MQTT QoS 0 vs QoS 1 vs QoS 2: Key Differences Explained

MQTT QoS 0 delivers messages once with no guarantee, QoS 1 ensures delivery at least once but may duplicate messages, and QoS 2 guarantees exactly one delivery with no duplicates using a four-step handshake.

Quick Comparison

Here is a quick table comparing MQTT QoS levels by key factors.

Factor	QoS 0	QoS 1	QoS 2
Delivery Guarantee	At most once (no guarantee)	At least once (possible duplicates)	Exactly once (no duplicates)
Message Duplication	Possible loss, no duplicates	Possible duplicates	No duplicates
Acknowledgment	No ACK	PUBACK	PUBREC, PUBREL, PUBCOMP
Network Overhead	Lowest	Medium	Highest
Use Case	Non-critical data	Important data, duplicates okay	Critical data, no duplicates allowed

Key Differences

QoS 0 is the simplest level where messages are sent once without any acknowledgment. This means messages can be lost if the network fails, but it has the lowest overhead and latency.

QoS 1 adds reliability by requiring the receiver to send a PUBACK acknowledgment. The sender retries until it gets this ACK, so messages arrive at least once but may be duplicated if the ACK is lost.

QoS 2 is the most reliable and complex level. It uses a four-step handshake (PUBREC, PUBREL, PUBCOMP) to ensure the message is delivered exactly once without duplicates. This adds more network traffic and latency but guarantees no message loss or duplication.

Code Comparison

Example of publishing a message with QoS 0 using Python's Paho MQTT client.

python

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883, 60)

# Publish with QoS 0
client.publish("test/topic", "Hello QoS 0", qos=0)
client.disconnect()

Output

Message published to 'test/topic' with QoS 0 (no acknowledgment expected)

QoS 1 Equivalent

Publishing the same message with QoS 1 to ensure delivery at least once.

python

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883, 60)

# Publish with QoS 1
client.publish("test/topic", "Hello QoS 1", qos=1)
client.disconnect()

Output

Message published to 'test/topic' with QoS 1 (PUBACK expected, retries if no ACK)

When to Use Which

Choose QoS 0 when you want the fastest delivery and can tolerate message loss, like sensor data that updates frequently.

Choose QoS 1 when message delivery is important but duplicates are acceptable, such as notifications or commands where retry is better than loss.

Choose QoS 2 for critical messages where duplicates cannot be tolerated, like financial transactions or device configuration updates.

Key Takeaways

MQTT QoS 0 sends messages once without delivery guarantee or acknowledgment.

QoS 1 ensures messages arrive at least once but may cause duplicates due to retries.

QoS 2 guarantees exactly one delivery with a four-step handshake to avoid duplicates.

Use QoS 0 for non-critical, fast data; QoS 1 for reliable but duplicate-allowed messages; QoS 2 for critical, no-duplicate messages.