Wildcard subscriptions (+ and #) in IOT Protocols - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the '+' wildcard represent in MQTT topic subscriptions?

The '+' wildcard matches exactly one level in the topic hierarchy. It acts like a placeholder for any single topic name at that level.

beginner

What does the '#' wildcard represent in MQTT topic subscriptions?

The '#' wildcard matches any number of levels, including zero, in the topic hierarchy. It must be the last character in the subscription.

beginner

How would you subscribe to all topics under 'home/kitchen' using wildcards?

Use the subscription 'home/kitchen/#' to receive messages from 'home/kitchen' and all its subtopics.

intermediate

Can the '+' wildcard be used multiple times in a single MQTT subscription?

Yes, '+' can be used multiple times to match one level at each position in the topic hierarchy.

intermediate

Why must the '#' wildcard be the last character in an MQTT subscription?

Because '#' matches all remaining levels, placing it last ensures the subscription covers all subtopics beyond that point without ambiguity.

What does the MQTT subscription 'home/+/temperature' match?

A Only the topic 'home/+/temperature'

B All topics starting with 'home/temperature'

C All topics with 'home' as first level, any second level, and 'temperature' as third level

D All topics under 'home' regardless of levels

The '+' wildcard matches exactly one level, so 'home/+/temperature' matches topics like 'home/kitchen/temperature' or 'home/livingroom/temperature'.

Which MQTT subscription matches all topics under 'office' including sublevels?

A office/+

B office/#

C #/office

D +/office/#

'office/#' matches 'office' and all its subtopics at any level.

Is 'home/#/temperature' a valid MQTT subscription?

A No, '#' must be the last character

B Yes, it matches all topics under 'home' with 'temperature' at the end

C Yes, but only matches 'home/#/temperature' literally

D No, '+' should be used instead of '#'

The '#' wildcard must be the last character in the subscription.

What does the subscription '+/sensor/#' match?

A All topics with 'sensor' anywhere

B Only topics starting with '+/sensor/'

C Only topics with exactly two levels

D Any topic with any first level, 'sensor' as second level, and any subtopics after

'+' matches one level, so the first level can be anything, then 'sensor', then any subtopics with '#'.

Which wildcard matches exactly one topic level in MQTT?

A +

B #

C *

D ?

'+' matches exactly one level; '#' matches multiple levels; '*' and '?' are not MQTT wildcards.

Explain how the '+' and '#' wildcards work in MQTT topic subscriptions and give an example of each.

Think about how you can listen to specific or broad topic patterns.

Describe why the '#' wildcard must be the last character in an MQTT subscription and what happens if it is not.

Consider how MQTT interprets topic hierarchies.

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