Use QOS1 for rarely sent discovery and availability messages #18756
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Specifically discovery and availability depending on the set up, it's possible for these messages to be silently discarded by the broker. Sending them as QOS1 means the mqtt library used will resend them until it gets an ACK. Ensuring delivery is particularly important for messages that are rarely sent like discovery or availability as they are only sent when there are changes, and nondelivery or dropped messages for them impede proper functioning in home assistant.
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Specifically discovery and availability depending on the set up, it's possible for these messages to be silently discarded by the broker.
Sending them as QOS1 means the underlying MQTT library used will resend them until it gets an ACK.
Ensuring delivery is particularly important for messages that are rarely sent like discovery or availability as they are only sent when there are changes, and if the broker silently drops the message for whatever reason (congestion, or broker clustering issues), this will impede proper functioning of Z2M with home assistant due to missing/stale retained messages.
And at-least-once semantics seem to be an appropriate fit for discovery and availability messages.
Specifically on my setup with a EMQ X broker on kubernetes. I've observed some discovery messsages just outright going missing or getting dropped by broker and sending them at QOS1 seems to resolve it.