Topic design patterns for IoT in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Publish-Subscribe Pattern in IoT

Which statement best describes the publish-subscribe pattern commonly used in IoT messaging?

A Devices publish messages to a broker, and subscribers receive messages from the broker.

B Devices store messages locally and share them when requested.

C Devices send messages directly to each other without a broker.

D Devices use a centralized database to read and write messages synchronously.

💡 Hint

Think about how messages flow through a middleman in this pattern.

The publish-subscribe pattern uses a broker to decouple message senders (publishers) from receivers (subscribers). Publishers send messages to the broker, which then forwards them to subscribers interested in those messages.

💻 Command Output

intermediate

MQTT Topic Wildcard Matching Output

Given an MQTT broker subscription to the topic home/+/temperature, which published topic will NOT match this subscription?

A home/livingroom/temperature

B home/livingroom/humidity

C home/kitchen/temperature

D home/bedroom/temperature

💡 Hint

The + wildcard matches exactly one level in the topic hierarchy.

The subscription home/+/temperature matches topics with exactly three levels where the middle level can be any single word. Option B has 'humidity' instead of 'temperature' at the last level, so it does not match.

❓ Configuration

advanced

Configuring Retained Messages in MQTT

Which MQTT client configuration will ensure that the last message published on a topic is stored by the broker and sent immediately to new subscribers?

A Subscribe with the clean session flag set to true.

B Subscribe with QoS level 0.

C Publish messages with QoS level 2 only.

D Publish messages with the retain flag set to true.

💡 Hint

Consider how the broker knows to keep the last message for a topic.

Setting the retain flag to true when publishing tells the broker to store that message as the last known good value for the topic. New subscribers receive this retained message immediately upon subscribing.

❓ Troubleshoot

advanced

Troubleshooting MQTT Topic Subscription Issues

An IoT device subscribes to the topic devices/+/status but does not receive messages published to devices/device1/status. What is the most likely cause?

A The broker does not support wildcard subscriptions.

B The subscription used the wildcard <code>#</code> instead of <code>+</code>.

C The device subscribed with a clean session flag set to false.

D The device published messages with QoS 0 instead of QoS 1.

💡 Hint

Think about broker capabilities and wildcard support.

If the broker does not support wildcard subscriptions, the device will not receive messages even if the subscription topic is correct. QoS levels and clean session flags do not prevent wildcard matching if supported.

🔀 Workflow

expert

Designing a Scalable IoT Topic Hierarchy

You need to design an MQTT topic hierarchy for a smart city project with thousands of sensors across multiple districts and sensor types. Which topic design pattern will best support scalability and easy filtering?

A Use a single topic for all sensors, like <code>all_sensors</code>.

B Use a flat topic structure with sensor IDs only, like <code>sensor123</code>.

C Use a hierarchical topic structure like <code>city/district/sensor_type/sensor_id</code>.

D Use random topic names generated at runtime for each sensor.

💡 Hint

Think about how to organize topics for filtering by location and type.

A hierarchical topic structure allows subscribers to filter messages by city, district, sensor type, or specific sensor ID. This supports scalability and efficient message routing.