MQTT Principles: Publish/Subscribe, Brokers and Clients

Why MQTT?

MQTT is a lightweight messaging protocol designed for unreliable networks and constrained devices. It minimizes overhead while ensuring timely delivery, making it ideal for industrial IoT.

Key Features:

• Publish/Subscribe decouples producers (publishers) from consumers (subscribers).
• Brokers handle message routing and store messages if needed (retained messages).
• QoS levels guarantee delivery from at-most-once to exactly-once.

Architecture Overview

1. Publisher: Sends messages to a topic without knowing recipients.
2. Broker: Central server routes messages based on topic subscriptions.
3. Subscriber: Receives messages for topics it has subscribed to.

flowchart LR
  Pub[Publisher] -->|"planta/area1/sensor"| Broker((Broker))
  Broker -->|"planta/area1/sensor"| Sub[Subscriber]

Connecting as a Client

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    print("Connected with code", rc)
    client.subscribe("planta/area1/sensor")

def on_message(client, userdata, msg):
    print(msg.topic, msg.payload)

client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.tls_set()  # optional TLS
client.username_pw_set("user","pass")
client.connect("broker.local", 8883)
client.loop_forever()

Advantages for IIoT

• Low bandwidth usage and small packet sizes.
• Supports intermittent connectivity and offline queuing.
• Simple API available in most languages and platforms.

MQTT Topics and Hierarchies

Topics in MQTT follow a hierarchical structure using / delimiters:

• Levels: Each level defines a namespace segment (e.g., planta/area1/maquina/sensor).
• Wildcards: + matches a single level, # matches all remaining levels.

Quality of Service (QoS) Levels

MQTT defines three QoS levels:

• QoS 0 (At most once): No acknowledgment, best-effort delivery.
• QoS 1 (At least once): Acknowledged delivery, may result in duplicates.
• QoS 2 (Exactly once): Two-phase handshake ensures single delivery.

Payload Formats

• JSON is the standard for IIoT payloads:

{ "sensor_id": "S1", "value": 23.5, "unit": "°C" }

• Use binary or Protobuf for high-throughput or low-bandwidth scenarios.

MQTT Versions: v3.1.1 vs v5

Security Considerations

• TLS/SSL: Encrypt connections with certificates.
• Authentication: Username/password or client certificates.
• Authorization (ACLs): Restrict topic access per client.

Understanding these principles is the first step to building reliable IIoT systems with MQTT.