Topics and topic hierarchy in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding MQTT Topic Hierarchy

In MQTT, topics are structured in a hierarchy separated by slashes. Which of the following represents a valid topic hierarchy for a smart home system?

A home.livingroom.temperature

B home/livingroom/temperature

C home\livingroom\temperature

D home-livingroom-temperature

💡 Hint

Think about how MQTT topics use slashes to separate levels.

MQTT topics use forward slashes '/' to separate levels in the hierarchy. Backslashes, dots, or dashes are not valid separators.

💻 Command Output

intermediate

MQTT Topic Subscription Wildcards Output

What topics will a client receive if it subscribes to home/+/temperature?

A home/livingroom/temperature and home/kitchen/temperature

B home/livingroom/temperature only

C home/livingroom/humidity and home/kitchen/temperature

D home/livingroom/temperature and home/livingroom/humidity

💡 Hint

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

The '+' wildcard matches exactly one level, so home/+/temperature matches topics like home/livingroom/temperature and home/kitchen/temperature, but not home/livingroom/humidity.

❓ Configuration

advanced

Configuring MQTT Broker Topic Access Control

You want to restrict clients so they can only publish to topics under devices/ and subscribe to topics under sensors/. Which configuration snippet correctly enforces this in a typical MQTT broker ACL file?

A

user client
publish devices/
subscribe sensors/

B

user client
publish sensors/#
subscribe devices/#

C

user client
publish #
subscribe #

D

user client
publish devices/#
subscribe sensors/#

💡 Hint

Remember that '#' matches all subtopics and the ACL syntax requires specifying publish and subscribe permissions.

The ACL line publish devices/# allows publishing to all topics starting with devices/. Similarly, subscribe sensors/# allows subscribing to all topics under sensors/. Other options either reverse permissions or use incorrect syntax.

❓ Troubleshoot

advanced

Troubleshooting MQTT Topic Subscription Failure

A client subscribes to home/+/temperature but does not receive messages published to home/livingroom/humidity. What is the most likely cause?

A The client is publishing to <code>home/livingroom/humidity</code> instead of <code>home/livingroom/temperature</code>

B The MQTT broker does not support the '+' wildcard

C The client subscribed to <code>home/#</code> instead of <code>home/+/temperature</code>

D The client subscribed with QoS 0 but messages are published with QoS 2

💡 Hint

Check if the published topic matches the subscribed topic pattern exactly.

The subscription home/+/temperature matches topics with exactly three levels where the last is 'temperature'. Publishing to home/livingroom/humidity will not match and thus no messages are received.

🔀 Workflow

expert

Designing a Topic Hierarchy for Scalable IoT Deployment

You are designing an MQTT topic hierarchy for a city-wide sensor network with multiple sensor types in many locations. Which topic hierarchy design best supports easy filtering by location and sensor type?

A sensor_type/city/location/sensor_id

B city/sensor_type/location/sensor_id

C city/location/sensor_type/sensor_id

D sensor_id/city/location/sensor_type

💡 Hint

Consider which topic levels you want to filter on first when subscribing.

Placing location before sensor_type allows subscribers to filter all sensors in a location easily, or filter by sensor type within a location. This is more intuitive for city-wide deployments.

Practice

1. What is the main purpose of topics in IoT protocols like MQTT?

easy

A. To encrypt messages for security

B. To store device firmware

C. To organize messages in a clear, folder-like structure

D. To control device power settings

Solution

1. Step 1: Understand the role of topics

   Topics act like folders to organize messages so devices know where to send or receive data.

2. Step 2: Compare options with topic function

   Only organizing messages matches the purpose of topics; encryption and firmware storage are unrelated.

3. Final Answer:

   To organize messages in a clear, folder-like structure -> Option C

4. Quick Check:

   Topics organize messages = B [OK]

Hint: Topics are like folders for messages [OK]

Common Mistakes:

• Confusing topics with security features
• Thinking topics store device data
• Mixing topics with device settings

2. Which of the following is the correct way to write a topic with three levels?

easy

A. home-kitchen-temperature

B. home.kitchen.temperature

C. home\kitchen\temperature

D. home/kitchen/temperature

Solution

1. Step 1: Recall topic level separator

   Topics use slashes (/) to separate levels, like folders in a path.

2. Step 2: Check each option's separator

   Only home/kitchen/temperature uses slashes correctly; others use dots, backslashes, or dashes which are incorrect.

3. Final Answer:

   home/kitchen/temperature -> Option D

4. Quick Check:

   Topic levels separated by '/' = A [OK]

Hint: Use slashes (/) to separate topic levels [OK]

Common Mistakes:

• Using dots instead of slashes
• Using backslashes which are invalid
• Using dashes instead of separators

3. Given the topic building1/floor2/room3/sensor, which part represents the third level?

medium

A. room3

B. floor2

C. building1

D. sensor

Solution

1. Step 1: Split the topic by slashes

   Splitting 'building1/floor2/room3/sensor' by '/' gives ['building1', 'floor2', 'room3', 'sensor'].

2. Step 2: Identify the third element

   The third element (index 2) is 'room3'.

3. Final Answer:

   room3 -> Option A

4. Quick Check:

   Third level = room3 = C [OK]

Hint: Count levels separated by '/' to find position [OK]

Common Mistakes:

• Counting from zero incorrectly
• Choosing first or second level by mistake
• Confusing last level with third

4. Identify the error in this topic string: home//livingroom/temperature

medium

A. Double slash creates an empty level

B. Missing slash between 'home' and 'livingroom'

C. Topic levels must use dots, not slashes

D. Topic cannot contain the word 'temperature'

Solution

1. Step 1: Analyze the topic string

   The string 'home//livingroom/temperature' has two slashes together, meaning an empty level between them.

2. Step 2: Understand topic level rules

   Empty levels are invalid because each level should have a name; double slashes cause this error.

3. Final Answer:

   Double slash creates an empty level -> Option A

4. Quick Check:

   Double slash = empty level error = A [OK]

Hint: Avoid double slashes to prevent empty topic levels [OK]

Common Mistakes:

• Ignoring empty levels caused by double slashes
• Thinking dots are required separators
• Believing certain words are forbidden in topics

5. You want to subscribe to all sensors in any room on floor 1 of building 2 using MQTT topics. Which subscription topic should you use?

hard

A. building2/floor1/*/sensor

B. building2/floor1/+/sensor

C. building2/floor1/room1/sensor

D. building2/floor1/#/sensor

Solution

1. Step 1: Understand MQTT wildcards

   The '+' wildcard matches exactly one level, while '#' matches all remaining levels.

2. Step 2: Analyze the subscription needs

   We want any room on floor1, so '+' replaces the room level. '#' would match too many levels, and '*' is invalid in MQTT.

3. Final Answer:

   building2/floor1/+/sensor -> Option B

4. Quick Check:

   '+' wildcard for one level = D [OK]

Hint: Use '+' for single-level wildcard in MQTT topics [OK]

Common Mistakes:

• Using '#' when only one level is needed
• Using '*' which is not a valid MQTT wildcard
• Specifying exact room instead of wildcard