MQTT(7) Conventions and miscellaneous MQTT(7)NAMEmqtt - MQ Telemetry Transport
SYNOPSIS
MQTT
DESCRIPTION
MQTT is a lightweight publish/subscribe messaging protocol. It is
useful for use with low power sensors, but is applicable to many
scenarios.
This manual describes some of the features of MQTT version 3.1, to
assist end users in getting the most out of the protocol. For more
complete information on MQTT, see http://mqtt.org/.
PUBLISH/SUBSCRIBE
The MQTT protocol is based on the principle of publishing messages and
subscribing to topics, or "pub/sub". Multiple clients connect to a
broker and subscribe to topics that they are interested in. Clients
also connect to the broker and publish messages to topics. Many clients
may subscribe to the same topics and do with the information as they
please. The broker and MQTT act as a simple, common interface for
everything to connect to. This means that you if you have clients that
dump subscribed messages to a database, to Twitter, Cosm or even a
simple text file, then it becomes very simple to add new sensors or
other data input to a database, Twitter or so on.
TOPICS/SUBSCRIPTIONS
Messages in MQTT are published on topics. There is no need to configure
a topic, publishing on it is enough. Topics are treated as a hierarchy,
using a slash (/) as a separator. This allows sensible arrangement of
common themes to be created, much in the same way as a filesystem. For
example, multiple computers may all publish their hard drive
temperature information on the following topic, with their own computer
and hard drive name being replaced as appropriate:
· sensors/COMPUTER_NAME/temperature/HARDDRIVE_NAME
Clients can receive messages by creating subscriptions. A subscription
may be to an explicit topic, in which case only messages to that topic
will be received, or it may include wildcards. Two wildcards are
available, + or #.
+ can be used as a wildcard for a single level of hierarchy. It could
be used with the topic above to get information on all computers and
hard drives as follows:
· sensors/+/temperature/+
As another example, for a topic of "a/b/c/d", the following example
subscriptions will match:
· a/b/c/d
· +/b/c/d
· a/+/c/d
· a/+/+/d
· +/+/+/+
The following subscriptions will not match:
· a/b/c
· b/+/c/d
· +/+/+
# can be used as a wildcard for all remaining levels of hierarchy. This
means that it must be the final character in a subscription. With a
topic of "a/b/c/d", the following example subscriptions will match:
· a/b/c/d
· #
· a/#
· a/b/#
· a/b/c/#
· +/b/c/#
Zero length topic levels are valid, which can lead to some slightly
non-obvious behaviour. For example, a topic of "a//topic" would
correctly match against a subscription of "a/+/topic". Likewise, zero
length topic levels can exist at both the beginning and the end of a
topic string, so "/a/topic" would match against a subscription of
"+/a/topic", "#" or "/#", and a topic "a/topic/" would match against a
subscription of "a/topic/+" or "a/topic/#".
QUALITY OF SERVICE
MQTT defines three levels of Quality of Service (QoS). The QoS defines
how hard the broker/client will try to ensure that a message is
received. Messages may be sent at any QoS level, and clients may
attempt to subscribe to topics at any QoS level. This means that the
client chooses the maximum QoS it will receive. For example, if a
message is published at QoS 2 and a client is subscribed with QoS 0,
the message will be delivered to that client with QoS 0. If a second
client is also subscribed to the same topic, but with QoS 2, then it
will receive the same message but with QoS 2. For a second example, if
a client is subscribed with QoS 2 and a message is published on QoS 0,
the client will receive it on QoS 0.
Higher levels of QoS are more reliable, but involve higher latency and
have higher bandwidth requirements.
· 0: The broker/client will deliver the message once, with no
confirmation.
· 1: The broker/client will deliver the message at least once, with
confirmation required.
· 2: The broker/client will deliver the message exactly once by using
a four step handshake.
RETAINED MESSAGES
All messages may be set to be retained. This means that the broker will
keep the message even after sending it to all current subscribers. If a
new subscription is made that matches the topic of the retained
message, then the message will be sent to the client. This is useful as
a "last known good" mechanism. If a topic is only updated infrequently,
then without a retained message, a newly subscribed client may have to
wait a long time to receive an update. With a retained message, the
client will receive an instant update.
CLEAN SESSION / DURABLE CONNECTIONS
On connection, a client sets the "clean session" flag, which is
sometimes also known as the "clean start" flag. If clean session is set
to false, then the connection is treated as durable. This means that
when the client disconnects, any subscriptions it has will remain and
any subsequent QoS 1 or 2 messages will be stored until it connects
again in the future. If clean session is true, then all subscriptions
will be removed for the client when it disconnects.
WILLS
When a client connects to a broker, it may inform the broker that it
has a will. This is a message that it wishes the broker to send when
the client disconnects unexpectedly. The will message has a topic, QoS
and retain status just the same as any other message.
SEE ALSOmosquitto(8), mosquitto_pub(1), mosquitto_sub(1)AUTHOR
Roger Light <roger@atchoo.org>
Mosquitto Project 03/24/2014 MQTT(7)
