Subscribing to control commands in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output when subscribing to a control topic with QoS 1?

You subscribe to the MQTT topic device/control with QoS level 1. The broker sends a retained message with payload ON. What will the client receive?

IOT Protocols

client.subscribe('device/control', qos=1)
# Broker sends retained message 'ON' on 'device/control'

A The client receives the message 'ON' multiple times without acknowledgment.

B The client receives the message 'ON' exactly once and sends PUBACK to broker.

C The client does not receive any message because QoS 1 ignores retained messages.

D The client receives the message 'ON' but does not send any acknowledgment.

💡 Hint

QoS 1 guarantees message delivery at least once with acknowledgment.

QoS 1 ensures the client receives the message at least once and sends PUBACK to confirm receipt. Retained messages are sent immediately upon subscription.

❓ Configuration

intermediate

Which MQTT client configuration enables automatic resubscription after reconnect?

You want your MQTT client to automatically resubscribe to control topics after a network reconnect. Which configuration option should you enable?

A Set <code>qos</code> to 0 to avoid resending messages.

B Set <code>keep_alive</code> to 0 to disable session expiration.

C Set <code>retain</code> to true to keep messages on the broker.

D Set <code>clean_session</code> to false to keep subscriptions after reconnect.

💡 Hint

Session persistence controls subscription retention.

Setting clean_session to false tells the broker to keep the session and subscriptions after disconnect, enabling automatic resubscription.

❓ Troubleshoot

advanced

Why does the client not receive control commands after reconnect?

An MQTT client subscribes to device/control with QoS 1 and clean_session=true. After a network drop and reconnect, the client does not receive any control commands sent during offline. Why?

A Because <code>clean_session=true</code> clears subscriptions on disconnect, so no resubscription occurs.

B Because QoS 1 does not guarantee message delivery after reconnect.

C Because the broker does not support retained messages on this topic.

D Because the client did not send PUBACK for previous messages.

💡 Hint

Check session persistence settings.

With clean_session=true, the broker deletes the session and subscriptions on disconnect, so the client must resubscribe manually after reconnect to receive messages.

🔀 Workflow

advanced

Order the steps for subscribing and receiving control commands via MQTT

Arrange the following steps in the correct order for an MQTT client to subscribe and receive control commands reliably.

A 1,3,2,4

B 2,1,3,4

C 1,2,3,4

D 2,3,1,4

💡 Hint

Think about connection before subscription.

First connect with session persistence, then subscribe, receive messages, and acknowledge them to ensure reliable delivery.

✅ Best Practice

expert

Which practice ensures reliable control command delivery in MQTT for IoT devices?

Select the best practice to ensure IoT devices reliably receive control commands even after temporary disconnections.

A Use persistent sessions (clean_session=false), subscribe with QoS 1 or 2, and use retained messages for last known command.

B Use clean_session=true and QoS 0 to minimize network traffic and avoid duplicates.

C Subscribe with QoS 0 and rely on the device to poll the broker for missed commands.

D Disable retained messages and use QoS 2 with clean_session=true for guaranteed delivery.

💡 Hint

Think about session persistence and message delivery guarantees.

Persistent sessions keep subscriptions active, QoS 1 or 2 ensures delivery, and retained messages provide the last command on reconnect.