Trigger retransmission in case of a disconnect

The TCP load balancer of the Arduino IoT Cloud terminates a TCP connnection after 350 seconds. Unfortunately this termination is only detected after transmitting the next data set to the Arduino IoT Cloud. This has the unfortunate side effect that the data set sent to the cloud is lost. This change buffers every transmission to the server and requests a retransmission in case of loosing the connection. Note: This can only happen if the keep-alive interval is set to a value > 350 seconds

Author: aentinger

src/ArduinoIoTCloudTCP.cpp

@@ -44,7 +44,9 @@ static unsigned long getTime() {
 
 ArduinoIoTCloudTCP::ArduinoIoTCloudTCP():
   _connection(NULL),
-
+  _mqtt_data_buf{0},
+  _mqtt_data_len{0},
+  _mqtt_data_request_retransmit{false},
   _sslClient(NULL),
   #ifdef BOARD_ESP
   _password(""),
@@ -182,7 +184,6 @@ int ArduinoIoTCloudTCP::connect() {
     if (_mqttClient->subscribe(_shadowTopicIn) == 0) {
       return CONNECT_FAILURE_SUBSCRIBE;
     }
-
     _syncStatus = ArduinoIoTSynchronizationStatus::SYNC_STATUS_WAIT_FOR_CLOUD_VALUES;
     _lastSyncRequestTickTime = 0;
   }
@@ -203,6 +204,11 @@ void ArduinoIoTCloudTCP::update() {
 
   if(connectionCheck() != ArduinoIoTConnectionStatus::CONNECTED) return;
 
+  if(_mqtt_data_request_retransmit && (_mqtt_data_len > 0)) {
+    writeProperties(_mqtt_data_buf, _mqtt_data_len);
+    _mqtt_data_request_retransmit = false;
+  }
+
   // MTTQClient connected!, poll() used to retrieve data from MQTT broker
   _mqttClient->poll();
 
@@ -231,8 +237,15 @@ void ArduinoIoTCloudTCP::update() {
 void ArduinoIoTCloudTCP::sendPropertiesToCloud() {
   uint8_t data[MQTT_TRANSMIT_BUFFER_SIZE];
   int const length = Thing.encode(data, sizeof(data));
-  if (length > 0) {
-    writeProperties(data, length);
+  if (length > 0)
+  {
+    /* If properties have been encoded store them in the back-up buffer
+     * in order to allow retransmission in case of failure.
+     */
+    _mqtt_data_len = length;
+    memcpy(_mqtt_data_buf, data, _mqtt_data_len);
+    /* Transmit the properties to the MQTT broker */
+    writeProperties(_mqtt_data_buf, _mqtt_data_len);
   }
 }
 
@@ -362,6 +375,7 @@ ArduinoIoTConnectionStatus ArduinoIoTCloudTCP::connectionCheck() {
     case ArduinoIoTConnectionStatus::CONNECTED: {
         if (!_mqttClient->connected()) {
           _iotStatus = ArduinoIoTConnectionStatus::DISCONNECTED;
+          _mqtt_data_request_retransmit = true;
           printConnectionStatus(_iotStatus);
           execCloudEventCallback(_on_disconnect_event_callback, 0 /* callback_arg - e.g. could be error code casted to void * */);
         }

src/ArduinoIoTCloudTCP.h

@@ -95,6 +95,9 @@ class ArduinoIoTCloudTCP: public ArduinoIoTCloudClass {
     TcpIpConnectionHandler * _connection;
     String _brokerAddress;
     uint16_t _brokerPort;
+    uint8_t _mqtt_data_buf[MQTT_TRANSMIT_BUFFER_SIZE];
+    int _mqtt_data_len;
+    bool _mqtt_data_request_retransmit;
 
     #ifdef BOARD_HAS_ECCX08
     BearSSLClient* _sslClient;