Sumatif Assesment: Wildcard, QoS, Retain in MQTT Protocol

A smart building uses the topic structure building/floor/room/sensor (e.g., building/5/conference_room/temperature). A client needs to subscribe to only the 'temperature' sensor data from all rooms on the 5th floor. Which topic subscription should it use?

• building/5/room/temperature

• building/5/#

• 
  building/5/+/temperature

• building/+/+/temperature

Answer explanation

This correctly uses the single-level wildcard + to match any room, but specifically on floor 5 and for the 'temperature' sensor.

A home automation system publishes data under topics like home/garage/door/status and home/garage/temp. What subscription is required to receive all messages published under the 'garage' category, regardless of how many sub-levels there are?

• home/garage/+/+

• home/garage/*

• home/garage/+

• home/garage/#

Answer explanation

This correctly uses the multi-level wildcard # to match all topics that begin with home/garage/, including all sub-levels.

A developer tries to subscribe to factory/#/status to get all status messages (e.g., factory/line1/status and factory/line2/machineA/status). Why will this subscription fail?

• The topic filter is too long for the broker.

• The # wildcard must be the last character in the topic subscription.

• Wildcards cannot be combined with specific topic names.

• The + wildcard should be used instead of #.

Answer explanation

This is the correct reason. A topic like factory/#/status is invalid because # must only appear at the very end of the subscription string.

An application publishes ambient temperature data from 1,000 sensors every 10 seconds. This data is for a non-critical dashboard, and occasional missed messages are acceptable. Which QoS level is most appropriate?

• QoS 3 (This level does not exist)

• QoS 2 (Exactly once)

• QoS 1 (At least once)

• QoS 0 (At most once)

Answer explanation

This is the best choice. It has the lowest overhead ('fire and forget') and is suitable for high-frequency, non-critical data where losing a message is acceptable.

A financial app uses MQTT to process a 'transfer funds' instruction. It is critical that the instruction is processed exactly once. A duplicate instruction would cause a double transfer. Which QoS level is required?

• QoS 2

• 
  QoS 0

• .

  QoS 1

Answer explanation

This is the correct choice. Its 4-part handshake ensures the message is delivered and processed by the recipient exactly one time, preventing both loss and duplication.

A mobile app sends a command to unlock a smart door. It is critical that the door receives the command. However, the door's software is idempotent (it can safely ignore a second 'unlock' command if it receives it twice). What is the minimum sufficient QoS level?

• QoS 2

• QoS 0

• QoS 0 or QoS 1

• QoS 1

Answer explanation

This guarantees 'at least once' delivery. Since the door can handle duplicates, this is the minimum sufficient level to ensure the command arrives.

A new smart thermostat connects to the network and subscribes to home/thermostat/setting. It needs to know the last set temperature immediately, without waiting for a new publish. Which MQTT feature should be used?

• A Persistent Session

• QoS 2 Publishing

• A wildcard subscription

• The RETAIN flag on the message

Answer explanation

Correct. When a message is published with the RETAIN flag, the broker stores it and delivers it instantly to any new client that subscribes to that topic.

A message with the RETAIN flag was published to device/123/status with the payload 'Online'. How can an administrator clear this retained message so new subscribers no longer receive 'Online'?

• The message will be cleared automatically after 24 hours.

• Publish a message with an empty payload and RETAIN=true to that topic.

• Disconnect all subscribers from the topic.

• Publish a new message with QoS 0 to the same topic.

Answer explanation

This is the standard MQTT method. The broker sees the RETAIN=true flag but an empty payload, and it deletes the retained message for that topic.

A publisher sends 'Ready' to robot/1/status with QoS=1 and RETAIN=true. A new client, Client_A, subscribes to robot/1/status with QoS=1. What happens immediately?

• Client_A immediately receives 'Ready', but it is delivered at QoS 0.

• Client_A receives an error because QoS 1 and Retain cannot be used together.

• Client_A receives nothing until the next time the publisher sends an update.

• Client_A immediately receives 'Ready', and this delivery follows QoS 1 rules.

Answer explanation

This is correct. The retained message is delivered, and the QoS level of the delivery is the lower of the original publish QoS (1) and the subscription QoS (1).

A broker has a retained message 'Stopped' on the topic factory/line/motor. A client subscribes using the wildcard topic factory/line/#. What will the client receive immediately upon subscribing?

• It will only receive new messages published to topics matching factory/line/#.

• Nothing, because wildcards do not trigger retained messages.

• An error, as this is an invalid combination of features.

• The retained message 'Stopped' from the topic factory/line/motor.

Answer explanation

This is correct. The subscription factory/line/# matches the topic factory/line/motor, and since that topic has a retained message, the broker sends it.