1MQTT(7) Conventions and miscellaneous MQTT(7)
2
6 mqtt - MQ Telemetry Transport
7
9 MQTT
10
12 MQTT is a lightweight publish/subscribe messaging protocol. It is
13 useful for use with low power sensors, but is applicable to many
14 scenarios.
15 This manual describes some of the features of MQTT version 3.1.1/3.1,
17 to assist end users in getting the most out of the protocol. For more
18 complete information on MQTT, see http://mqtt.org/.
19
21 The MQTT protocol is based on the principle of publishing messages and
22 subscribing to topics, or "pub/sub". Multiple clients connect to a
23 broker and subscribe to topics that they are interested in. Clients
24 also connect to the broker and publish messages to topics. Many clients
25 may subscribe to the same topics and do with the information as they
26 please. The broker and MQTT act as a simple, common interface for
27 everything to connect to. This means that you if you have clients that
28 dump subscribed messages to a database, to Twitter, Cosm or even a
29 simple text file, then it becomes very simple to add new sensors or
30 other data input to a database, Twitter or so on.
31
33 Messages in MQTT are published on topics. There is no need to configure
34 a topic, publishing on it is enough. Topics are treated as a hierarchy,
35 using a slash (/) as a separator. This allows sensible arrangement of
36 common themes to be created, much in the same way as a filesystem. For
37 example, multiple computers may all publish their hard drive
38 temperature information on the following topic, with their own computer
39 and hard drive name being replaced as appropriate:
40 · sensors/COMPUTER_NAME/temperature/HARDDRIVE_NAME
42 Clients can receive messages by creating subscriptions. A subscription
44 may be to an explicit topic, in which case only messages to that topic
45 will be received, or it may include wildcards. Two wildcards are
46 available, + or #.
47 + can be used as a wildcard for a single level of hierarchy. It could
49 be used with the topic above to get information on all computers and
50 hard drives as follows:
51 · sensors/+/temperature/+
53 As another example, for a topic of "a/b/c/d", the following example
55 subscriptions will match:
56 · a/b/c/d
58 · +/b/c/d
60 · a/+/c/d
62 · a/+/+/d
64 · +/+/+/+
66 The following subscriptions will not match:
68 · a/b/c
70 · b/+/c/d
72 · +/+/+
74 # can be used as a wildcard for all remaining levels of hierarchy. This
76 means that it must be the final character in a subscription. With a
77 topic of "a/b/c/d", the following example subscriptions will match:
78 · a/b/c/d
80 · #
82 · a/#
84 · a/b/#
86 · a/b/c/#
88 · +/b/c/#
90 Zero length topic levels are valid, which can lead to some slightly
92 non-obvious behaviour. For example, a topic of "a//topic" would
93 correctly match against a subscription of "a/+/topic". Likewise, zero
94 length topic levels can exist at both the beginning and the end of a
95 topic string, so "/a/topic" would match against a subscription of
96 "+/a/topic", "#" or "/#", and a topic "a/topic/" would match against a
97 subscription of "a/topic/+" or "a/topic/#".
98
100 MQTT defines three levels of Quality of Service (QoS). The QoS defines
101 how hard the broker/client will try to ensure that a message is
102 received. Messages may be sent at any QoS level, and clients may
103 attempt to subscribe to topics at any QoS level. This means that the
104 client chooses the maximum QoS it will receive. For example, if a
105 message is published at QoS 2 and a client is subscribed with QoS 0,
106 the message will be delivered to that client with QoS 0. If a second
107 client is also subscribed to the same topic, but with QoS 2, then it
108 will receive the same message but with QoS 2. For a second example, if
109 a client is subscribed with QoS 2 and a message is published on QoS 0,
110 the client will receive it on QoS 0.
111 Higher levels of QoS are more reliable, but involve higher latency and
113 have higher bandwidth requirements.
114 · 0: The broker/client will deliver the message once, with no
116 confirmation.
117 · 1: The broker/client will deliver the message at least once, with
119 confirmation required.
120 · 2: The broker/client will deliver the message exactly once by using
122 a four step handshake.
123
125 All messages may be set to be retained. This means that the broker will
126 keep the message even after sending it to all current subscribers. If a
127 new subscription is made that matches the topic of the retained
128 message, then the message will be sent to the client. This is useful as
129 a "last known good" mechanism. If a topic is only updated infrequently,
130 then without a retained message, a newly subscribed client may have to
131 wait a long time to receive an update. With a retained message, the
132 client will receive an instant update.
133
135 On connection, a client sets the "clean session" flag, which is
136 sometimes also known as the "clean start" flag. If clean session is set
137 to false, then the connection is treated as durable. This means that
138 when the client disconnects, any subscriptions it has will remain and
139 any subsequent QoS 1 or 2 messages will be stored until it connects
140 again in the future. If clean session is true, then all subscriptions
141 will be removed for the client when it disconnects.
142
144 When a client connects to a broker, it may inform the broker that it
145 has a will. This is a message that it wishes the broker to send when
146 the client disconnects unexpectedly. The will message has a topic, QoS
147 and retain status just the same as any other message.
148
150 mosquitto(8), mosquitto_pub(1), mosquitto_sub(1)
151
153 Roger Light <roger@atchoo.org>
154Mosquitto Project 08/19/2020 MQTT(7)