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How to publish and subscribe to a topic in mosquitto

In recent post, we learned how to run mosquitto as a docker container. Now, we will learn about how to carry out messages using its publish/subscribe model.

In order to publish/subscribe to a topic in mosquitto, we need mosquitto-clients to be installed in our machine. We can do it by the following command:

shell
$ sudo apt-get install mosquitto-clients

This will make both mosquitto_pub and mosquitto_sub available to use.

Publish message under a topic

In MQTT protocol, messages should be published and subscribed under respective topic names. For instance, if we are going to publish a message related to weather, it should be published like the following: /topic/weather/nl/utrecht.

Lets do the same, using mosquitto_pub:

shell
$ mosquitto_pub -t '/topic/weather/nl/utrecht' -m '{ "city":"Utrecht", "temp": "20" }'
$ mosquitto_pub -t '/topic/weather/nl/rotterdam' -m '{ "city":"Rotterdam", "temp": "30" }'
  • -t -- The topic under which we publish a message.
  • -m -- The message we want to publish.

mosquitto_pub publishes the messages under the specified topic via default port 1883 of localhost.

We can publish anything as long as we keep it as a string. The above is just a string representation of a Weather JSON object. People subscribe to it, can parse it using JSON.parse() to use as a JSON.

Subscribe to a topic

Since people publishing already, lets subscribe to a topic and hear the gossip O_o

shell
$ mosquitto_sub -t '/topic/weather/nl/utrecht'
{ "city":"Utrecht", "temp": "20" }

If there are messages being published, you can see it being logged in your terminal.

We can subscribe to all the topics by specifying the -t option as the following: mosquitto_sub -t '#'. However, it is advised to subscribe to as specifici topic as possible like mosquitto_sub -t '/topic/weather/nl/#' to subscribe to the entire country's weather.

shell
$ mosquitto_sub -t '/topic/weather/nl/#'
{ "city":"Utrecht", "temp": "20" }
{ "city":"Rotterdam", "temp": "30" }