Subscribe to a topic

Problem

You want to subscribe to messages on an MQTT topic.

Solution

Use the MQTT Input node to subscribe to the broker and receive messages published to matching topics.

Example

[{"id":"8024cb4.98c5238","type":"mqtt in","z":"eda2a949.74ea98","name":"","topic":"sensors/#","qos":"2","broker":"61de5090.0f5d9","x":240,"y":180,"wires":[["15d727dd.33e808"]]},{"id":"15d727dd.33e808","type":"debug","z":"eda2a949.74ea98","name":"","active":true,"console":"false","complete":"false","x":390,"y":180,"wires":[]},{"id":"61de5090.0f5d9","type":"mqtt-broker","z":"","broker":"localhost","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"willTopic":"","willQos":"0","willPayload":"","birthTopic":"","birthQos":"0","birthPayload":""}]

Discussion

The MQTT Input node must be hardcoded with the topic filter to use - it cannot be changed dynamically.

One possible workaround is to set the topic to an environment variable such as $(MY_TOPIC). When the Node-RED runtime starts it will substitute the environment variable value into that property of the node. This does allow the topic to be changed, although doing so does require a restart of Node-RED to pickup changes to the environment variable.

You can also use MQTT wildcards, + for a single topic level or # for multiple. This allows you to receive multiple topics with a single node. The messages will be sent from the node with msg.topic set to the actual topic received.