1wildcard(3)                Library Functions Manual                wildcard(3)
2
3
4

NAME

6       wildcard - Subscription wildcards Every MQTT message includes a topic
7       that classifies it. MQTT servers use topics to determine which
8       subscribers should receive messages published to the server.
9
10       Consider the server receiving messages from several environmental
11       sensors. Each sensor publishes its measurement data as a message with
12       an associated topic. Subscribing applications need to know which sensor
13       originally published each received message. A unique topic is thus used
14       to identify each sensor and measurement type. Topics such as
15       SENSOR1TEMP, SENSOR1HUMIDITY, SENSOR2TEMP and so on achieve this but
16       are not very flexible. If additional sensors are added to the system at
17       a later date, subscribing applications must be modified to receive
18       them.
19
20       To provide more flexibility, MQTT supports a hierarchical topic
21       namespace. This allows application designers to organize topics to
22       simplify their management. Levels in the hierarchy are delimited by the
23       '/' character, such as SENSOR/1/HUMIDITY. Publishers and subscribers
24       use these hierarchical topics as already described.
25
26       For subscriptions, two wildcard characters are supported:
27
28       • A '#' character represents a complete sub-tree of the hierarchy and
29         thus must be the last character in a subscription topic string, such
30         as SENSOR/#. This will match any topic starting with SENSOR/, such as
31         SENSOR/1/TEMP and SENSOR/2/HUMIDITY.
32       • A '+' character represents a single level of the hierarchy and is
33         used between delimiters. For example, SENSOR/+/TEMP will match
34         SENSOR/1/TEMP and SENSOR/2/TEMP.
35       Publishers are not allowed to use the wildcard characters in their
36       topic names.
37       Deciding on your topic hierarchy is an important step in your system
38       design.
39
40
41
42Paho MQTT C Client Library      Fri Jul 22 2022                    wildcard(3)
Impressum