MQTT Explained with Examples

MQTT (Message Queuing Telemetry Transport) is a lightweight publish/subscribe messaging protocol designed for constrained devices and unreliable networks. Devices publish messages to topics on a broker, and any client subscribed to a matching topic receives them.

The core idea: broker, topics, clients

• Broker: the server that receives all messages and forwards them to subscribers (for example Mosquitto).
• Client: any device or app that connects to the broker, publishing, subscribing or both.
• Topic: a hierarchical string such as farm/field1/soil/moisture that labels a message.

Publishers and subscribers do not know about each other, which makes it easy to add a dashboard or a logger without changing the device.

Example flow

Wildcards: + matches one level (home/+/temp), # matches all remaining levels (home/#).

Quality of Service (QoS)

Retained messages and Last Will

A retained message is stored by the broker and sent to new subscribers immediately, so a dashboard shows the last known value as soon as it connects. A Last Will and Testament is a message the broker publishes if a client disconnects unexpectedly, which is the standard way to detect that a device went offline.

Security basics

• Use authentication and per-client access control lists.
• Use TLS (port 8883 is the registered port for MQTT over TLS; plain MQTT uses 1883).
• Never expose an unauthenticated broker to the internet.

Minimal ESP32 publish example

#include <WiFi.h>
#include <PubSubClient.h>

WiFiClient net;
PubSubClient mqtt(net);

void setup() {
  WiFi.begin("YOUR_WIFI", "YOUR_PASSWORD");
  while (WiFi.status() != WL_CONNECTED) delay(500);
  mqtt.setServer("broker.local", 1883);
}

void loop() {
  if (!mqtt.connected()) mqtt.connect("esp32-node-01");
  mqtt.loop();
  mqtt.publish("home/livingroom/temp", "27.4");
  delay(10000);
}

In real projects add reconnection back-off, a Last Will and credentials. The MQTT and Node-RED course and the ESP32 course cover these patterns. Compare protocols in MQTT vs HTTP for IoT.

References

• OASIS, MQTT Version 5.0: docs.oasis-open.org
• MQTT.org: mqtt.org
• Eclipse Mosquitto broker: mosquitto.org