devices subscribed to home/temperature receive the messages.

3. Final Answer:

   Any device subscribed to home/temperature -> Option C

4. Quick Check:

   Subscribers to topic get messages [OK]

Hint: Subscribers to the exact topic get messages [OK]

Common Mistakes:

• Assuming all devices get messages
• Thinking publishing device receives its own messages
• Confusing similar topic names

4. A subscriber is not receiving messages from the broker. Which of these is a likely cause?

medium

A. Publisher is sending messages to the subscriber directly

B. Subscriber is publishing messages instead of subscribing

C. Broker is encrypting messages incorrectly

D. Subscriber is not subscribed to the correct topic

Solution

1. Step 1: Check subscription correctness

   If the subscriber is not subscribed to the right topic, it won't get messages.

2. Step 2: Rule out other causes

   Publishers don't send messages directly; encryption issues or wrong roles are less common causes here.

3. Final Answer:

   Subscriber is not subscribed to the correct topic -> Option D

4. Quick Check:

   Correct topic subscription needed [OK]

Hint: Check subscriber's topic subscription first [OK]

Common Mistakes:

• Thinking publisher sends messages directly
• Blaming encryption without evidence
• Confusing subscriber and publisher roles

5. In a large IoT system using publish-subscribe, how can you ensure that only devices in the 'factory/floor1' area receive temperature alerts without affecting others?

hard

A. Publish all alerts to factory/temperature and let devices filter messages locally

B. Publish temperature alerts to topic factory/floor1/temperature and have devices subscribe to it

C. Send alerts directly to each device's IP address

D. Use a single topic temperature for all devices

Solution

1. Step 1: Use topic hierarchy for targeted messaging

   Publishing to factory/floor1/temperature targets only devices subscribed to that specific area.

2. Step 2: Avoid broad topics or direct device addressing

   Broad topics or direct IP messaging reduce scalability and flexibility.

3. Final Answer:

   Publish temperature alerts to topic factory/floor1/temperature and have devices subscribe to it -> Option B

4. Quick Check:

   Use specific topics for targeted delivery [OK]

Hint: Use detailed topic paths for precise message delivery [OK]

Common Mistakes:

• Using broad topics causing unwanted message delivery
• Trying direct device IP messaging in pub-sub
• Ignoring topic hierarchy benefits