What is MQTT and How it Works?


The Internet of Things (IoT) has transformed the way we live and work, with more and more of our everyday devices and appliances connecting to the internet and communicating with each other. From smart home assistants to wearable fitness trackers, the IoT enables an exciting new era of convenience, efficiency, and insight through connected technology.

But facilitating reliable communication between all these networked devices brings formidable challenges. Traditional HTTP protocols used for web traffic are often too heavyweight for resource-constrained IoT devices. Moreover, the need for real-time data streaming means we cannot rely on periodic client/server polling.

This is where MQTT comes in. MQTT (Message Queuing Telemetry Transport) is a lightweight publish/subscribe messaging protocol optimized for machine-to-machine (M2M) communication in IoT environments. Its efficient, low-bandwidth operation makes it ideal for connecting devices with limited capabilities or unreliable connections.

In this comprehensive guide, we will unveil the world of MQTT and how it establishes efficient, real-time communication for the IoT. We’ll cover:

  • The core concepts behind MQTT
  • How the publish/subscribe architecture works
  • The role of the MQTT broker
  • Quality of Service levels in MQTT
  • MQTT protocol messages and topic structure
  • Key benefits of using MQTT for IoT
  • Real-world applications leveraging MQTT
  • The future of MQTT in emerging technologies
  • FAQs about MQTT

So let’s dive in and illuminate this transformative technology!

Delving into the Essentials of MQTT

MQTT stands for Message Queuing Telemetry Transport. It is an open-source protocol initially developed in 1999 by Andy Stanford-Clark of IBM and Arlen Nipper of Arcom (now Eurotech).

MQTT provides a lightweight method of exchanging structured data and control information between devices using a publish/subscribe (pub/sub) messaging pattern. These characteristics make it ideal for machine-to-machine communication in IoT environments.

Publish-Subscribe Architecture

The pub/sub messaging pattern is the heart of MQTT. It decouples the sending and receiving of messages.

Publishers generate messages and publish them to specific topics. Subscribers express interest in various topics and receive messages published to those topics.

This allows highly scalable one-to-many message distribution with minimal network overhead. Devices publish updates to a topic, without targeting any particular recipient. Other devices subscribe to topics of interest and seamlessly receive relevant updates.

Publish-subscribe architecture of MQTT

Role of the Broker

what is mqtt broker

The publish-subscribe delivery of MQTT happens through an MQTT broker.

The broker is responsible for filtering messages based on topic, and distributing them to subscribed clients. It acts as a central hub, accepting published messages and dispatching them accordingly.

Using a broker removes the need for direct connections between every client. Publishers simply send messages to the broker. Subscribers connect to the broker to receive messages. This keeps the network topology simple even for large numbers of clients.

The broker also handles other key tasks like authentication, permissions, and persistence of messages. MQTT brokers can scale to handle huge numbers of simultaneously connected devices. Popular choices include Mosquitto, HiveMQ and AWS IoT Core.

Message Delivery Quality of Service

Message Delivery Quality of Service

Since IoT devices often use unreliable networks like WiFi or cellular, MQTT build in features to ensure reliable message delivery. Clients can specify a Quality of Service (QoS) level when publishing messages:

  • QoS 0 – No guarantee of delivery. Message sent only once.
  • QoS 1 – Message delivered at least once. May receive duplicates.
  • QoS 2 – Message delivered exactly once. Uses a 4 step handshake.

Higher levels provide more certainty of delivery, at the cost of more processing overhead. So clients can choose the optimal QoS level for each use case. Settings like telemetrics can use QoS 0, while critical control messages require QoS 2.

This gives MQTT flexibility to operate efficiently across different environments, from a satellite network to a noisy shop floor.

Key Components of MQTT Architecture

The MQTT ecosystem consists of three core components: publishers, subscribers, and the broker.


Publishers are MQTT clients that send messages to the broker. They package the payload data along with a topic name that classifies the message.

For example, a publisher could be a temperature sensor that periodically sends temperature readings to the topic “home/bedroom/temperature”. Or an event-based publisher like a motion sensor that sends real-time alerts whenever it detects movement.

Publishers establish connections with the broker and send messages using MQTT protocol commands like CONNECT, PUBLISH and DISCONNECT.


Subscribers are clients that connect to the broker and subscribe to various topics of interest. The broker delivers messages from any publisher to the subscribers of those topics.

For instance, a subscriber could be a smart AC system that subscribes to “home/bedroom/temperature” to adjust cooling accordingly. Or a security app subscribed to motion alerts from sensors.

Subscribers register subscriptions with the broker and receive messages using protocol commands like CONNECT, SUBSCRIBE and UNSUBSCRIBE. The subscriptions can include wildcard patterns to receive messages from a range of topics.


The MQTT broker manages connections from publishers and subscribers and routes messages between them. It is responsible for:

  • Accepting incoming network connections using TCP/IP or WebSockets
  • Authenticating clients and authorizing access
  • Receiving published messages and dispatching them to subscribed clients
  • Maintaining session information and subscriptions for connected clients
  • Queueing messages offline until clients reconnect
  • Communicating with other brokers to form a federated network

Thus, the broker forms the core of any MQTT system. It needs to be highly scalable and reliable to handle huge numbers of devices and messages.

Popular open source brokers like Mosquitto, HiveMQ and EMQ can handle thousands of clients on commodity hardware. There are also fully managed cloud brokers like AWS IoT Core and Google Cloud IoT Core.

Exploring MQTT Protocol Messages

MQTT relies on a simple, lightweight protocol to facilitate real-time messaging between clients. Let’s explore the structure of MQTT protocol messages and the key concepts involved.

MQTT Topics

The topic is a crucial part of any MQTT message. Topics provide a hierarchical structure to classify and organize messages.

When publishing a message, the client sets the topic to categorize the information. The broker uses topics to filter messages and deliver them to subscribers.

For example, a topic like “home/kitchen/candle_flame_detector” clearly identifies the source and purpose of the message. MQTT topic elements are separated by forward slashes, with arbitrary nesting depth.

Topics can use wildcards like # and + for broader subscriptions:

  • home/kitchen/# – Receive all messages under kitchen
  • +/candle_flame_detector – Receive all flame detector messages

Message Payloads

The payload contains the actual data being transmitted in the MQTT message. It can be any content ranging from a few bytes to kilobytes of data.

For sensor readings, the payload would contain the measured value and metadata like units, accuracy, etc. Control messages might have action directives or parameter updates in the payload.

To optimize for low bandwidth usage, payloads should be kept as small and efficient as possible. MQTT functions well across low-throughput networks since just the topic and small payload need to be transmitted.

Quality of Service

As discussed before, the Quality of Service (QoS) level is an important parameter in each MQTT message. It determines delivery guarantees and underlying communication overhead.

The three QoS levels are:

QoS 0 – Fire and forget. Message delivered at most once. Default for many non-critical applications.

QoS 1 – Ensures delivery at least once using acknowledgements. May receive duplicate messages.

QoS 2 – Delivered exactly once by handling re-transmissions. Uses a 4 step handshake. Critical for control messages.

The broker stores QoS 1 and 2 messages until it gets acknowledgement of successful delivery to subscribed clients. This prevents data loss but incurs more processing overhead.

Benefits of Adopting MQTT for IoT Applications

MQTT is designed to meet the specific demands of machine-to-machine communication in IoT systems. Let’s examine some key advantages:


MQTT uses a minimal packet header design, with fixed lengths of just 2 bytes for control packets. This allows for greater efficiency than HTTP headers which use ASCII strings.

Control packets like CONNECT and PUBLISH have a maximum payload length of just 256 MB. This optimizes MQTT for low-bandwidth networks and embedded devices with limited processing and memory.

Low Bandwidth Usage

MQTT minimizes network bandwidth usage by reducing the amount of data transmitted. This allows it to work across constrained networks and reduces operational costs like cellular data usage.

Protocol overhead is slashed by eliminating unnecessary transmissions. For example, PUBLISH packets omit the subscriber address – the broker handles delivery. Keepalive pings monitor connection health without transmitting heavy data.

Bandwidth usage scales linearly with number of clients, rather than exponentially. This enables support for large device densities.

Reliable Message Delivery

Quality of Service levels allow reliable transmission even on unreliable networks like satellite links or noisy factory floors.

For critical messages, MQTT guarantees delivery using handshakes and acknowledgements. Stored messages are forwarded when clients reconnect after a network dropout.

These features establish MQTT as a robust platform for industrial automation, remote equipment monitoring and other mission-critical applications.


MQTT scales to gigantic numbers of clients and messages by using efficient networking patterns.

The publish-subscribe architecture removes overhead from one-to-one messaging between clients. Wildcard subscriptions and topic-based filtering optimize data distribution.

Commercial and open-source brokers leverage techniques like clustering to handle millions of simultaneous clients on commodity servers.


MQTT offers unmatched flexibility and customizability for different environments and use cases.

Clients can select from multiple Quality of Service levels to tune delivery guarantees. Topic hierarchies for messaging can be designed to suit any ontology. Payloads efficiently transmit any data format.

This versatility allows MQTT to connect diverse devices across a huge array of industries – from healthcare to oil rigs!


MQTT enables secure communication using TLS/SSL encryption for the underlying TCP connections.

Access control is managed using client usernames and passwords. Authorization policies determine allowed topic accesses and operations for each user. Audit logs record security events.

MQTT brokers can integrate with external identity providers via SASL authentication. Encrypted client certificates allow connecting approved devices.

Open Standard

MQTT is an open ISO standard (ISO/IEC PRF 20922) with openly published documentation. This ensures interoperability between clients and brokers from different vendors.

There are open source broker implementations like Mosquitto and HiveMQ. Code for MQTT client libraries is freely available for all major languages and platforms.

An open standard avoids vendor lock-in and enables widespread adoption. The MQTT community continues to drive evolution of the protocol.

Alotcer MQTT in Action: IoT Applications

MQTT is gaining tremendous traction across industries as the Internet of Things expands. Let’s explore some major IoT domains that are leveraging MQTT:

Smart Home Automation

In smart homes, MQTT connects and controls lighting, climate, entertainment, security and other systems:

  • Smart thermostats publish temperature readings and adjust HVAC accordingly
  • Motion sensors send real-time intruder alerts
  • Lighting systems subscribe to time schedules to control bulbs
  • Voice assistants use MQTT for system commands and notifications

Home automation platforms like Home Assistant use MQTT as the backbone for device integration. It provides a unified pipeline for efficient home network communication.

Industrial IoT

AR7091 Industrial IoT Gateway

For industrial environments, MQTT delivers key data between sensors, controllers and monitoring systems:

  • Sensors publish real-time operational data such as temperatures or tank levels
  • Alarms are raised for out of bounds measurements
  • Control systems adjust equipment parameters based on analyzed metrics
  • Dashboards visualize aggregates and analytics for operators

MQTT withstands the challenges of factories – range, interference, noise – to enable Industry 4.0. Reliable real-time data enables automation and intelligence.


In the fitness and health domain, wearables leverage MQTT to transmit tracked activity data:

  • Fitness trackers publish accelerometer data, GPS coordinates, heart rate readings, etc.
  • Data is aggregated into activity summaries for display to users
  • Alerts are raised for arrhythmia detection or lack of activity
  • Doctor-defined diagnostic rules respond to biomarker thresholds

MQTT allows efficient streaming of biometrics data and alerts. This facilitates development of personalized digital medicine apps.

These are just a few examples. MQTT also powers smart cities, supply chain logistics, renewable energy, and more!

The Future of MQTT: Embracing New Technologies

MQTT has proven its mettle as an IoT messaging protocol. But it also shows promise as enabler for cutting edge technological paradigms:

MQTT for Edge Computing

Edge computing pushes processing out to local devices rather than centralized cloud servers. MQTT is optimal for streaming data to and from edge nodes:

  • Real-time metrics are published from nearby equipment to edge gateways
  • The gateways filter and analyze the data locally and send aggregated results to the cloud
  • Cloud systems can publish configuration updates back to field devices via MQTT

By reducing network trips, MQTT unlocks the potential for real-time edge intelligence.

Integration with AI/ML

MQTT is a natural fit to support emerging AI/ML technologies in the IoT space:

  • MQTT streams high frequency data from IoT devices to feed AI models
  • MQTT delivers inferences and directives from AI systems back to operational equipment
  • During model training, MQTT efficiently transmits datasets from distributed sensors to the cloud
  • Anomaly alerts are published based on real-time deviations detected by AI agents

MQTT delivers the high speed, reliable data pipelines required to enable AI-driven automation.

The lightweight yet powerful nature of MQTT makes it the ideal messaging backbone to unlock new possibilities as technology progresses.

Commonly Asked Questions about MQTT

What is MQTT used for?

MQTT is optimized for IoT environments due to its lightweight architecture and reliability. It connects low-power, resource-constrained devices where bandwidth is limited. Typical use cases involve telemetry, event notifications, and controlling equipment.

How does MQTT ensure message delivery?

MQTT provides different Quality of Service (QoS) levels. QoS 1 ensures delivery at least once by using acknowledgements. QoS 2 delivers messages exactly once by using a 4 step handshake between sender and receiver.

Is MQTT suitable for real-time applications?

Yes, MQTT is designed for low-latency real-time messaging. Messages are delivered immediately rather than using scheduled polling. This allows fast responses for time-sensitive applications.

What security measures does MQTT employ?

MQTT brokers secure connections using TLS/SSL. Access can be controlled through username/password authentication and ACLs. Messages can also be end-to-end encrypted by clients for added privacy.

How to choose the right QoS level in MQTT?

QoS 0 for non-critical data like telemetry. QoS 1 for important alerts and updates where duplicates are acceptable. QoS 2 for commands and actions where exactly one delivery is vital. Balance reliability vs overhead.


As the Internet of Things revolution continues to accelerate, MQTT has established itself as the protocol of choice for reliable, real-time messaging between devices and systems. Its efficient publish-subscribe architecture, built for unreliable networks, makes it a perfect fit for connecting the massive number of embedded devices coming online.

MQTT streamlines communication flows between sensors, controllers, applications and the cloud. It enables exciting new possibilities like real-time analytics and automation powered by AI. With MQTT, businesses can reap benefits such as improved operational efficiency, reduced costs and new revenue opportunities.

In this guide, we went deep into how MQTT actually works – its components like the broker and subscribers, the protocol message format, different Quality of Service levels and the overall benefits it brings to the table. We saw MQTT in action across diverse verticals like smart homes, industrial automation and wearables. And we looked at how it can support emerging paradigms like edge computing and AI.

MQTT continues to evolve, with an active community driving new features and integrations. It enables engineers to focus on their core application logic rather than the challenges of device connectivity and messaging. As your IoT projects take off, MQTT delivers the resilient, scalable and extensible communication layer to set your innovations in motion.

So get started experimenting with this lightweight messaging protocol that is powering the future of connected things!