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<refentry xml:id="mqtt" xmlns:xlink="http://www.w3.org/1999/xlink">
<refmeta>
<refentrytitle>mqtt</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo class="source">Mosquitto Project</refmiscinfo>
<refmiscinfo class="manual">Conventions and miscellaneous</refmiscinfo>
</refmeta>
<refnamediv>
<refname>mqtt</refname>
<refpurpose>MQ Telemetry Transport</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>MQTT</command>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><command>MQTT</command> is a lightweight publish/subscribe
messaging protocol. It is useful for use with low power sensors, but
is applicable to many scenarios.</para> <para>This manual describes
some of the features of MQTT version 3.1.1/3.1, to assist end users in
getting the most out of the protocol. For more complete information on
MQTT, see <uri type="webpage">http://mqtt.org/</uri>.</para>
</refsect1>
<refsect1>
<title>Publish/Subscribe</title>
<para>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.</para>
</refsect1>
<refsect1>
<title>Topics/Subscriptions</title>
<para>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:</para>
<itemizedlist>
<listitem><para>sensors/COMPUTER_NAME/temperature/HARDDRIVE_NAME</para></listitem>
</itemizedlist>
<para>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, <option>+</option> or <option>#</option>.</para>
<para><option>+</option> 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:</para>
<itemizedlist>
<listitem><para>sensors/+/temperature/+</para></listitem>
</itemizedlist>
<para>As another example, for a topic of "a/b/c/d", the following
example subscriptions will match:</para>
<itemizedlist mark="circle">
<listitem><para>a/b/c/d</para></listitem>
<listitem><para>+/b/c/d</para></listitem>
<listitem><para>a/+/c/d</para></listitem>
<listitem><para>a/+/+/d</para></listitem>
<listitem><para>+/+/+/+</para></listitem>
</itemizedlist>
<para>The following subscriptions will not match:</para>
<itemizedlist mark="circle">
<listitem><para>a/b/c</para></listitem>
<listitem><para>b/+/c/d</para></listitem>
<listitem><para>+/+/+</para></listitem>
</itemizedlist>
<para><option>#</option> 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:</para>
<itemizedlist mark="circle">
<listitem><para>a/b/c/d</para></listitem>
<listitem><para>#</para></listitem>
<listitem><para>a/#</para></listitem>
<listitem><para>a/b/#</para></listitem>
<listitem><para>a/b/c/#</para></listitem>
<listitem><para>+/b/c/#</para></listitem>
</itemizedlist>
<para>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/#".</para>
</refsect1>
<refsect1>
<title>Quality of Service</title>
<para>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.</para>
<para>Higher levels of QoS are more reliable, but involve higher
latency and have higher bandwidth requirements.</para>
<itemizedlist>
<listitem><para>0: The broker/client will deliver the message once, with no confirmation.</para></listitem>
<listitem><para>1: The broker/client will deliver the message at least once, with confirmation required.</para></listitem>
<listitem><para>2: The broker/client will deliver the message exactly once by using a four step handshake.</para></listitem>
</itemizedlist>
</refsect1>
<refsect1>
<title>Retained Messages</title>
<para>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.</para>
</refsect1>
<refsect1>
<title>Clean session / Durable connections</title>
<para>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.</para>
</refsect1>
<refsect1>
<title>Wills</title>
<para>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.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member>
<citerefentry>
<refentrytitle><link xlink:href="mosquitto-8.html">mosquitto</link></refentrytitle>
<manvolnum>8</manvolnum>
</citerefentry>
</member>
<member>
<citerefentry>
<refentrytitle><link xlink:href="mosquitto_pub-1.html">mosquitto_pub</link></refentrytitle>
<manvolnum>1</manvolnum>
</citerefentry>
</member>
<member>
<citerefentry>
<refentrytitle><link xlink:href="mosquitto_sub-1.html">mosquitto_sub</link></refentrytitle>
<manvolnum>1</manvolnum>
</citerefentry>
</member>
</simplelist>
</refsect1>
<refsect1>
<title>Author</title>
<para>Roger Light <email>roger@atchoo.org</email></para>
</refsect1>
</refentry>