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.