node-red/packages/node_modules/@node-red/nodes/locales/en-US/io/10-mqtt.html

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<script type="text/x-red" data-help-name="mqtt in">
<p>Connects to a MQTT broker and subscribes to messages from the specified topic.</p>
    <h3>Outputs</h3>
    <dl class="message-properties">
       <dt>payload <span class="property-type">string | buffer</span></dt>
       <dd>a string unless detected as a binary buffer.</dd>
       <dt>topic <span class="property-type">string</span></dt>
       <dd>the MQTT topic, uses / as a hierarchy separator.</dd>
       <dt>qos <span class="property-type">number</span> </dt>
       <dd>0, fire and forget - 1, at least once - 2, once and once only.</dd>
       <dt>retain <span class="property-type">boolean</span></dt>
       <dd>true indicates the message was retained and may be old.</dd>
    </dl>
    <h3>Details</h3>
    The subscription topic can include MQTT wildcards, + for one level, # for multiple levels.</p>
    <p>This node requires a connection to a MQTT broker to be configured. This is configured by clicking
    the pencil icon.</p>
    <p>Several MQTT nodes (in or out) can share the same broker connection if required.</p>
</script>

<script type="text/x-red" data-help-name="mqtt out">
    <p>Connects to a MQTT broker and publishes messages.</p>
    <h3>Inputs</h3>
    <dl class="message-properties">
       <dt>payload <span class="property-type">string | buffer</span></dt>
       <dd> most users prefer simple text payloads, but binary buffers can also be published.</dd>

       <dt class="optional">topic <span class="property-type">string</span></dt>
       <dd> the MQTT topic to publish to.</dd>

       <dt class="optional">qos <span class="property-type">number</span></dt>
       <dd>0, fire and forget - 1, at least once - 2, once and once only. Default 0.</dd>

       <dt class="optional">retain <span class="property-type">boolean</span></dt>
       <dd>set to true to retain the message on the broker. Default false.</dd>
    </dl>
    <h3>Details</h3>
    <code>msg.payload</code> is used as the payload of the published message.
    If it contains an Object it will be converted to a JSON string before being sent.
    If it contains a binary Buffer the message will be published as-is.</p>
    <p>The topic used can be configured in the node or, if left blank, can be set by <code>msg.topic</code>.</p>
    <p>Likewise the QoS and retain values can be configured in the node or, if left
    blank, set by <code>msg.qos</code> and <code>msg.retain</code> respectively. To clear a previously
    retained topic from the broker, send a blank message to that topic with the retain flag set.</p>
    <p>This node requires a connection to a MQTT broker to be configured. This is configured by clicking
    the pencil icon.</p>
    <p>Several MQTT nodes (in or out) can share the same broker connection if required.</p>
</script>

<script type="text/x-red" data-help-name="mqtt-broker">
    <p>Configuration for a connection to an MQTT broker.</p>
    <p>This configuration will create a single connection to the broker which can
       then be reused by <code>MQTT In</code> and <code>MQTT Out</code> nodes.</p>
    <p>The node will generate a random Client ID if one is not set and the
       node is configured to use a Clean Session connection. If a Client ID is set,
       it must be unique to the broker you are connecting to.</p>
    <h4>Birth Message</h4>
    <p>This is a message that will be published on the configured topic whenever the
       connection is established.</p>
    <h4>Close Message</h4>
    <p>This is a message that will be published on the configured topic before the
       connection is closed normally, either by re-deploying the node, or by shutting down.</p>
    <h4>Will Message</h4>
    <p>This is a message that will be published by the broker in the event the node
       unexpectedly loses its connection.</p>
    <h4>WebSockets</h4>
    <p>The node can be configured to use a WebSocket connection. To do so, the Server
       field should be configured with a full URI for the connection. For example:</p>
    <pre>ws://example.com:4000/mqtt</pre>

</script>