What is the QoS parameter of the mqtt in node?

General
1

I don't understand the meaning of the QoS parameter in the mqtt in node. Is it some kind of filter?

2

This is generally considered the best explanation: What is MQTT Quality of Service (QoS) 0,1, & 2? – MQTT Essentials: Part 6 (hivemq.com).

3

I'm not asking what the QoS values mean, I understand what they mean when publishing messages.

I'm asking what the purpose of setting a QoS is when receiving messages.

4

Oooh, that's the QoS of the subscription, right?

5

Yes, QoS levels of publishers and subscribers can be different, which could have effects on message delivery.

For reference, search for the paragraph "Downgrade of QoS" in the link provided above.

6

The subscription QoS will be ignored if it's higher than one published to the broker according to spec

If it's lower than the published one , then it will be obeyed

e.g pub as 0, sub as 2 - broker will use 0 - pub as 2, sub as 1, broker will use 1

But the mosquito broker can be (mis) configured to not do this :slight_smile:

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