11_MQTT_Exercise.md

11. Exercise for (10_MQTT)

We have modified the Transport/MqttMessageBus.cpp in two places:

#define OFFLINE_MESSAGE "{ \"online\": false }"
#define ONLINE_MESSAGE "{ \"online\": true }"

void MqttMessageBus::ReconnectMqtt()
{
  // Loop until we're reconnected
   while (!_mqttClient.connected())
   {
     // Create a random client ID
     String clientId = String(_deviceId) + "-" + String(random(0xffff), HEX);
     Serial.printf("[MQTT] Attempting MQTT connection (ClientId: %s)...\n", clientId.c_str());

     // Attempt to connect
     String statusTopic = String(_deviceId) + "/status";
     // boolean connect(const char* id, const char* willTopic, uint8_t willQos, boolean willRetain, const char* willMessage);
     if (_mqttClient.connect(clientId.c_str(), statusTopic.c_str(), MQTTQOS0, true, OFFLINE_MESSAGE))
     {
       // ...

       // boolean publish(const char* topic, const char* payload, boolean retained);
       PublishInternal(statusTopic.c_str(), ONLINE_MESSAGE, true);
     }
     else
     {
       // ...
     }
   }
}

In short we pass the last will message during connect() and specify the retain flag. Also soon after device connect we also send an online message with retain flag to that same topic.

Notice that we extracted PublishInternal method from public Publish:

bool MqttMessageBus::Publish(const char* topic, JsonObject& message)
{
  String payload = _serializationProvider->Serialize(message);
  return PublishInternal(topic, payload.c_str(), false);
}

bool MqttMessageBus::PublishInternal(const char* topic, const char* payload, bool retain)
{
  Serial.printf("[MQTT] Publish to topic: '%s', message: '%s'\n", topic, payload);
  return _mqttClient.publish(topic, payload, retain);
}

Lets test this...

After the device start, we subscribe to the my_device_id/status topic:

$ mosquitto_sub -t my_device_id/status
{ "online": true }

This is expected as the message was retained, which means any new subscriber will receive the last good known value.

When we now unplug the device, after few seconds Mosquitto detects, the client is offline and sends the last will message (LWT)

{ "online": false }