If you have several participants who are interested in each other’s measurements or events, you can use the MQTT protocol for this. In the following, I will present the basics.
The Mqtt protocol is based on publish and subscribe with asynchronous communication. Therefore it can also be used in networks with high latency. It can also be operated with low bandwidth.
At the center is an MQTT broker. It receives published messages and forwards them to the subscribing clients. The MQTT topics are used for this purpose. Each message is published to a topic. The topics look like a file path and can be chosen almost freely. The only exception are names beginning with $, because these are used for MQTT-own telemetry data. An example for such a topic would be “My/Test/Topic”. Attention, the topic is case sensitive. Every level of the topic can be subscribed to. For example “My/Test/Topic/#”, “My/Test/#” or “My/#”. In the latter case, a message published to “My/Productive/Things” would also be received by the subscriber. This way you can build your own message hierarchy using the Topics.
In the picture a rough structure of the MQTT infrastructure is shown. Two clients have subscribed to a topic. If the sensor sends data to the topic, the broker forwards it to the clients. One of the clients writes the data into a database, for example, and then processes it graphically with a tool such as Grafana.
How to send messages
For the code examples I used Python with the package paho-mqtt. First, an MQTT client must be created and connected.
self.client = mqtt.Client()
self.client.connect("hostname-broker.de", 1883)
self.client.loop_start()Afterwards, the client can send messages to the MQTT broker at any time using the publish command. A topic and the actual message are sent as payload. The payload can have any structure. For example Json format or xml. In the code example json is used
self.client.publish(topic="own/test/topic", payload=json.dumps(payload))How to subscribe topics
Even when subscribing, an MQTT client must first be created and a connection established. However, the on_connect and on_message functions are also used here. These are always called when the client establishes a connection or a new message arrives. It makes sense to make the subscriptions in the on_connect method, since they are created so with a new connection also always new and are not lost.
self.client = mqtt.Client()
self.client.on_connect = on_connect
self.client.on_message = on_message
self.client.connect("hostname-broker.de", 1883)
self.client.loop_start()Here you can see an example on_connect method that outputs the result code of the connection setup and subscribes to a topic. For this, only the respective topic must be specified.
def on_connect(client, userdata, flags, rc):
print(Connected with result code " + str(rc))
self.client.subscribe("own/test/topic/#")In the on_message method you can specify what should happen to an incoming message.
Conclusion
MQTT is a simple way to exchange data between a variety of devices. You can customize it very much and have a lot of freedom. All messages are TSL encrypted and you can set up client authentication in the broker, which is why it is also considered secure. For asynchronous communication, this is definitely a technology to consider.
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