The messages are lost

[sigmashig]

I'm using AsyncMqttClient on ESP32 + Ardion-ESP. I found, that a lot of messages are lost. I would try to describe a problem:

1. After establishing the connection, I subscribe to 25 topics.
2. MQTT server is configured as 'Publish the last message on subscribe in 1000ms'. And I see in the server log, that message is really sent:

2023-04-30 08:24:18.155 - info: mqtt.0 (6565) Client [Board_7] subscribes on "mqtt.0.HeatPump.Board_7.Config.SSID"
2023-04-30 08:24:18.188 - info: mqtt.0 (6565) Client [Board_7] publishOnSubscribe

On the client, I provide a callback function for onMessage with just one line: put received topic+message to the Serial for testing.
I didn't get a lot of messages, but when I look at server log I see:

2023-04-30 08:24:18.403 - debug: mqtt.0 (6565) Client [Board_7] send to this client "HeatPump/Board_7/Config/SSID": deleted
As a result, I have got messages just for 4-5 topics from 25 subscriptions.
What am I doing wrong?
UPDATE:
It happens when I run subscribe with QoS=2. I received all messages, when I subscribes with QoS=0. Why? I thought, that QoS=2 guarantees just that messages are delivered, but in real life it is not.

[nanobrad]

The QoS value used during subscription is the highest QoS you will accept. Meaning, if you subscribe with QoS of 0, then you will not be sent messages published with a QoS of 1 or 2. This is part of the protocol.

[bertmelis]

Two principles here:

• the publisher defines the QoS of a message (the publisher is not aware of subscribers)
• a subscriber defines the maximum QoS he wants to receive (or able to).

When a message is published, it travels to the broker with a certain QoS. The broker then distributes it to the subscribers and possibly downgrading the QoS to match the clients subscription. QoS is, however, not upgraded.
So a message sent with qos 2 can end at a client with qos 0.
A message with qos 0 will be received and qos 0, also when the subscription is made with qos 2.

[jbaudoux]

I was facing similar issue and end-up on this thread. I'm subscribing to about 40 topics. Some topics where not receiving the messages.
I analyzed the broker log and spotted that not all subscriptions had been registered.
I solved the issue by calling yield() every few subscription. I think you need to have the subscription in loop to have it working. Calling delay(10) was also working.