MQTT MESSAGING

OVERVIEW

MQTT (Message Queuing Telemetry Transport) is a lightweight publish/subscribe messaging protocol designed for constrained devices on unreliable networks. It runs over TCP, uses a central broker, and provides three QoS levels from fire-and-forget to exactly-once delivery.

QOS LEVELS

TOPIC ARCHITECTURE

Naming Convention

{org}/{site}/{zone}/{device_type}/{device_id}/{data_type}

2nth/hillcrest-dc/zone-a/temp-sensor/ts-001/telemetry
2nth/hillcrest-dc/zone-a/temp-sensor/ts-001/status
2nth/hillcrest-dc/zone-a/temp-sensor/ts-001/cmd

Wildcards

• + single level: 2nth/+/zone-a/+/+/telemetry
• # multi-level: 2nth/hillcrest-dc/#
• Subscribe-only — cannot publish to wildcard topics

Retained Messages & Last Will

Retained: Broker stores last message per topic. New subscribers get current state immediately. Essential for device status and config.

LWT (Last Will and Testament): Set at CONNECT time. Broker auto-publishes if client disconnects ungracefully — instant offline detection.

BROKER SELECTION

Open Source

Managed Cloud

ESP32 MQTT CLIENT

ESP-IDF (Production)

#include "mqtt_client.h"

esp_mqtt_client_config_t cfg = {
    .broker.address.uri = "mqtts://mqtt.2nth.io:8883",
    .broker.verification.certificate = server_cert_pem,
    .credentials.username = "device01",
    .credentials.authentication.password = "secret",
    .session.last_will = {
        .topic = "device/status",
        .msg = "{\"online\":false}",
        .qos = 1, .retain = true,
    },
};
esp_mqtt_client_handle_t client = esp_mqtt_client_init(&cfg);
esp_mqtt_client_start(client);

// Publish telemetry
esp_mqtt_client_publish(client,
    "device/telemetry",
    "{\"temp\":24.5,\"heap\":280000}", 0, 0, 0);

Arduino (PubSubClient)

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

WiFiClient espClient;
PubSubClient mqtt(espClient);
mqtt.setServer("mqtt.2nth.io", 1883);
mqtt.connect("esp32-device", "user", "pass");
mqtt.subscribe("device/cmd");
mqtt.publish("device/telemetry", "{\"temp\":24.5}");

SECURITY

Always use TLS (port 8883) in production. Plaintext MQTT on 1883 is unacceptable for anything beyond local development.

SCALING PATTERNS

Bridge Pattern (Edge to Cloud)

Edge Broker (NanoMQ)              Cloud Broker (EMQX)
  Local devices publish             Aggregated telemetry
  Low-latency local rules           Dashboard / API
  Offline buffering                 Rules engine -> DB
  Bridge forwards to cloud -------> Fleet management

Shared Subscriptions (v5)

# Three workers share the load — each msg to only one
Worker-1: SUBSCRIBE $share/workers/telemetry/#
Worker-2: SUBSCRIBE $share/workers/telemetry/#
Worker-3: SUBSCRIBE $share/workers/telemetry/#

Sparkplug B (Industrial)

Standardised topic namespace with birth/death certificates, Protobuf payloads, and state management for industrial IoT. Topic format: spBv1.0/{group}/{type}/{edge_node}/{device}

PAYLOAD FORMATS

KEY GITHUB REPOSITORIES