1qos(3)                     Library Functions Manual                     qos(3)
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6       qos - Quality of service
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8       The MQTT protocol provides three qualities of service for delivering
9       messages between clients and servers: 'at most once', 'at least once'
10       and 'exactly once'.
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12       Quality of service (QoS) is an attribute of an individual message being
13       published. An application sets the QoS for a specific message by
14       setting the MQTTClient_message.qos field to the required value.
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16       A subscribing client can set the maximum quality of service a server
17       uses to send messages that match the client subscriptions. The
18       MQTTClient_subscribe() and MQTTClient_subscribeMany() functions set
19       this maximum. The QoS of a message forwarded to a subscriber thus might
20       be different to the QoS given to the message by the original publisher.
21       The lower of the two values is used to forward a message.
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23       The three levels are:
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25       QoS0, At most once: The message is delivered at most once, or it may
26       not be delivered at all. Its delivery across the network is not
27       acknowledged. The message is not stored. The message could be lost if
28       the client is disconnected, or if the server fails. QoS0 is the fastest
29       mode of transfer. It is sometimes called 'fire and forget'.
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31       The MQTT protocol does not require servers to forward publications at
32       QoS0 to a client. If the client is disconnected at the time the server
33       receives the publication, the publication might be discarded, depending
34       on the server implementation.
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36       QoS1, At least once: The message is always delivered at least once. It
37       might be delivered multiple times if there is a failure before an
38       acknowledgment is received by the sender. The message must be stored
39       locally at the sender, until the sender receives confirmation that the
40       message has been published by the receiver. The message is stored in
41       case the message must be sent again.
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43       QoS2, Exactly once: The message is always delivered exactly once. The
44       message must be stored locally at the sender, until the sender receives
45       confirmation that the message has been published by the receiver. The
46       message is stored in case the message must be sent again. QoS2 is the
47       safest, but slowest mode of transfer. A more sophisticated handshaking
48       and acknowledgement sequence is used than for QoS1 to ensure no
49       duplication of messages occurs.
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53Paho MQTT C Client Library      Thu Jan 19 2023                         qos(3)
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