What Is MQTT? The IoT Messaging Protocol

What Is MQTT?

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for IoT devices operating on low-bandwidth and unreliable networks. Developed by IBM in 1999, the protocol was published as an OASIS standard in 2014. It runs over TCP/IP, and its message size is minimal.

The MQTT Publish-Subscribe (Pub/Sub) Model

Unlike the traditional client-server model, MQTT operates through a broker (intermediary):

• Publisher: The device or application that produces data. It tags the data with a topic and sends it to the broker.
• Broker: Receives messages and delivers them to the relevant subscriber(s). Popular examples: Mosquitto, HiveMQ, EMQX.
• Subscriber: Subscribes to the topics it is interested in; the broker delivers each new message as it arrives.

Thanks to this architecture, the publisher and the subscriber do not need to know one another; their connections are independent. This characteristic is especially valuable for field devices with intermittent internet connectivity.

QoS Levels

• QoS 0 — At most once. Fast, but carries a risk of loss.
• QoS 1 — At least once. Through an acknowledgment (ACK) mechanism, messages can be re-sent. Argus EMS uses this level.
• QoS 2 — Exactly once. A four-way handshake; the most reliable but the slowest.

MQTT and Its Use in Energy Monitoring Systems

In industrial energy monitoring systems, MQTT is the most efficient way to move field data to a central server. In the Argus EMS architecture:

• The Field Agent on the on-site PC converts data read from Modbus/M-Bus devices into MQTT messages
• It sends them over a TLS-encrypted connection on port 8883 to the Mosquitto broker on the Linux server
• The MQTT subscriber receives the messages and writes them to the PostgreSQL database
• If the connection drops, the Field Agent holds the data in a SQLite queue and sends it in bulk once the connection is restored

MQTT TLS Security

In production systems, MQTT must always be encrypted with TLS:

• Port 1883 (unencrypted) is kept open only to localhost
• Port 8883 (TLS) is used for external connections
• Certificate validation is enabled
• For client authentication, a username/password or a client certificate is used

Retain and Last Will Messages

Retain: The broker stores the last message; a new subscriber immediately receives the most recent value. Ideal for publishing device status.
Last Will: If the connection is unexpectedly lost, the broker publishes this message. The Argus EMS Field Agent uses this method to notify the central server of a dropped connection.