MQTT with QoS levels in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does MQTT stand for and what is its main use?

MQTT stands for Message Queuing Telemetry Transport. It is a lightweight messaging protocol used for sending data between devices, especially in IoT (Internet of Things) systems like Raspberry Pi projects.

beginner

What are the three QoS levels in MQTT?

The three QoS (Quality of Service) levels in MQTT are:<br>1. QoS 0 - At most once delivery (fire and forget)<br>2. QoS 1 - At least once delivery (message is retried until acknowledged)<br>3. QoS 2 - Exactly once delivery (ensures message is received only once)

beginner

Explain QoS 0 in MQTT with a real-life example.

QoS 0 means the message is sent once without waiting for confirmation. It's like sending a postcard: you drop it in the mailbox but don't know if it arrives. This is good for non-critical data where occasional loss is okay.

intermediate

How does QoS 1 ensure message delivery?

QoS 1 makes sure the message arrives at least once by resending it until the receiver sends an acknowledgment. It's like calling a friend and leaving a voicemail until they confirm they got your message.

intermediate

Why is QoS 2 the most reliable but also the slowest QoS level?

QoS 2 guarantees the message is received exactly once by using a four-step handshake between sender and receiver. This extra checking makes it very reliable but slower, like sending a registered letter that requires a signature on delivery.

Which MQTT QoS level sends a message only once without confirmation?

A QoS 2

B QoS 1

C QoS 0

D QoS 3

QoS 0 sends the message once without waiting for any acknowledgment.

What does QoS 1 guarantee in MQTT?

A Message is sent at least once

B Message is sent at most once

C Message is sent exactly once

D Message is sent twice

QoS 1 guarantees the message is delivered at least once, possibly more if retries happen.

Which QoS level uses a four-step handshake to ensure message delivery?

A QoS 2

B QoS 1

C QoS 3

D QoS 0

QoS 2 uses a four-step handshake to guarantee exactly once delivery.

In MQTT, which QoS level is best for critical messages that must not be lost or duplicated?

A QoS 1

B QoS 2

C QoS 0

D None

QoS 2 ensures messages are delivered exactly once, making it best for critical data.

What is a downside of using QoS 2 in MQTT?

A Messages may be lost

B It does not guarantee delivery

C Messages are not acknowledged

D It is slower due to extra handshakes

QoS 2 is slower because it uses multiple steps to ensure exactly once delivery.

Describe the three MQTT QoS levels and when you might use each one.

Think about message delivery guarantees and real-life examples.

Explain why QoS 2 is more reliable but slower than QoS 0 and QoS 1 in MQTT.

Compare the delivery process and message confirmation.