MQTT what is the purpose or usage of Last Will Testament?

Question

I\'m surely missing something about how the whole MQTT protocol works, as I can\'t grasp the usage pattern of Last Will Testament messages: what\'s their purpose?
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Answer 1

A in-depth article about Last-Will-and-Testament messages is available in the MQTT Essentials Blog Post series: http://www.hivemq.com/mqtt-essentials-part-9-last-will-and-testament/.

To summarize the blog post:

The Last Will and Testament feature is used in MQTT to notify other clients about an ungracefully disconnected client.

MQTT is often used in scenarios were unreliable networks are very common. Therefore it is assumed that some clients will disconnect ungracefully from time to time, because they lost the connection, the battery is empty or any other imaginable case. It would be good to know if a connected client has disconnected gracefully (which means with a MQTT DISCONNECT message) or not, in order to take appropriate action.