Wildcard subscriptions (+ and #) in IOT Protocols - Step-by-Step Execution

Process Flow - Wildcard subscriptions (+ and #)

Start Subscription

↓

Check Topic Filter

↓

Match Single Level

| No↓

Match Multi Level

No↓

Exact Match

↓

Deliver Messages

This flow shows how MQTT wildcard subscriptions use '+' to match one topic level and '#' to match multiple levels, deciding which messages to deliver.

Execution Sample

IOT Protocols

Subscribe to: sensors/+/temperature
Publish to: sensors/livingroom/temperature
Publish to: sensors/kitchen/humidity

Subscribe with '+' wildcard to receive temperature from any room, then publish messages to different topics to see which match.

Process Table

Step	Subscription Topic Filter	Published Topic	Match Condition	Match Result	Message Delivered
1	sensors/+/temperature	sensors/livingroom/temperature	'+' matches 'livingroom' level	Match	Yes
2	sensors/+/temperature	sensors/kitchen/humidity	'temperature' != 'humidity' at last level	No Match	No
3	sensors/#	sensors/kitchen/humidity	'#' matches all remaining levels	Match	Yes
4	sensors/#	devices/kitchen/humidity	First level 'sensors' != 'devices'	No Match	No
5	home/+/temperature	home/livingroom/temperature	'+' matches 'livingroom'	Match	Yes
6	home/+/temperature	home/livingroom/kitchen/temperature	'+' matches only one level, extra levels present	No Match	No

💡 All published topics checked against subscription filters; messages delivered only if match conditions met.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
Subscription Topic Filter	sensors/+/temperature	sensors/+/temperature	sensors/+/temperature	sensors/#	sensors/#	home/+/temperature	home/+/temperature	home/+/temperature
Published Topic	sensors/livingroom/temperature	sensors/kitchen/humidity	sensors/kitchen/humidity	devices/kitchen/humidity	devices/kitchen/humidity	home/livingroom/temperature	home/livingroom/kitchen/temperature	home/livingroom/kitchen/temperature
Match Result	Match	No Match	Match	No Match	No Match	Match	No Match	No Match
Message Delivered	Yes	No	Yes	No	No	Yes	No	No

Key Moments - 3 Insights

Why does '+' only match one level and not multiple levels?

The '+' wildcard matches exactly one topic level. For example, in row 6 of the execution table, 'home/+/temperature' does not match 'home/livingroom/kitchen/temperature' because '+' matches only 'livingroom', not multiple levels like 'livingroom/kitchen'.

How does '#' differ from '+' in matching topics?

The '#' wildcard matches zero or more levels at the end of the topic. For example, in row 3, 'sensors/#' matches 'sensors/kitchen/humidity' because '#' covers all remaining levels after 'sensors'.

Can '#' be used in the middle of a topic filter?

No, '#' must be the last character in the topic filter. It matches all remaining levels from that point. This is why in the examples, '#' appears only at the end.

Visual Quiz - 3 Questions

Look at the execution table, at step 2, why is the message not delivered?

A Because '+' does not match 'kitchen'

B Because the last level 'humidity' does not match 'temperature'

C Because '#' wildcard is missing

D Because the published topic has too many levels

💡 Hint

Check the 'Match Result' and 'Match Condition' columns at step 2 in the execution table.

At which step does the '#' wildcard allow matching multiple topic levels?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look for '#' in the 'Subscription Topic Filter' column and see where it matches multiple levels.

If the subscription was 'home/#' instead of 'home/+/temperature', which published topic would match at step 6?

A sensors/livingroom/temperature

B home/livingroom/temperature

C home/livingroom/kitchen/temperature

D devices/kitchen/humidity

💡 Hint

Refer to how '#' matches multiple levels in the variable_tracker and execution_table.

Concept Snapshot

MQTT wildcard subscriptions:
- '+' matches exactly one topic level
- '#' matches zero or more levels at the end
- '+' and '#' help subscribe to multiple topics easily
- '#' must be last in filter
- Messages delivered only if topic matches filter

Full Transcript

This lesson shows how MQTT wildcard subscriptions work using '+' and '#'. The '+' wildcard matches exactly one level in the topic path, like a single folder in a file path. The '#' wildcard matches all remaining levels, like a catch-all folder. We traced examples where subscriptions with '+' matched topics with one varying level, and '#' matched multiple levels. We saw why some messages are delivered and others are not, based on these rules. This helps understand how to subscribe to many topics efficiently in IoT messaging.

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