waterish_os_rev3_public/libraries/MQTT_Client

MQTT Client

MQTT Client is a library that can publish strings to a topic of a MQTT broker. For using it you have to include the library Cooperative Multitasking too.

Example

The following example publishes the string "Hello" to the topic "amotzek/hello" of the broker broker.hivemq.com repeatedly. The message can be observed in HiveMQs Websockets client.

The example assumes that you connect to the internet via Wifi101, so you need the SSID and the password of your WLAN. You also need an Arduino board that is compatible with Wifi101, e.g. the MKR1000. If your board connects to the internet in a different way, you have to pass a different client (for example an EthernetClient) to the constructor of MQTTClient.

#include <Client.h>
#include <WiFi101.h>
#include <CooperativeMultitasking.h>
#include <MQTTClient.h>

char ssid[] = "...";
char pass[] = "...";
char host[] = "broker.hivemq.com";
char clientid[] = "...";
char username[] = "...";
char password[] = "...";
char topicname[] = "amotzek/hello";

CooperativeMultitasking tasks;
WiFiClient wificlient;
MQTTClient mqttclient(&tasks, &wificlient, host, 1883, clientid, username, password);
MQTTTopic topic(&mqttclient, topicname);

void setup() {
  Serial.begin(9600);
  //
  while (!Serial) {
    delay(1000);
  }
  //
  WiFi.begin(ssid, pass);
  delay(10000); // wait until WiFi connection is established
}

void loop() {
  if (mqttclient.connect()) {
    topic.publish("Hello");
    //
    while (tasks.available()) {
      tasks.run(); // receive connect acknowledgement, send publish, receive publish acknowledgement
    }
    //
    mqttclient.disconnect();
  }
  //
  switch (WiFi.status()) {
    case WL_CONNECT_FAILED:
    case WL_CONNECTION_LOST:
    case WL_DISCONNECTED: WiFi.begin(ssid, pass); // reconnect WiFi if necessary
  }
  //
  delay(30000);
}

Details

To create an instance of class MQTTClient you need a reference to an instance of class CooperativeMultitasking, a reference to an instance of a network connection (e.g. from WiFi101 or Ethernet), the host name of the MQTT broker, its port number (typically 1883) and a client id. If the broker requires that, you also have to pass a user name and a password.

To create an instance of class MQTTTopic you have to pass a reference to an instance of MQTTClient and the topic name.

The method connect() of MQTTClient creates a network connection to the broker.

You can publish a string by calling the method publish("...") of MQTTTopic. The method can be called independently of an established connection to the MQTT broker. It will delay publishing your message until a connection becomes available.

The message is published with QoS 1. That means that MQTTClient waits for a publish acknowledgement from the MQTT broker. If the acknowledgement does not arrive in time, MQTTClient retries publishing the message up to 15 times until it gives up.

connect and publish work asynchronously. They create tasks that are managed by the instance of CooperativeMultitasking. That's why you have to call it's method run() until all tasks are finished.