1qos(3)                     Library Functions Manual                     qos(3)
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6       qos - Quality of service The MQTT protocol provides three qualities of
7       service for delivering messages between clients and servers: 'at most
8       once', 'at least once' and 'exactly once'.
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10       Quality of service (QoS) is an attribute of an individual message being
11       published. An application sets the QoS for a specific message by
12       setting the MQTTClient_message.qos field to the required value.
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14       A subscribing client can set the maximum quality of service a server
15       uses to send messages that match the client subscriptions. The
16       MQTTClient_subscribe() and MQTTClient_subscribeMany() functions set
17       this maximum. The QoS of a message forwarded to a subscriber thus might
18       be different to the QoS given to the message by the original publisher.
19       The lower of the two values is used to forward a message.
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21       The three levels are:
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23       QoS0, At most once: The message is delivered at most once, or it may
24       not be delivered at all. Its delivery across the network is not
25       acknowledged. The message is not stored. The message could be lost if
26       the client is disconnected, or if the server fails. QoS0 is the fastest
27       mode of transfer. It is sometimes called 'fire and forget'.
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29       The MQTT protocol does not require servers to forward publications at
30       QoS0 to a client. If the client is disconnected at the time the server
31       receives the publication, the publication might be discarded, depending
32       on the server implementation.
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34       QoS1, At least once: The message is always delivered at least once. It
35       might be delivered multiple times if there is a failure before an
36       acknowledgment is received by the sender. The message must be stored
37       locally at the sender, until the sender receives confirmation that the
38       message has been published by the receiver. The message is stored in
39       case the message must be sent again.
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41       QoS2, Exactly once: The message is always delivered exactly once. The
42       message must be stored locally at the sender, until the sender receives
43       confirmation that the message has been published by the receiver. The
44       message is stored in case the message must be sent again. QoS2 is the
45       safest, but slowest mode of transfer. A more sophisticated handshaking
46       and acknowledgement sequence is used than for QoS1 to ensure no
47       duplication of messages occurs.
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51Paho MQTT C Client Library      Thu Jul 22 2021                         qos(3)
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