Wildcard subscriptions (+ and #) in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the '+' wildcard in MQTT topic subscriptions

In MQTT, what does the '+' wildcard match in topic subscriptions?

A It matches zero or more topic levels at the position of the '+'

B It matches exactly one topic level at the position of the '+'

C It matches the entire topic string regardless of levels

D It matches only the last topic level in the topic string

💡 Hint

Think about how '+' works compared to '#' in MQTT topics.

The '+' wildcard matches exactly one topic level at its position. It cannot match multiple levels or zero levels.

🧠 Conceptual

intermediate

Understanding the '#' wildcard in MQTT topic subscriptions

In MQTT, what does the '#' wildcard match in topic subscriptions?

A It matches zero or more topic levels including the current and all following levels

B It matches exactly one topic level at the position of the '#'

C It matches only the first topic level in the topic string

D It matches only topics without any sub-levels

💡 Hint

Consider how '#' differs from '+' in matching topic levels.

The '#' wildcard matches zero or more topic levels starting from its position to the end of the topic.

💻 Command Output

advanced

Output of MQTT subscription matching with '+' wildcard

Given the subscription topic filter home/+/temperature, which published topic will match this subscription?

A home/temperature

B home/kitchen/livingroom/temperature

C home/kitchen/temperature

D home//temperature

💡 Hint

Remember '+' matches exactly one level, no more, no less.

The '+' wildcard matches exactly one topic level. 'home/kitchen/temperature' matches because 'kitchen' is one level. 'home/kitchen/livingroom/temperature' has two levels between 'home' and 'temperature', so it does not match.

💻 Command Output

advanced

Output of MQTT subscription matching with '#' wildcard

Given the subscription topic filter devices/sensors/#, which published topic will NOT match this subscription?

A devices/sensors

B devices/sensors/temperature

C devices/sensors/temperature/room1

D devices/actuators/temperature

💡 Hint

The '#' wildcard matches zero or more levels after its position.

The subscription 'devices/sensors/#' matches any topic starting with 'devices/sensors' and any sub-levels. 'devices/actuators/temperature' does not start with 'devices/sensors' so it does not match.

❓ Troubleshoot

expert

Troubleshooting MQTT subscription with mixed wildcards

A client subscribes to the topic filter building/+/floor/#. Which of the following published topics will NOT be received by the client?

A building/room1/temperature/floor

B building/room2/floor

C building/room3/floor/room4/humidity

D building/room1/floor/temperature

💡 Hint

Check the position of the wildcards and the topic levels carefully.

The subscription 'building/+/floor/#' expects the second level to be any single level (matched by '+'), then the third level to be 'floor', followed by zero or more levels (matched by '#'). 'building/room1/temperature/floor' has 'temperature' as the third level, so it does not match.

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