Question
“Dropped message: QoS 0 channel not writable” this error message is observed in the HiveMQ control center.
Answer
This is not a broker error, as QoS 0 messages, according to the MQTT specification, have no delivery guarantee and can be dropped. The notification is displayed in the Control Center in order to notify about a possible issue with slow-consuming clients.
When your clients are consuming slowly or cannot handle the load on subscribed topics individually then a possible solution is to use shared subscriptions.