What Is MQTT? How Robots and IoT Devices Exchange Messages.

What Is MQTT?

MQTT is a lightweight messaging protocol used to exchange information between devices and applications through a publish-subscribe communication model.

It is commonly used in the Internet of Things, industrial monitoring, and connected equipment.

For example, a factory robot may publish its battery status or operating condition to an MQTT broker.

Monitoring applications can then receive those updates without needing separate direct connections to the robot.

Key Takeaways

• MQTT uses a publish-subscribe model built around clients, topics, and a broker.
• It is designed to support lightweight messaging, including on constrained devices and networks.
• MQTT offers different delivery-quality levels, but these do not automatically guarantee safety-critical or real-time robot behavior.

How Does MQTT Work?

Imagine a warehouse containing 100 mobile robots.

Each robot needs to report information about its operating condition.

Rather than connecting every robot directly to every monitoring application, the robots publish messages to an MQTT broker.

A message may be associated with a topic such as:

warehouse/robot17/battery

Applications interested in battery measurements subscribe to the relevant topic.

When the robot publishes an update, the broker distributes the information to matching subscribers.

This makes communication easier to organize as devices and applications are added.

What Are MQTT Quality of Service Levels?

MQTT defines three common delivery-quality levels.

QoS 0 — At most once: A message is sent without the protocol's acknowledgment-and-retry mechanism.

QoS 1 — At least once: Delivery is acknowledged and may involve retransmission, so duplicates can occur.

QoS 2 — Exactly once: Uses a more involved protocol exchange to provide exactly-once message delivery within the defined MQTT messaging context.

Higher QoS levels introduce additional communication overhead.

Importantly, exactly-once message delivery does not automatically mean a physical task will execute exactly once.

Applications must still manage command processing and device behavior.

MQTT vs. DDS

MQTT and DDS can both support publish-subscribe communication.

MQTT commonly uses a broker-based architecture and is widely adopted for IoT telemetry.

DDS provides distributed data communication with configurable policies suited to many demanding applications.

For example, a robotics company might use DDS within a robot's software architecture and MQTT to send selected fleet status information to a monitoring platform.

The technologies can complement each other.

What Are MQTT's Limitations?

A broker can become an important dependency.

Network outages, authentication problems, and application failures can interrupt information exchange.

Traditional MQTT communication also does not provide a complete machine-safety system.

Security and operational reliability depend on authentication, encryption, broker configuration, and application design.

Frequently Asked Questions

Does MQTT require the public internet?

No. MQTT can operate entirely within a local network.

Is MQTT suitable for controlling robot motors?

It is generally more appropriate for telemetry and application messaging than for hard real-time motor control.

The Bottom Line

MQTT provides a lightweight way for connected machines and applications to exchange messages. It is especially useful for industrial telemetry and fleet monitoring.