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AcuLink 810 Data Aquisition Server
9.5.4 Last Will & Testament
The AcuLink supports Last Will and Testament messages via the MQTT protocol. These set-
tings can be configured under the Last Will & Testament tab.
The last will and testament message is used to notify other clients regarding other discon-
nected clients. The message is an MQTT message that contains a topic, a QoS level and a
payload.
Topic: Refers to the path used to access the MQTT message.
QoS: Stands for Quality of Service and refers to the reliability of the message delivery be-
tween the publisher and subscriber. There are three types of quality of service:
1. QoS 0 - is the lowest level, and is defined as "at most once" delivery. This level has the
fastest message delivery but the success rate of delivery is not reliable.
2. QoS 1 - is defined as "at least once" delivery. These types of messages are reliable and are
guaranteed, however the messaged may be sent as duplicates several times.
3. QoS 2 - Is the highest level, and is defined as "exactly once" delivery. These messages
are more reliable and are guaranteed to be sent once without any duplicates. This type
of messaging sends the most reliable message however it has a slower message delivery.
Each client can optionally specify its own LWT message when it connects to a broker. The
broker stores this message so that if the client disconnects ungracefully, the broker will
send the disconnected client's LWT message to all the other clients that are subscribed to
that last will message topic.
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V: 1.08 Revised: May 2020
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