MQTT Quality of Service (QoS) Levels: Ensuring Delivery in IIoT

In IIoT environments, communications can be intermittent or critical. MQTT offers three QoS levels to balance reliability and performance.

QoS 0: At most once

• Fire-and-forget delivery. The message is sent once and not retried.
• Useful for non-critical or high-frequency data (e.g., continuous telemetry).
• Does not guarantee delivery; messages may be lost.

QoS 1: At least once

• Delivery with ACK; the publisher retries until confirmation is received.
• May produce duplicates; the subscriber must handle idempotence.
• Good balance between reliability and network load.

QoS 2: Exactly once

• Four-step exchange (PUBLISH, PUBREC, PUBREL, PUBCOMP).
• Guarantees unique delivery without duplicates.
• Higher overhead; use only for critical transactions.

Example with Paho-MQTT

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.local", 1883)

# Publish with QoS 2
topic = "plant/area1/sensor/temperature"
payload = "23.5"
client.publish(topic, payload, qos=2)

How to Choose?

• QoS 0: High-frequency, loss-tolerant data.
• QoS 1: Important data that can tolerate duplicates.
• QoS 2: Critical transactions (control commands, billing).

In the next post, we’ll explore payload formats (JSON vs. binary) and when to use them in IIoT.