October 2019
Intermediate to advanced
350 pages
9h 5m
English
Content preview from Hands-On Internet of Things with MQTT
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Persistent sessions/clean sessions
We have already discussed QoS, which makes it possible to receive messages, even when the recipient is offline. When a device is unavailable, the MQTT server will keep track of it, and re-send the message once the device goes back online again.
To make this possible, the server needs to do some bookkeeping. For every message that was sent using this feature, the server needs to keep track of whether the message has already been delivered to the recipient or not. Once it has been delivered, the server will forget about it. Imagine a mail. If you are not at home when they try to deliver a package to you, they will try again another day. Once it has been delivered successfully, they will forget about it.
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