MQTT

If you're wondering what MQTT is exactly and why it dominates industrial IoTITermIoT (Internet of Things)The IoT (Internet of Things) is the network of physical objects with sensors, software and connectivity that collect and exchange data and act autonomously.View profile, here's the short answer: it's the lightest pub/sub protocol that exists for unreliable networks. If you're starting from scratch, it's worth first understanding what IoT is and how the ecosystem works; if you already have that foundation, read on.

Executive summary

• MQTT (Message Queuing Telemetry Transport) is a lightweight pub/sub messaging protocol standardized under ISO/IEC 20922 and maintained by OASIS.
• It runs over TCP/IP with tiny headers (2 fixed bytes), designed for devices with limited CPU, constrained battery, and unreliable networks.
• Architecture: clients (publishers and subscribers) talk to a central broker that distributes messages across hierarchical topics.
• It is the dominant protocol in industrial IoT (Industry 4.0), smart home (Home Assistant, Tasmota, Shelly), and large-scale messaging (WhatsApp and Facebook Messenger use it internally).
• Versions: MQTT 3.1.1 (ubiquitous, 2014) and MQTT 5.0 (2019, adds shared subscriptions, reason codes, message expiry).
• Ports: 1883 (plain TCP), 8883 (TLS), 80/443 (WebSockets).
• Don't use it when: you need synchronous request/reply (use HTTP/CoAPProtocolCoAPCoAP (RFC 7252) brings GET/POST/PUT/DELETE to devices with 8 KB of RAM over UDP. How Observe and DTLS work, and when to pick it over MQTT.View profile), you have large messages (>256 MB), or you can't maintain a broker.

What MQTT is and what it's for

MQTT stands for Message Queuing Telemetry Transport. It's a machine-to-machine (M2M) communication protocol designed in 1999 by Andy Stanford-Clark (IBM) and Arlen Nipper (Cirrus Link) to monitor oil pipelines via satellite — environments with scarce bandwidth and connections that keep dropping. At Plataforma IoT we cover it alongside the rest of the IoT protocols that make up the connectivity ecosystem.

Unlike HTTP (which is point-to-point, request/response, with verbose headers), MQTT uses an asynchronous publish/subscribe model with an intermediary:

[Device A] → publishes to topic "factory/line1/temperature" → [Broker] → distributes to [Device B, C, D] subscribed

The broker (Mosquitto, EMQXETermEMQXEMQX is a highly scalable open-source MQTT broker (written in Erlang) for IoT production, with clustering and multi-protocol support.View profile, HiveMQ, AWS IoT Core, etc.) receives the messages and forwards them to every client subscribed to the corresponding topic. The sender doesn't know — and doesn't need to know — who's listening. That decouples producers from consumers and makes it possible to scale to millions of devices.

How MQTT works

1. Connection

A client opens a TCP connection to the broker (port 1883 by default) and sends a CONNECT packet with:

• client_id (unique per device)
• optional username and password
• keep_alive (in seconds: how often the client sends a PINGREQ to keep the session alive)
• optional will message: a message the broker will publish if the client disconnects abruptly
• clean_session: whether subscriptions are preserved between connections

The broker responds with CONNACK (success or an error code).

2. Topics

Topics are hierarchical UTF-8 strings separated by /:

factory/lineA/machine01/temperature
factory/lineA/machine01/vibration
home/livingroom/light/state

Subscribers can use wildcards:

• + (single level): factory/+/machine01/temperature receives the temperature of machine01 on any line.
• # (multi level): factory/# receives everything under factory.

3. Publish / Subscribe

To publish: a PUBLISH packet with topic, payload (any bytes — text, JSON, binary up to 256 MB), qos, and retain.

To subscribe: a SUBSCRIBE packet with a list of topics and the desired QoSMTermMQTT QoSMQTT QoS (Quality of Service) defines a message's delivery guarantee at three levels: 0 (at most once), 1 (at least once) and 2 (exactly once).View profile for each one.

4. QoS (Quality of Service)

MQTT defines 3 levels:

Security note: QoS levels are independent of encryption. MQTT without TLS transmits everything in plain text, including credentials. For production deployments, see our IoT security guide, where we detail authentication and encryption best practices for brokers.

Note: QoS 2 adds significant latency. In industrial IoT, QoS 1 plus consumer-side idempotency is almost always better than QoS 2.

5. Retained messages

If you publish with retain=true, the broker stores the last message on the topic. When a new client subscribes, it immediately receives that last value. Useful for current state (e.g., "last known temperature").

6. Last Will & Testament (LWT)

If the client disconnects abruptly (network drop, crash), the broker automatically publishes the "will message" the client declared in CONNECT. A typical pattern: device/123/status with the value online retained, plus an LWT offline retained — this enables real-time presence.

Real-world use cases

Leading MQTT brokers

Pros and cons

Pros

• Lightweight: a fixed 2-byte header, ideal for microcontrollers (ESP32, nRF52).
• Resilient on poor networks: designed for intermittent connections.
• Asynchronous: decouples producers and consumers, scales well.
• Mature standard: ISO/IEC 20922, a huge ecosystem.
• TLS support: end-to-end encryption at the transport layer.

Cons

• Requires a broker: it isn't peer-to-peer; the broker is a single point of failure unless you cluster it.
• Not secure by default: without TLS, everything travels in the clear. Without auth, anyone can publish. You have to configure it.
• Hard to debug: binary payloads, multiple publishers — you need tools (MQTT Explorer, mosquitto_sub).
• QoS 2 is expensive: the 4-way handshake adds latency.
• No automatic discovery: there's no standard way to list active topics (in MQTT 5 you can do it with $SYS/, but it depends on the broker).

MQTT vs HTTP vs CoAP

Summary: MQTT wins when you have many devices sending continuous data. HTTP wins for request/response APIs and catalogs. CoAP wins on extremely constrained devices that can't sustain TCP. If you're still evaluating which protocol fits your project, the IoT platforms section can help you understand the full stack.

Getting started: minimal setup

1. Spin up a local broker (Mosquitto)

# macOS
brew install mosquitto
brew services start mosquitto

# Docker (cross-platform)
docker run -d --name mosquitto -p 1883:1883 eclipse-mosquitto

2. Subscribe to a topic

mosquitto_sub -h localhost -t "test/topic" -v

3. Publish from another terminal

mosquitto_pub -h localhost -t "test/topic" -m "Hello IoT"

You'll see the message appear in the subscriber. You now have pub/sub working.

MQTT is also the protocol of choice for edge computing in IoT patterns: the broker can run on a local gateway and process data before pushing it to the cloud, reducing latency and transmission cost.

4. Connect an MCU (ESP32 + Arduino)

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

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  WiFi.begin("ssid", "pass");
  client.setServer("192.168.1.10", 1883);
  client.connect("esp32-01");
  client.publish("home/livingroom/temperature", "22.5");
  client.subscribe("home/livingroom/light/cmd");
}

void callback(char* topic, byte* payload, unsigned int length) {
  // process incoming command
}

Primary sources

• OASIS MQTT specification — official spec (accessed: 2026-05)
• mqtt.org — official OASIS site (accessed: 2026-05)
• Eclipse Mosquitto — open source reference broker
• HiveMQ MQTT Essentials — educational series
• MQTT 5 features (accessed: 2026-05)