October 2019
Intermediate to advanced
350 pages
9h 5m
English
Content preview from Hands-On Internet of Things with MQTT
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Quality of Service
MQTT supports a feature called QoS, which makes it possible for clients to receive messages that were sent while the device was offline. You can see this feature being used in messenger apps (for example, Facebook Messenger, which also uses MQTT under the hood). When somebody sends you a message while you are offline, you still receive the message later on, when you come back online; it does not get lost.
There are three different settings for QoS:
- QoS 0: Fire and forget
- QoS 1: Deliver at least once
- QoS 2: Deliver exactly once
When publishing messages, as well as when subscribing to messages, each client can independently decide which QoS to use. So, simply publishing with QoS 1 or QoS 2 does not make sure each subscriber ...
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