Define what must survive
A live temperature display and a production event ledger have different needs. Decide which messages may be replaced by a newer state and which must be retained as events. Define maximum tolerated age and loss for each class. These requirements belong in the data contract, before choosing client settings.
Test the complete delivery chain
An MQTT acknowledgement does not by itself prove that a business record was stored. Trace one message from publisher to broker, subscriber and final persistence. Give events stable identifiers where deduplication is required. Compare the count at each boundary after reconnecting, and inspect retry behavior at the application layer.
Exercise short and long disconnects
In an approved test environment, interrupt the client connection, restart the broker and exceed the configured session or message expiry. Observe queue limits and storage growth. Verify that replay retains the original event timestamp and does not flood a slow consumer without a bounded recovery policy.
Make stale state obvious
A retained message can be useful initial state, but it is not proof of current device health. Display its source time and quality alongside the value. Define what happens when no update arrives. Acceptance evidence should include a disconnect timeline, missing-event count, duplicate count and the time until normal operation resumes.
Put it into practice
Use the related tool to check your assumptions, then bring the results to your project discussion.