Wildcard subscriptions (+ and #) in IOT Protocols - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to all topics under 'home/kitchen' using a single-level wildcard.

IOT Protocols

client.subscribe("home/kitchen/[1]")

Drag options to blanks, or click blank then click option'

A*

B#

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '#' instead of '+' subscribes to all subtopics recursively, not just one level.

The single-level wildcard '+' matches exactly one topic level. So 'home/kitchen/+' subscribes to all topics directly under 'home/kitchen'.

2fill in blank

medium

Complete the code to subscribe to all topics under 'home' recursively using a multi-level wildcard.

IOT Protocols

client.subscribe("home/[1]")

Drag options to blanks, or click blank then click option'

A#

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '#' subscribes only to one level, missing deeper topics.

The multi-level wildcard '#' matches any number of topic levels, including zero. So 'home/#' subscribes to all topics under 'home' and its subtopics recursively.

3fill in blank

hard

Fix the error in the subscription to match all topics under 'office' and its subtopics.

IOT Protocols

client.subscribe("office/[1]")

Drag options to blanks, or click blank then click option'

A+/#

B+

C#/+

D#

Attempts:

3 left

💡 Hint

Common Mistakes

Placing '#' in the middle of the topic filter causes syntax errors.

The multi-level wildcard '#' must be at the end of the topic filter and matches all subtopics. 'office/#' correctly subscribes to all topics under 'office'.

4fill in blank

hard

Fill both blanks to subscribe to all sensors in any room under 'building1'.

IOT Protocols

client.subscribe("building1/[1]/[2]")

Drag options to blanks, or click blank then click option'

A+

Bsensors

C#

Droom

Attempts:

3 left

💡 Hint

Common Mistakes

Using '#' in the middle of the topic filter is invalid.

'building1/+/sensors' subscribes to the sensors topic in any room ('+') under 'building1'. Using '+' wildcard for room names and 'sensors' as the topic name is correct.

5fill in blank

hard

Fill all three blanks to subscribe to all temperature readings in any floor and any room under 'campus'.

IOT Protocols

client.subscribe("campus/[1]/[2]/[3]")

Drag options to blanks, or click blank then click option'

A+

Btemperature

Dfloor

Attempts:

3 left

💡 Hint

Common Mistakes

Using '#' instead of '+' for room level subscribes too broadly.

'campus/+/+/temperature' subscribes to temperature readings in any room ('+') on any floor ('+') under 'campus'.

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