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.