feat: Add MQTT message retention and client status topic (#22)

Author: fligneul

README.md

@@ -108,6 +108,7 @@ APS2MQTT configuration can be provided by a yaml config file or by environment v
 | MQTT_BROKER_PASSWD | User password of the MQTT broker | "itsasecret" | "" |
 | MQTT_CLIENT_ID | Client ID if the MQTT client | "MyAwesomeClient" | "APS2MQTT" |
 | MQTT_TOPIC_PREFIX | Topic prefix for publishing | "my-personal-topic" | "" |
+| MQTT_RETAIN | Retain MQTT messages | True | False |
 | MQTT_BROKER_SECURED_CONNECTION | Use secure connection to MQTT broker | True | False |
 | MQTT_BROKER_CACERTS_PATH | Path to the cacerts file | "/User/johndoe/.ssl/cacerts" | None |
 
@@ -150,6 +151,7 @@ mqtt:
   MQTT_BROKER_PORT: 1883
   MQTT_BROKER_USER: 'johndoe'
   MQTT_BROKER_PASSWD: 'itsasecret'
+  MQTT_RETAIN: True
 ```
 
 #### Secured connection
@@ -194,6 +196,8 @@ services:
 
 The aps2mqtt retrieve from the whole PV array as a whole as well as each individual inverter in detail.
 
+* aps/status - current status of the service, `online` or `offline`. The `offline` message is sent using LWT
+
 ### ECU data
 
 * aps/[ECU_ID]/power - total amount of power (in W) being generated right now

aps2mqtt/config.py

@@ -15,6 +15,7 @@ def __init__(self, cfg):
         self.broker_passwd = cfg.get("MQTT_BROKER_PASSWD", "")
         self.client_id = cfg.get("MQTT_CLIENT_ID", "APS2MQTT")
         self.topic_prefix = cfg.get("MQTT_TOPIC_PREFIX", "")
+        self.retain = str2bool_exc(str(cfg.get("MQTT_RETAIN", False)))
         self.secured_connection = str2bool_exc(
             str(cfg.get("MQTT_BROKER_SECURED_CONNECTION", False))
         )

aps2mqtt/mqtthandler.py

@@ -21,12 +21,14 @@ def __init__(self, mqtt_config):
             mqtt_config.topic_prefix + "/" if len(mqtt_config.topic_prefix.strip()) > 0 else ""
         )
         self.client = None
+        self.status_topic = self.topic_prefix + "aps/status"
 
     def on_connect(self, client, userdata, flags, reason_code, properties):
         """Callback function on broker connection"""
-        del client, userdata, flags, properties
+        del userdata, flags, properties
         if reason_code == 0:
             _LOGGER.info("Connected to MQTT Broker!")
+            self._publish(client, self.status_topic, "online", retain=True)
         else:
             _LOGGER.error("Failed to connect: %s", reason_code)
 
@@ -35,10 +37,13 @@ def on_disconnect(self, client, userdata, flags, reason_code, properties):
         del client, userdata, flags, properties
         _LOGGER.info("Disconnected from MQTT Broker: %s", reason_code)
 
-    def _publish(self, client, topic, msg):
-        result = client.publish(topic, msg)
+    def _publish(self, client, topic, msg, retain=False):
+        # If mqtt_retain is True in config, all messages are retained.
+        # Otherwise, only LWT uses retain.
+        actual_retain = retain or self.mqtt_config.retain
+        result = client.publish(topic, msg, retain=actual_retain)
         if result.rc == 0:
-            _LOGGER.debug("Send `%s` to topic `%s`", msg, topic)
+            _LOGGER.debug("Send `%s` to topic `%s` (retain=%s)", msg, topic, actual_retain)
         else:
             _LOGGER.error("Failed to send message to topic %s: %s", topic, result.rc)
 
@@ -72,6 +77,9 @@ def connect_mqtt(self):
         else:
             _LOGGER.debug("Use unsecured connection")
 
+        _LOGGER.debug("Set LWT on topic '%s'", self.status_topic)
+        self.client.will_set(self.status_topic, "offline", qos=1, retain=True)
+
         self.client.on_connect = self.on_connect
         self.client.on_disconnect = self.on_disconnect
 
@@ -82,7 +90,13 @@ def connect_mqtt(self):
         )
         self.client.connect_async(self.mqtt_config.broker_addr, self.mqtt_config.broker_port)
         self.client.loop_start()
-        atexit.register(self.client.loop_stop)
+        atexit.register(self.disconnect)
+
+    def disconnect(self):
+        if self.client.is_connected():
+            _LOGGER.info("Publishing 'offline' status on graceful exit.")
+            self._publish(self.client, self.status_topic, "offline", retain=True)
+        self.client.loop_stop()
 
     def publish_values(self, data):
         """Publish ECU data to MQTT"""