What happens if there is an (for example) blackout and the broker is down?
What information does the broker persist and how?
Does the broker persist Last Will, Retained Messages, …)?
Does “Persistent Session” also refer to the failure of the brokers? (Or only to the connection failures of MQTT clients?)
Hey tlichten,
I’m happy to see you join our community. Welcome!
To answer your question:
HiveMQ persists all session relevant data. This includes subscriptions, last will and testament not yet completed message flows for in transit messages of QoS 1 or 2. Retained messages are also written to disk.
This means that even the event of an unexpected shutdown or network connection loss, clients with persistent sessions can resume their sessions without data loss.
I hope helps you with the implementation of any MQTT related plans you might have.
Feel free to ask if anything else seems unclear 
Regards,
Finn of the HiveMQ team
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