MQTT and IoT Platforms

Why MQTT

HTTP works for occasional uploads. For many devices sending frequent small messages, and for controlling devices remotely, MQTT is lighter: devices publish and subscribe to topics through a broker.

greenhouse/1/temperature      ← device publishes 22.4
greenhouse/1/pump/set         → device subscribes; dashboard publishes "on"

Brokers

• Mosquitto (self-hosted, lightweight, runs on a Raspberry Pi).
• Hosted brokers from cloud IoT services.

Secure them: username/password or certificates per device, TLS encryption (port 8883), and access-control lists so each device can only use its own topics.

ESP32 with an MQTT library

#include <WiFiClientSecure.h>
#include <PubSubClient.h>

WiFiClientSecure net;
PubSubClient mqtt(net);

void onMessage(char* topic, byte* payload, unsigned int len) {
  String msg; for (unsigned i = 0; i < len; i++) msg += (char)payload[i];
  if (String(topic) == "greenhouse/1/pump/set") setPump(msg == "on");
}

void setup() {
  // connect Wi-Fi first...
  net.setCACert(ROOT_CA_PEM);
  mqtt.setServer(MQTT_HOST, 8883);
  mqtt.setCallback(onMessage);
}

void loop() {
  if (!mqtt.connected()) {
    mqtt.connect("greenhouse-1", MQTT_USER, MQTT_PASS);
    mqtt.subscribe("greenhouse/1/pump/set");
  }
  mqtt.loop();
  // publish readings on a millis() timer:
  // mqtt.publish("greenhouse/1/temperature", String(t, 1).c_str());
}

Dashboards and automation

• Home Assistant integrates MQTT devices into dashboards and automations ("if soil is dry for 10 minutes, run the pump for 30 seconds").
• ESPHome lets you define ESP32 devices in YAML and integrates them with Home Assistant without writing C++.
• Node-RED wires flows visually.
• Time-series storage and charts: InfluxDB or PostgreSQL with Grafana.

Exercise

Run Mosquitto with authentication and TLS, connect your ESP32 to publish readings and accept pump commands, and build a Home Assistant (or Node-RED) dashboard with an automation.

Check yourself

• What are topics, publishers and subscribers?
• How should an MQTT broker be secured?
• What does ESPHome simplify?