mqtt-client🔗 i

1Example🔗 i

Below, we list a simple example for the way, this module might be used. To send and receive messages we set up different contexts, two of which are mandatory: a client context and, nested in it, a connection context. Optionally, we can change some parameters using contexts. Here, for example, we change the quality of service (QOS) when publishing to qos-1.

First, we set up a client using mqtt/with-client . Herein, we provide the message broker’s URL, "localhost", and a client id, "client1", for our application.

Next, we set up a connection to an MQTT message broker using . All connection parameters have reasonable default values but, here, we set the keep-alive interval to 20 seconds and request a clean session.

Lastly, we set the QOS for publishing and subscribing to qos-1 using mqtt/with-publish-qos.

Within these three nested contexts, setting up a client, a connection, and QOS, we can subscribe to topics, send messages, and wait for messages to be received. Here, we use mqtt/subscribe to subscribe to the topic "some-topic". We use mqtt/publish to publish the message "Hello World". Finally, we receive a message using mqtt/with-message-recv .

The form mqtt/with-message-recv creates its own context, in which two named variables are defined designating the topic name on which the message was received and the message payload. Here, we call these variables topic and payload and display them.

#lang racket/base

 

(require mqtt-client)

 

(mqtt/with-client ("localhost""client1")

 

(mqtt/with-connection (#:keep-alive-interval20

#:clean-session#t)

 

(mqtt/with-qos (' qos-1)

 

(mqtt/subscribe "some-topic")

 

(mqtt/publish "some-topic""Helloworld")

 

(mqtt/with-message-recv (topicpayload)

(displayln topic)

(displayln payload)))))

2MQTT API🔗 i

The MQTT API can be used to exchange messages via MQTT in a functional style where the state of the application is managed in the form of scopes and construction and destruction of stateful objects is implicit.