1wildcard(3)                Library Functions Manual                wildcard(3)
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6       wildcard - Subscription wildcards
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8       Every MQTT message includes a topic that classifies it. MQTT servers
9       use topics to determine which subscribers should receive messages
10       published to the server.
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12       Consider the server receiving messages from several environmental
13       sensors. Each sensor publishes its measurement data as a message with
14       an associated topic. Subscribing applications need to know which sensor
15       originally published each received message. A unique topic is thus used
16       to identify each sensor and measurement type. Topics such as
17       SENSOR1TEMP, SENSOR1HUMIDITY, SENSOR2TEMP and so on achieve this but
18       are not very flexible. If additional sensors are added to the system at
19       a later date, subscribing applications must be modified to receive
20       them.
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22       To provide more flexibility, MQTT supports a hierarchical topic
23       namespace. This allows application designers to organize topics to
24       simplify their management. Levels in the hierarchy are delimited by the
25       '/' character, such as SENSOR/1/HUMIDITY. Publishers and subscribers
26       use these hierarchical topics as already described.
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28       For subscriptions, two wildcard characters are supported:
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30       • A '#' character represents a complete sub-tree of the hierarchy and
31         thus must be the last character in a subscription topic string, such
32         as SENSOR/#. This will match any topic starting with SENSOR/, such as
33         SENSOR/1/TEMP and SENSOR/2/HUMIDITY.
34       • A '+' character represents a single level of the hierarchy and is
35         used between delimiters. For example, SENSOR/+/TEMP will match
36         SENSOR/1/TEMP and SENSOR/2/TEMP.
37       Publishers are not allowed to use the wildcard characters in their
38       topic names.
39       Deciding on your topic hierarchy is an important step in your system
40       design.
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44Paho MQTT C Client Library      Thu Jan 19 2023                    wildcard(3)
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