Wildcard subscriptions (+ and #) in IOT Protocols - Mini Project: Build & Apply

Wildcard Subscriptions (+ and #) in MQTT Topics

📖 Scenario: You are setting up an MQTT client to subscribe to sensor data topics. MQTT topics use slashes to separate levels, like home/kitchen/temperature. You want to use wildcards to subscribe to multiple topics easily.The + wildcard matches exactly one level, and the # wildcard matches all remaining levels.

🎯 Goal: Learn how to create MQTT topic subscriptions using the + and # wildcards to receive messages from multiple topics efficiently.

📋 What You'll Learn

Create a list of MQTT topics to subscribe to

Add wildcard subscription patterns using + and #

Filter topics matching the wildcard subscriptions

Print the matched topics

💡 Why This Matters

🌍 Real World

MQTT is widely used in IoT devices to send sensor data. Wildcard subscriptions let you listen to many related topics easily, saving network and processing resources.

💼 Career

Understanding MQTT wildcards is important for IoT developers, DevOps engineers managing IoT infrastructure, and anyone working with message brokers and real-time data streams.

1

Create a list of MQTT topics

Create a list called topics with these exact MQTT topic strings: 'home/kitchen/temperature', 'home/kitchen/humidity', 'home/livingroom/temperature', 'office/room1/temperature', 'office/room2/humidity'.

IOT Protocols

# Create the list called topics with the exact MQTT topic strings
# Your code here

Hint

Use square brackets to create a list and include all topics as strings separated by commas.

2

Add wildcard subscription patterns

Create a list called subscriptions with these exact wildcard subscription strings: 'home/+/temperature' and 'office/#'.

IOT Protocols

topics = ['home/kitchen/temperature', 'home/kitchen/humidity', 'home/livingroom/temperature', 'office/room1/temperature', 'office/room2/humidity']
# Create the list called subscriptions with the exact wildcard strings
# Your code here

Hint

Use a list with the two wildcard subscription strings exactly as shown.

3

Filter topics matching the wildcard subscriptions

Create a function called match_topic that takes subscription and topic strings and returns True if the topic matches the subscription using MQTT wildcard rules for + and #. Then create a list called matched_topics that includes topics from topics matching any subscription in subscriptions.

IOT Protocols

topics = ['home/kitchen/temperature', 'home/kitchen/humidity', 'home/livingroom/temperature', 'office/room1/temperature', 'office/room2/humidity']
subscriptions = ['home/+/temperature', 'office/#']
# Define match_topic(subscription, topic) function
# Create matched_topics list with topics matching any subscription
# Your code here

Hint

Split both subscription and topic by '/' and compare each level. The '+' wildcard matches one level, '#' matches all remaining levels.

4

Print the matched topics

Write a print statement to display the matched_topics list.

IOT Protocols

topics = ['home/kitchen/temperature', 'home/kitchen/humidity', 'home/livingroom/temperature', 'office/room1/temperature', 'office/room2/humidity']
subscriptions = ['home/+/temperature', 'office/#']

def match_topic(subscription, topic):
    sub_levels = subscription.split('/')
    topic_levels = topic.split('/')
    for i, sub_level in enumerate(sub_levels):
        if sub_level == '#':
            return True
        if i >= len(topic_levels):
            return False
        if sub_level != '+' and sub_level != topic_levels[i]:
            return False
    return len(topic_levels) == len(sub_levels)

matched_topics = [topic for topic in topics if any(match_topic(sub, topic) for sub in subscriptions)]
# Print the matched_topics list
# Your code here

Hint

Use print(matched_topics) to show the list of matched topics.

Practice

1. What does the MQTT wildcard + represent in topic subscriptions?

easy

A. It matches zero or more topic levels.

B. It matches exactly one topic level.

C. It matches the entire topic string.

D. It matches only the last topic level.

Solution

1. Step 1: Understand the + wildcard meaning

   The + wildcard matches exactly one topic level in MQTT subscriptions.

2. Step 2: Compare with other wildcards

   The # wildcard matches zero or more levels, so it is different from +.

3. Final Answer:

   It matches exactly one topic level. -> Option B

4. Quick Check:

   + = one level [OK]

Hint: Remember: + means one level only [OK]

Common Mistakes:

• Confusing + with # wildcard
• Thinking + matches multiple levels
• Assuming + matches zero levels

2. Which of the following is the correct MQTT subscription syntax to match all topics under home/kitchen including sub-levels?

easy

A. home/kitchen/#

B. home/+/kitchen

C. home/kitchen/+

D. home/#/kitchen

Solution

1. Step 1: Identify the wildcard for all sub-levels

   The # wildcard matches all remaining topic levels, including zero or more sub-levels.

2. Step 2: Apply to the topic home/kitchen

   Using home/kitchen/# subscribes to home/kitchen and any deeper topics like home/kitchen/temperature.

3. Final Answer:

   home/kitchen/# -> Option A

4. Quick Check:

   # matches all sub-levels [OK]

Hint: Use # to catch all subtopics under a path [OK]

Common Mistakes:

• Using + instead of # for multiple levels
• Placing # in the middle of the topic
• Using invalid syntax like # in the middle

3. Given the subscription building/+/temperature, which of these topics will be matched?

medium

A. building/floor1/temperature

B. building/floor1/room1/temperature

C. building/temperature

D. building/floor1/humidity

Solution

1. Step 1: Understand the + wildcard position

   The + wildcard matches exactly one topic level between building and temperature.

2. Step 2: Check each topic against the pattern

   Topic building/floor1/temperature matches because floor1 fills the + wildcard. Others either have too many or too few levels or wrong last level.

3. Final Answer:

   building/floor1/temperature -> Option A

4. Quick Check:

   + matches one level only [OK]

Hint: + matches exactly one level between slashes [OK]

Common Mistakes:

• Matching topics with extra levels
• Ignoring the exact position of +
• Assuming + matches zero levels

4. You wrote the subscription sensor/#/temperature but it does not work. What is the problem?

medium

A. The subscription is missing a leading slash.

B. The + wildcard should be used instead of # here.

C. The topic levels are case sensitive and must be lowercase.

D. The # wildcard must be at the end of the topic filter.

Solution

1. Step 1: Recall the rule for # wildcard placement

   The # wildcard must be the last character in the topic filter and can only appear once.

2. Step 2: Analyze the subscription sensor/#/temperature

   Here, # is in the middle, which is invalid syntax and causes the subscription to fail.

3. Final Answer:

   The # wildcard must be at the end of the topic filter. -> Option D

4. Quick Check:

   # only at end [OK]

Hint: # must be last in topic filter [OK]

Common Mistakes:

• Placing # in the middle of topic
• Confusing + and # usage
• Ignoring MQTT topic syntax rules

5. You want to subscribe to all topics under office but only one level deep, such as office/room1 and office/room2, but not office/room1/temperature. Which subscription should you use?

hard

A. office/+/+

B. office/#

C. office/+

D. office

Solution

1. Step 1: Understand the requirement for one level deep

   You want to match topics exactly one level below office, like office/room1.

2. Step 2: Choose the correct wildcard

   The + wildcard matches exactly one level, so office/+ matches office/room1 but not office/room1/temperature.

3. Final Answer:

   office/+ -> Option C

4. Quick Check:

   One level deep = use + wildcard [OK]

Hint: Use + for exactly one sub-level [OK]

Common Mistakes:

• Using # which matches all levels
• Using multiple + wildcards incorrectly
• Not including any wildcard