CHAPTER 3 ■ COMMUNICATION PROTOCOLS
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installed on your machine, you can use the openly available MQTT broker from Eclipse
Foundation ( iot.eclipse.org ) or Mosquitto ( test.mosquitto.org ).
Listing 3-7. MQTT Setup
// IP address of the MQTT broker
char server[] = { "iot.eclipse.org" };
int port = 1883
char topic[] = { "codifythings/testMessage" };
As shown in Listing 3-8 , initialize the MQTT client. The callback() function
encapsulates all the details of receiving payload from broker.
Listing 3-8. MQTT Initialization and Callback Function
PubSubClient pubSubClient(server, 1883, callback, client);
void callback(char* topic, byte* payload, unsigned int length)
{
// Print payload
String payloadContent = String((char *)payload);
Serial.println("[INFO] Payload: " + payloadContent);
}
Standard Functions
Finally, the code in this last section is provided in Listing 3-9 . It implements Arduino’s
standard setup() and loop() functions .
In the setup() function, the code initializes the serial port and connects to the
Internet. If the MQTT broker is connected, it will subscribe to the codifythings/
testMessage topic. Once successfully subscribed, the code publishes a new message to
the codifythings/testMessage topic. The code subscribes to same topic to which it is
publishing. Therefore, as soon as a message is published, the callback() function will be
called. The loop() function simply waits for new messages from the MQTT broker.
Listing 3-9. Code for Standard Arduino Functions
void setup()
{
// Initialize serial port
Serial.begin(9600);
// Connect Arduino to internet
connectToInternet();